rice nutrient disorders nutrient management volume 1

8/11/2019 Rice Nutrient Disorders Nutrient Management Volume 1

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Achim Dobermann

International Rice Research Institute

Thomas Fairhurst

Potash & Phosphate Institute/Potash & Phosphate Institute of Canada

RiceNutrient Disorders &Nutrient Management


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Rice: Nutrient Disorders & Nutrient ManagementHandbook Series

A. DobermannT.H. Fairhurst

Copyright 2000

Potash & Phosphate Institute (PPI), Potash & Phosphate Institute of Canada (PPIC) andInternational Rice Research Institute (IRRI).

All rights reserved

No part of this handbook or the accompanying CD-ROM may be reproduced for use in anyother form, by any means, including but not limited to photocopying, electronic informationstorage or retrieval systems known or to be invented. For permission to produce reprints andexcerpts of this handbook, contact PPI.

Limits of liability

Although the authors have used their best efforts to ensure that the contents of this book arecorrect at the time of printing, it is impossible to cover all situations. The information is distributedon an as is basis, without warranty. Neither the authors nor the publishers shall be responsiblefor any liability, loss of profit or other damages caused or alleged to have been directly orindirectly caused by following guidelines in this book.

Typesetting & layout by Tham Sin Chee

First edition 2000

ISBN 981-04-2742-5

About the publishersPPIs mission is to develop and promote scientific information that is agronomically sound,economically advantageous, and environmentally responsible in advancing the worldwide useof phosphorus and potassium in crop production systems. PPI books are available at special

discounts for bulk purchases and member companies. Special editions, foreign languagetranslations, and excerpts can also be arranged - contact PPIs East and Southeast AsiaPrograms office for more information (details are on the back cover).

IRRls goal is to improve the well-being of present and future generations of rice farmers andconsumers, particularly those with low incomes. It was established in 1960 by the Ford andRockerfeller Foundations with the help and approval of the Government of the Philippines.Today it is one of 16 nonprofit international research centers supported by the ConsultativeGroup on International Agricultural Research (CGIAR).

Printed by Oxford Graphic Printers Pte Ltd


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Acknowledgments

We wish to acknowledge the following people and organizations:

Dr. Christian Witt (IRRI) for writing most of Sections 2.42.6, revising the chapters on N, P, and

K, and many other fruitful discussions and comments.Dr. Shaobing Peng (IRRI) and Dr. Helmut von Uexkull (Bonn, Germany) for reviewing the bookand for their suggestions on improvements.

Mrs. Corintha Quijano (IRRI) for providing slides and revising all chapters on nutritional disorders.

Dr. V. Balasubramanian (IRRI) for contributing to Section 5.9, and reviewing an earlier draft ofthe book.

Dr. Kenneth G. Cassman (University of Nebraska - Lincoln, USA), who initiated much of theresearch on improving nutrient management and nitrogen efficiency in rice. The framework forassessing N efficiency described in Section 5.6 is largely based on his work.

All scientists, support staff and farmers participating in the Reversing Trends of DecliningProductivity in Intensive, Irrigated Rice Systems (RTDP) project, for providing key data on N, P,and K efficiencies.

Dr. David Dawe (IRRI) for constantly reminding us that economists have a different view of theagricultural world.

Dr. Lawrence Datnoff (University of Florida, USA) for providing slides on Si deficiency.

Dr. Takeshi Shimizu (Osaka Prefecture Agriculture and Forestry Research Center, Japan) forcontributing slides on various nutritional disorders.

Dr. Ernst Mutert (PPI -PPIC) for encouraging us to take on this task.

Bill Hardy, Katherine Lopez, and Arleen Rivera (IRRI), and Tham Sin Chee (PPI -PPIC) foreditorial assistance.

Elsevier Science for permission to reprint the photograph from Crop Protection, Vol 16, DatnoffL, Silicon fertilization for disease management of rice in Florida; Dr. Helmut von Uexkull (PPI -PPIC), Dr. Pedro Sanchez (ICRAF) and Dr. Jose Espinosa (PPI -PPIC) for permission to reusetheir photographs.

The following organizations for funding different components of the RTDP project, includingfinancial support for the production of this book:

Swiss Agency for Development and Cooperation (SDC),

Potash and Phosphate Institute and Potash and Phosphate Institute of Canada (PPl -

International Fertilizer Industry Association (IFA),

International Potash Institute (IPI), and

International Rice Research Institute.

Finally, writing a book is impossible without family support and we were lucky to enjoy this at allstages. Thus, we thank Ilwa, Joan, and our kids for their hearty support and understanding.

PPIC),

Achim Dobermann and Thomas Fairhurst

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Foreword

Thirty years ago, persuading rice farmers to use modern varieties and their accompanyingfertilizer inputs was easy because the results, in terms of yield increases, were often spectacular.

At the same time, governments invested heavily in fertilizer subsidies, and made improvementsto irrigation facilities, infrastructure, and rice price support mechanisms that made riceintensification (increased input use, increased number of crops per year) economically attractive.

Further improvements in rice productivity, however, are likely to be much more incremental andknowledge-based. Future yield increases will mostly result from the positive interactions andsimultaneous management of different agronomic aspects such as nutrient supply, pest anddisease control, and water.

In many countries, fertilizer and other input subsidies have already been removed and it islikely that in the future, the maintenance of irrigation facilities will increasingly become theresponsibility of farmers rather than governments. This means that to achieve the requiredfuture increases in rice production, extension services will need to switch from distributingprescriptive packets of production technology to a more participatory or client-based servicefunction. Such an approach requires greater emphasis on interpreting farmers problems anddeveloping economically attractive solutions tailored to each farmers objectives. Yet extensionservices are generally ill-prepared for such a change.

This handbook provides a guide for detecting nutrient deficiency and toxicity symptoms, andmanaging nutrients in rice grown in tropical and subtropical regions. Some backgroundinformation on the function of nutrients in rice and the possible causes of nutrient deficienciesare included. Estimates of nutrient removal in grain and straw have been included to helpresearchers and extension workers calculate the amount of nutrients removed from the fieldunder different management systems. Specific nutrients are discussed in Chapter 3 - Mineral

Deficiencies.In most tropical and subtropical regions, rice farms are small, nutrients are managed by handand farmers do not have access to more resource-demanding forms of nutrient management,such as soil and plant tissue testing. Therefore, we describe a new approach to calculatingsite-specific nutrient management recommendations for N, P, and K in lowland rice. The conceptdescribed is based on ongoing, on-farm research in the Mega Project on Reversing Trends inDeclining Productivity in Intensive, Irrigated Rice Systems, a collaborative project betweenIRRl and researchers in China, India, Indonesia, the Philippines, Thailand, and Vietnam, Asthis work progresses, a more complete approach for site-specific nutrient management willevolve.

This handbook has been written primarily for irrigated and rainfed lowland rice systems, becausethese systems account for about 80% of the total harvested area of rice and 92% of global riceproduction. Where appropriate, we have included additional information particular to uplandrice or rice grown in flood-prone conditions. We hope that this book will help increase theimpact of new approaches to nutrient management at the farm level by bridging the gap betweentechnology development and field implementation.

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Contents

1

1.11.21.3

2

2.12.22.32.4

2.52.62.72.8

3

3.13.23.33.4

3.53.63.73.83.93.103.113.12

Topic PageRice Ecosystems ................................................................................ 2Irrigated Rice ............................................................................................................... 3Rainfed Lowland and Upland Rice .......................................................................... 6Flood -Prone Rice .......................................................................................................... 11

Nutrient Management ...................................................................... 12

Yield Gaps and Crop Management ........................................................................... 13The Nutrient Input -Output Budget in an Irrigated Rice Field ................................ 15Site -Specific Nutrient Management Strategy ........................................................... 18

Crop Nutrient Requirements-

The Nutritional Balance Concept .............................. 25

Managing Organic Manures. Straw. and Green Manure ............................................. 32

Estimating Indigenous N, P, and K Supplies ........................................................... 22

Recovery Efficiencies of Applied Nutrients ............................................................. 28

Economics of Fertilizer Use .................................................................................... 38

Mineral Deficiencies ....................................................................... 40

Nitrogen Deficiency .................................................................................................... 41

Phosphorus Deficiency .............................................................................................. 60

Potassium Deficiency ................................................................................................. 72Zinc Deficiency ............................................................................................................ 84

Sulfur Deficiency ......................................................................................................... 90Silicon Deficiency ....................................................................................................... 95

Magnesium Deficiency ............................................................................................... 99

Calcium Deficiency ................................................................................................... 102Iron Deficiency ......................................................................................................... 105Manganese Deficiency ............................................................................................. 109Copper Deficiency ................................................................................................... 113Boron Deficiency ..................................................................................................... 117

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Topic Page

4 Mineral Toxicities ...................................................................................... 120

4.1 Iron Toxicity .................................................................................................................... 1214.2 Sulfide Toxicity 1264.3 Boron Toxicity 1294.4 Manganese Toxicty 1324.5 Aluminum Toxicity 1354.6 Salinity .............................................................................................................................. 139

5 Tools and Information .............................................................................. 146

................................................................................................................................................................................................................................

..................................................................................................................................................................................................................

5.1

5.25.35.45.55.65.75.85.9

Soil Zones. the Fate of Fertilizer Nitrogen, and the Rhizosphere in Lowland

Paddy Soils .................................................................................................................... 147Diagnostic Key for Identifying Nutrient Deficiencies in Rice ......................................... 151Nutrient Concentrations in Plant Tissue ........................................................................... 152Grain Yield and Yield Components .................................................................................... 154

Assessing Nitrogen Efficiency ............................................................................................ 155Tools for Optimizing Topdressed N Applications .......................................................... 161Soil - and Season -Specific Blanket Fertilizer Recommendations ................................. 166Converting Fertilizer Recommendations into Fertilizer Materials ................................. 169Soil and Plant Sampling ................................................................................................ 172

Appendices .......................................................................................................... 182 A1 Glossary & Abbreviations ............................................................................................... 183 A2 Measurement Units & Useful Numbers .......................................................................... 186 A3 Sources of Information .................................................................................................... 190

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Table 1

Table 2

Table 3

Table 4

Table 5

Table 6

Table 7

Table 8

Table 9

Table 10

Table 11

Table 12

Table 13

Table 14

Table 15

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List of Figures

Maximum yield and yield gaps at the farm level ........................................................... 14

Components of the input -output balance of nutrients in a typical irrigated rice field .... 15Strategy for site -specific nutrient management in irrigated rice .................................... 20Estimation of indigenous nutrient supplies of N, P, and K from grain yield innutrient omission plots 23

Schematic relationship between grain yield and plant nutrient accumulation in totalaboveground plant dry matter of rice as affected by potential yield ............................. 26

Schematic relationship between actual plant P accumulation with grain and straw atmaturity of rice and potential P supply for a certain maximum P uptake potential ....... 29

Relationship between grain yield, total N uptake and maximum yield .......................... 50

Approximate recovery efficiency of topdressed N fertilizer for rice at differentgrowth stages ................................................................................................................. 54

Relationship between grain yield and total P uptake depending on maximum yield .... 67

Relationship between grain yield and total K uptake depending on maximum yield .... 79Nitrogen cycle and N transformations in a flooded rice soil ........................................ 148

Processes causing acidification of the rhizosphere of rice under submergedconditions ............................................................................................................... 149

Examples of different N response functions and associated N use efficiencies

...................................................................................................

at N rate of 120 kg ha -1 ................................................................................................ 159

List of Tables

Nutrient budget for an irrigated rice crop yielding 6 t ha -1 ............................................. 17

The effect of nutrient availability on the removal of N, P, and K (in kg) per ton of ricegrain for the linear part of the relationship between grain yield and nutrient uptake .. 26

Optimal internal use efficiency for N, P, and K in irrigated rice ..................................... 27

Typical nutrient contents of organic materials ............................................................... 34

Typical nutrient concentrations of rice straw at harvest ................................................ 34

Optimal ranges and critical levels of N in plant tissue .................................................. 42N uptake and N content of modern rice varieties .......................................................... 45

N fertilizer sources for rice ............................................................................................. 49

Optimal ranges and critical levels of P in plant tissue ................................................... 61

P uptake and P content of modern rice varieties ........................................................... 63P fertilizer sources for rice ............................................................................................. 66

Optimal ranges and critical levels of K in plant tissue ................................................... 74

K uptake and K content of modern rice varieties .......................................................... 76

K fertilizers for rice ......................................................................................................... 79

Optimal ranges and critical levels of Zn in plant tissue ................................................. 85


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Table 16

Table 17

Table 18

Table 19

Table 20

Table 21Table 22

Table 23

Table 24

Table 25

Table 26

Table 27

Table 28

Table 29

Table 30

Table 31

Table 32

Table 33

Table 34

Table 35

Table 36

Table 37

Table 38

Table 39

Table 40

Table 41

Table 42

Table 43

Table 44

Table 45

Table 46

Zn fertilizers for rice ....................................... ................................................................ 87

Optimal ranges and critical levels of S in plant tissue ................................................... 91

S fertilizers for rice ......................................................................................................... 93

Optimal ranges end critical levels of Si in plant tissue .................................................. 96Si fertilizers for rice ........................................................................................................ 97

Optimal ranges and critical levels of Mg in plant tissue ............................................... 100Mg fertilizers for rice ..................................................................................................... 101

Optimal ranges and critical levels of Ca in plant tissue .............................................. 103

Ca fertilizers for rice 104

Optimal ranges and critical levels of Fe in plant tissue ............................................... 106

Fe fertilizers for rice ......................................................................................................... 107

Optimal ranges and critical levels of Mn in plant tissue ............................................... 110

Mn fertilizers for rice 111

Optimal ranges and critical levels of Cu in plant tissue 114

Cu fertilizers for rice 115

Optimal ranges and critical levels of B in plant tissue .................................................. 117

B fertilizers for rice ........................................................................................................ 118

Optimal range and critical level for occurrence of Fe toxicity ..................................... 123

Optimal ranges and critical levels for occurrence of B toxicity ................................... 130

Optimal ranges and critical levels for occurrence of Mn toxicity ................................. 133

Optimal range and critical level for occurrence of Al toxicity ...................................... 136

Materials for treating Al toxicity in rice ......................................................................... 137

Optimal ranges and critical levels for occurrence of mineral deficiencies ortoxicities in rice tissues ................................................................................................ 152

Average nutrient removal of modern irrigated rice varieties end mineral

..........................................................................................................

.........................................................................................................

................................................

..........................................................................................................

concentrations in grain and straw ................................................................................ 153

Ranges of grain yield and yield components in irrigated rice ..................................... 154Current N use efficiencies in irrigated lowland rice fields in Asia ............................... 157

Proposed amounts of N to be applied each time the SPAD value is below thecritical level ...................................................................................................................... 162

Proposed amounts of N to be applied depending on SPAD values at criticalgrowth stages .................................................................................................................... 163

General soil- and season-specific fertilizer recommendations for irrigated rice ......... 167

Conversion factors for nutrient concentrations in fertilizers ........................................ 169

Molecular weights (g mol -1 ) for nutrients ..................................................................... 170

List of Procedures and Worked ExamplesBox 1

Box 2Key steps for preparing a site-specific N fertilizer recommendation ............................. 50

one average recovery efficiency for applied N 56Example 1 Preparing a site-specific N fertilizer recommendation using

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Box 3

Box 4

Box 5

Box 6

Box 7Box 8

Box 9

Box 10

Box 11

Box 12

Example 2 Preparing a site-specific N fertilizer recommendation usingmore than one recovery efficiency for applied N ........................................................... 57

Key steps for preparing a site-specific P fertilizer recommendation ............................. 67

Example 3 Preparation of a site-specific P fertilizer recommendation ...................... 69

Key steps for preparing a site-specific K fertilizer recommendation ............................. 79

Example 4 Site-specific K fertilizer recommendation ................................................. 81Converting fertilizer recommendations into fertilizer materials ................................... 171

experiments for the purpose of monitoring soil changes over time ............................ 172Procedure for regular soil sampling from small treatment plots in field

Procedure for obtaining one sample that represents the average nutrient contentfor a farmers field ........................................................................................................ 174

Procedure for measuring yield components and nutrient concentrations at physiological maturity .................................................................................................. 177Procedure for measuring grain yield at harvestable maturity ..................................... 180

List of Color Plates

Rice is grown in a range of contrasting farming systems .............................................................. 3

Fertilizer application and rice harvesting ........................................................................................... 8

Rice cultivation .......................................................................................................................................... 7

Nutrient omission plots .......................................................................................................................... 22

Nutritional balance ................................................................................................................................... 25

Straw management ................................................................................................................................... 32

Nitrogen deficiency symptoms in rice .................................................................................................... 41

Phosphorus deficiency symptoms in rice ............................................................................................... 60

Potassium deficiency symptoms in rice ............................................................................................... 73

Sulfur deficiency symptoms in rice ..................................................................................................... 90

Silicon deficiency symptoms in rice .................................................................................................... 95

Magnesium deficiency symptoms in rice .......................................................................................... 99

Calcium deficiency symptoms in rice ......................................................................................................... 102

Manganese deficiency symptoms in rice ............................................................................................... 109

Copper deficiency symptoms in rice ....................................................................................................... 113

Zinc deficiency symptoms in rice ......................................................................................................... 84

Iron deficiency symptoms in rice .............................................................................................................. 105

Iron toxicity symptoms in rice ................................................................................................................. 121

Sulfide toxicity symptoms in rice ............................................................................................................. 126

Boron toxicity symptoms in rice ............................................................................................................. 129

Manganese toxicity symptoms in rice ................................................................................................... 132

Aluminum toxicity symptoms in rice ..................................................................................................... 135

Salinity symptoms in rice ....................................................................................................................... 139

Leaf color chart ...................................................................................................................................... 164

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1Rice EcosystemsRice production systems differ widely in cropping intensity and yield,ranging from single -crop rainfed lowland and upland rice with smallyields (13 t ha -1 ), to triple -crop irrigated systems with an annual

grain production of up to 1518 t ha-1

. Irrigated and rainfed lowlandrice systems account for about 80% of the worldwide harvested ricearea and 92% of total rice production. To keep pace with populationgrowth, rice yields in both the irrigated and rainfed lowlandenvironments must increase by 25% over the next 20 years. Currently,upland and flood -prone rice account for less than 8% of the globalrice supply, and it is unlikely that production from these systems canbe significantly increased in the near future.

In this chapter

1.1 Irrigated Rice

1.2 Rainfed Lowland and Upland Rice

1.3 Flood -Prone Rice


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3

1.1 Irrigated Rice

Intensive, irrigated rice -based croppingsystems are found on alluvial floodplains,

terraces, inland valleys, and deltas in Asia.Irrigated rice is grown in puddled soil in bundedrice fields with one or more crops planted eachyear. Irrigation is the main water source in thedry season and is used to supplement rainfallin the wet season. Irrigated rice accounts for55% of the global harvested rice area andcontributes 75% of global rice production(~410 M t of rice per year).

AreaWorldwide, the total harvested area of irrigatedrice is about 79 M ha, with 43% (34 M ha) inEast Asia (China, Taiwan, Japan, Korea), 24

M ha in South Asia, and 15 M ha in Southeast Asia. The countries with the largest areas of

irrigated rice are China (31 M ha), India (19 Mha), Indonesia (7 M ha), and Vietnam (3 Mha).

Cropping systemsIrrigated rice systems are intensive croppingsystems with a total grain production of 1015 t ha -1 year 1. Cropping intensities range fromone (in the temperate regions) to three (in thetropical regions) crops grown per year.

Examples of intensive rice-based croppingsystems are rice -rice, rice -rice -rice, rice -rice -

pulses, rice -wheat, and rice -rice -maizerotations. In rice monocropping systems, 23

Rice is grown in a rangeof contrasting farmingsystems

(a), (b) Irrigated systems andirrigated terraces provide thelargest yields. (c) Rainfed ricefields may be affected by drought.(d) Deep water fields are prone toflooding. (e) In upland rice fields,low soil fertility status is the major

production constraint.


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4

short -duration crops are grown per year; atsome sites, up to seven crops are grown in 2years. Fallow periods between two cropsrange from a few days to 3 months. The majorirrigated rice -cropping systems are double -and triple -crop monoculture rice in the tropics,and rice -wheat rotations in the subtropics.Together, they cover a land area of 36 M ha in

Asia and account for ~50% of global riceproduction. Most irrigated rice land is plantedto modern semidwarf indica and japonicavarieties, which have a large yield potentialand respond well to N fertilizer. In China, hybridrice varieties are used in >50% of the irrigatedrice area, and yields are about 1015% largerthan for conventional rice varieties.

Recent changes in production technologyinclude the following:

the change from transplanting to direct

increased use of herbicides for weed

the introduction of mechanized land

seeding,

control, and

preparation and harvesting techniques.

Yields and major constraints

The global average yield of irrigated rice is 5 tha -1 per crop, but national, regional, andseasonal yield averages vary widely. Largeyields (more than 5 - 6 t ha -1) are obtained inthe USA, Australia, China, Egypt, Japan,Indonesia, Vietnam, and the Republic ofKorea. Medium yields (45 t ha -1) occur inBangladesh, northwestern and southern India,Lao PDR, Malaysia, Myanmar, the Philippines,Sri Lanka, and Thailand. Yields are smaller

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5

yield increases and result in poor N useefficiency.

Problems with weeds, insects, anddiseases

Weeds are mainly a problem in areas wheredirect-seeded rice is grown and hand weedingis not possible due to labor scarcity. This hasled to the use of herbicides as a standardpractice in regions such as California (USA),South Vietnam, Malaysia, Central Thailand,and Central Luzon (Philippines). In mostcases, insecticide application is not necessaryduring the first 40 days after planting, andintegrated pest management techniques usingsmaller amounts of insecticide have beenwidely adopted in recent years. The need forlarger N fertilizer rates to maintain or increaseyields, however, often results in greater pestand disease pressure. The large leaf arearequired to achieve high yields results in adense canopy that provides a microclimateenvironment that favors the development andspread of many rice pests and diseases. K orSi deficiency increases susceptibility to pests,particularly when coupled with excessive Nsupply.

Sustainability and environmental problems

There have been reports of declining yields insome long-term, double- and triple-crop riceexperiments in Asia, where the bestmanagement practices have been rigorouslyfollowed. There is also anecdotal evidence ofdiminishing returns to N fertilizer use infarmers fields. In many countries, the rate ofincrease in rice yields has decreased in recentyears, and this may be related to decliningfactor productivity from applied inputs. Itremains unclear whether yield or productivitydecline is widespread in Asia. Where theyoccur, they are caused mainly by soil nutrientdepletion, changes in soil organic matter, oraccumulation of toxic substances in soil,particularly in systems with short and wet fallowperiods between two crops.

Global methane (CH 4) emissions from floodedrice fields are about 4050 Tg year -1, or ~10%of total global methane emissions. In irrigatedrice areas, controlled water supply andintensive soil preparation contribute toimproved rice growth but result in theproduction and emission of larger amounts ofCH 4 . Improved water management techniquescan reduce the emission of CH 4 from rice fields,but feasible management practices that reduceCH 4 emissions without increasing N losses andreducing yield have yet to be developed.

As much as 6070% of applied fertilizer N maybe lost as gaseous N, mainly because of NH 3volatilization and denitrification. Nitrous oxideemissions occur as a result of nitrification-

denitrification during periods of alternate soilwetting and drying. In irrigated rice systemswith proper water control, N 2O emissions areusually small except where excessive amountsof N fertilizer are applied to fertile rice soils. Inpoorly drained, puddled lowland rice soils,little nitrification takes place and NO 3 leachinglosses are therefore usually


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1.2 Rainfed Lowland and Upland Rice

Rainfed lowland rice grows in bunded fieldsthat are flooded for at least part of the cropping

season with water to a depth that may exceed50 cm for no more than 10 consecutive days.The rainfed lowland rice ecosystem can bedivided into five subecosystems:

favorable rainfed lowland,

drought -prone,submergence -prone,drought - and submergence -prone, andmedium -deep water.

Rainfed lowlands are characterized by lack ofwater control, with floods and drought beingpotential problems. Rainfed rice accounts for~25% of the worlds total rice land, with a totalproduction of ~85 M t of rice per year (17% ofthe global rice supply).

Upland rice is grown with small amounts ofexternal inputs in unbunded fields. The soilmay be cultivated when dry and planted bydirect seeding. Upland rice is also dibbled

directly into the uncultivated soil after landclearing and burning. Surface water does notaccumulate for any significant time during thegrowing season. Landforms for upland ricevary from low -lying valley bottoms toundulating and steep sloping lands with highsurface runoff and lateral water movement.Upland rice constitutes only 10% of the globalrice area and 3.8% of total world riceproduction.

Area and most importantcountriesRainfed lowland rice is grown on ~36 M ha, ofwhich ~34 M ha are found in Asia. It is themost common system in the subhumidsubtropics (eastern India, Myanmar, Thailand)and large parts of the humid tropics(Bangladesh, Cambodia, Lao PDR). These areregions where modern rice technologies haveyet to make an important impact on productivity

and past increases in production have comefrom an expansion in the area planted. The

countries with the largest rainfed lowland riceareas are India (12.8 M ha), Thailand (6.7 Mha), and Bangladesh (4.4 M ha).

Only ~17 M ha are planted to upland riceworldwide. India (6.2 M ha), Brazil (3.1 M ha),and Indonesia (1.4 M ha) have the largestupland rice areas.

Cropping systems

Usually only one crop is grown each year in

rainfed lowland rice systems and yields aresmall. In some areas farmers grow ricefollowed by mungbean, soybean, wheat,maize, or vegetables as a secondary crop. Aparticular farmer may cultivate rainfed lowlandrice at several positions in a toposequencesuch that on one farm some fields may bedrought -prone while others may be affectedby flooding in the same season. Because ofunstable yields and the high risk of crop failure,rainfed lowland rice farmers are usually poorand typically grow traditional, photoperiod -sensitive cultivars that do not respond well tomineral fertilizer.

Upland rice is an important crop in shiftingcultivation (or slash -and -burn) farmingsystems in Indonesia, Lao PDR, thePhilippines, northern Thailand, and Vietnamin Asia, and in forested areas of Latin Americaand West Africa. Farmers plant rice as a solecrop or mixed with other crops such as maize,

yam, beans, cassava, or bananas. An area isfarmed for 13 years until weed and pestinfestations increase because of a decline insoil fertility.

Permanent cultivation of upland rice aspracticed in Asia and Latin America ischaracterized by orderly intercropping, relaycropping, and sequential cropping with a rangeof crop species.


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7

Rice cultivation

(a), (b) In irrigated rice, land is prepared by plowing and puddlingoperations to destroy the soilstructure. (c) In upland rice, seedis dibbled into cultivated anduncultivated soil after land clearingand burning. (d) Large seedbedsare required for transplanted rice.(e), (f) Poor maintenance ofirrigation equipment and channelsmay result in water shortagesduring critical growth periods.(g), (h) Transplanted rice requiresmore labor inputs than direct -seeded rice. (i), (j) Hand weedingis essential to reduce competitionfrom weeds during the early stagesof crop establishment up to canopy

closure.


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8

Fertilizer application and

rice harvesting(k), (I) In Asia, basal and top -dressed fertilizers are broadcast byhand. (m) Rice is usuallyharvested by hand. (n) Wherefields are large, however, combineharvesters have been introducedsuccessfully. (o), (p) In Vietnamand the Philippines, threshing isdone in the field using mobile ricethreshers. This practice leavesmost of the straw as heaps in thefield, which are often burned in

situ.


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9

Yields and major constraints

The world average yield for rainfed lowlandrice is 2.3 t ha -1 per crop, but under favorableconditions, yields of >5 t ha -1 can be achieved.Yields of upland rice have increased slowly

over the last 30 years and average about 1 tha -1 in most countries, except in some largeand partly mechanized farms in Latin America,where yields can reach 2-3 t ha -1. Adverseclimatic conditions, poor soils, and a lack ofsuitable and adapted modern technology arethe major constraints to increasing theproductivity of rice in rainfed lowlands anduplands. The income of most farmers is smalland they have limited and difficult access tocredit, inputs, and information about moderntechnologies. Rice farming in rainfed lowlandsis risk -prone because crops can be affectedby droughts, floods, pest and diseaseoutbreaks, and weeds, as well as soilconstraints. Growing conditions are diverseand unpredictable because most rainfedlowland rice fields depend on erratic rainfall.

Many upland rice soils are acid, vulnerable toerosion, and highly P -fixing. In most cases, Pdeficiency must be corrected before a

response to N is obtained.

Fertilizer use and fertilizer useefficiencyBecause of higher risk and reduced efficiency,most rainfed lowland rice farmers applyfertilizer N to their rice crops in much smalleramounts than in irrigated rice systems. Theapplication of N fertilizer, however, is notcommon in upland rice, where mineral fertilizer

may not be available. K fertilizer is notcommonly applied to rainfed lowland andupland rice although a response to K hasfrequently been shown, particularly on coarse -textured soils. A smaller yield potential andgreater uncertainty because of climatic andabiotic stress are two reasons why input useis less in rainfed lowland and uplandenvironments. For example, in rainfedsystems, N use efficiency is mainly governedby environmental factors such as drought and

flooding, which are beyond the farmers

control. On acid soils in upland rice systems,P deficiency and AI toxicity limit growth andyield. Reduced Si availability under uplandconditions increases the susceptibility of riceplants to diseases (e.g., blast) and this reducesthe amount of N that can be used safely. Theseconstraints limit the returns to investments inN fertilizer in contrast to irrigated systemswhere N use efficiency is higher, moreconsistent, and more reliable. In addition,rainfed soils are characterized by intermittentwetting and drying cycles, even during the wetseason, which result in an accumulation ofnitrate because of nitrification and thesubsequent loss of N by denitrification orleaching. Slow -release fertilizers may havepotential to increase N use efficiency in theseenvironments.

Problems with weeds, insects, anddiseasesWeeds are the main production constraint inrainfed lowland and upland rice systemsbecause fields are direct -seeded and do notbenefit from the presence of a water mulch toreduce the weed population. Moreover, weedsare also more competitive than rice when soilfertility is poor. Small farmers often cannotafford to implement weed control measures.Estimates of yield losses caused bycompetition from weeds range from 30% to100%. Other pest problems include blast,brown spot, nematodes, stem borers, and ricebugs. Nematode infestations can result in yieldlosses of up to 30%.

Environmental problems

Methane emissions are smaller and morevariable in rainfed lowland rice than in irrigatedrice because of periodic droughts during thegrowing season. Upland rice is not a sourceof CH 4 emissions. Nitrate leaching is commonin rainfed rice systems or rice -nonrice croppingsystems, particularly on coarse -textured soils,which may result in the contamination ofgroundwater systems. In addition to theeconomic loss from N leaching, cumulativeN2O fluxes are 3-4 times larger during the


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10

fallow period than during the cropping period.With sufficient residual soil moisture, NO 3losses can be reduced by growing nitratecatch crops which take up and retain NO 3 inaboveground biomass during the fallow period.Nitrate accumulation can be reduced bydelaying the application of N fertilizer until theonset of permanent flooding, and by splittingthe recommended N dose.

Future challenges

The rainfed lowlands offer tremendous scopefor increased rice production because the areaunder this system continues to increase andyields are small. Rainfed rice varieties for thefuture should be more responsive to mineral

fertilizer but should retain the stress toleranceand grain quality built into traditional varieties.Farmers would then be motivated to invest inmore productive land preparation and fertilitymanagement practices that result in higheryields.

The major requirement for improving theproductivity of upland rice is to develop suitabletechniques for managing P and soil acidity.Until these problems have been resolved,

investments in breeding improved varieties willhave little impact on productivity in the uplandrice ecosystem.


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1.3 FIood - Prone Rice

Flood -prone rice is grown in inland and tidal(coastal) wetland areas where the depth of

floodwater is >50 cm throughout the growingseason. Around 12 M ha of rice lands in Southand Southeast Asia are subject to uncontrolledflooding. Rice grown under such conditionsmust be adapted to temporary submergenceof 110 days, long periods (15 months) ofstanding (stagnant) water ranging in depthfrom 50 to 400 cm or more, or daily tidalfluctuations that sometimes also causecomplete submergence. Rice yields are verysmall (~1.5 t ha -1) and very variable mainly due

to poor soils and the unpredictable incidenceof drought and flooding. The flood -proneecosystem accounts for only 4% of global riceproduction but is important for food security insome areas.

Further reading

Cassman KG, Pingali PL. 1995. lntersificationof irrigated rice systems: Learning from thepast to meet future challenges. GeoJournal

35:299 -305.Dowling NG, Greenfield SM, Fischer KS,editors. 1998. Sustainability of rice in the globalfood system. Davis, Calif. (USA): Pacific BasinStudy Center and Manila (Philippines):International Rice Research Institute.

Hossain M, Fischer KS. 1995. Rice researchfor food security and sustainable agriculturaldevelopment in Asia: Achievements and futurechallenges. GeoJournal 35:286 -298.

IRRl (International Rice Research Institute).1997. Rice Almanac. 2nd ed. Los Baos(Philippines): IRRI.

Zeigler RS, Puckridge DW. 1995. Improvingsustainable productivity in rice -based rainfedlowland systems of South and Southeast Asia.GeoJournal 35307 -324.


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2NutrientManagement

In this chapter

2.1 Yield Gaps and Crop Management

2.2 The Nutrient Input -Output Budget in an Irrigated Rice Field

2.3 Site -Specific Nutrient Management Strategy

2.4 Estimating Indigenous N, P, and K Supplies

2.5 Crop Nutrient Requirements The Nutritional BalanceConcept

2.6 Recovery Efficiencies of Applied Nutrients

2.7 Managing Organic Manures, Straw, and Green Manure

2.8 Economics of Fertilizer Use


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2.1 Yield Gaps and Crop Management

Currently, most rice farmers, even those inirrigated areas, achieve less than 60% of the

climatic and genetic yield potential of aparticular site. To understand why yields infarmers' fields are only a fraction of thepotential or maximum yield, a simple modelcan be used to illustrate the particular factorsaccounting for the yield gap (Figure 1).

Maximum yield, Y max At Y max , grain yield is limited by climate andgenotype only, and all other factors arenonlimiting. Ymax fluctuates from year to year(10%) because of climatic factors. For mostrice-growing environments in tropical Southand Southeast Asia, the Ymax of currentlygrown high-yielding rice varieties is about 10t ha -1 in the dry season (high solar radiation),and 78 t ha -1 in the wet (monsoon) season,when high humidity leads to greater diseasepressure and the amount of solar radiation issmaller due to greater cloud cover.Experimentally, Ymax can be measured only inmaximum yield trials with complete control ofall growth factors other than solar radiation.

Important points:

Climate cannot be manipulated, but Ymaxvaries depending on the planting(sowing) date.Grow rice varieties adapted to prevailingclimatic conditions (i.e., select genotypeswith the highest Ymax under a givenclimatic regime).

Attainable yield, Y a

At Ya , grain yield is smaller than Ymax due tolimited water and nutrient supply. In irrigatedrice, water is usually not a limiting factor(except when the temperature of the irrigationwater is very high (i.e., geothermal influence)or very low (i.e., at high altitudes), thus Y arepresents the attainable yield limited bynutrient supply.

The maximum economic Ya achieved by thebest farmers is about 7080% of the potential

Ymax because the internal efficiency of nutrientuse decreases when Y a >80% of Ymax (Section2.5). At this point on the yield response curve,larger and larger amounts of N, P, or K mustbe taken up by the rice plant to produce a givenincrement in grain yield.

Important points:

In irrigated rice, Yield Gap 1 ( Ymax - Y a ) ismainly caused by an insufficient supply ofN, P, K, and other nutrients. To increase

and maintain Ya at >7080% of Ymax ,emphasis must be given to improving soilfertility and ameliorating all constraints tonutrient uptake, balanced nutrition, andhigh N use efficiency.

In rainfed lowland and upland rice, YieldGap 1 is usually caused by insufficientwater as well as soil infertility. Therefore,a combined approach of improving waterand nutrient management is required toreduce Yield Gap 1. The selection of

varieties resistant to biotic and abioticstresses (drought, weeds, soil stresses),and improvements in soil fertility andwater and nutrient use efficiency areimportant.

Actual yield, Y

Ya is reduced to Y due to pests and diseases,toxicities, and constraints other than climate,water, or nutrient supply. Yield Gap 2 ( Ya - Y)

results from a reduction in nutrient useefficiency. For example, if Yield Gap 2 is large,the rice plant may take up a large amount ofnutrients, but they are not converted efficientlyinto profitable harvest products (grain) so thatthe overall profitability of the cropping systemremains less than optimal. Crop managementin rice must minimize Yield Gap 2 to achieveefficient nutrient use.


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(a) In a well -managed field, yield gap 2 is close to zero so that the actual yield approaches Y a at a level of about80% of Y max . Nutrient efficiency and profit are high.

(b) Yield losses are large because of poor crop management, inadequate pest control, or mineral toxicities.

(c) Yield loss because of poor nutrient management.

(d) Yield loss because of poor nutrient and crop management.

Figure 1. Maximum yield and yield gaps at the farm level.

Important points:

Ameliorate all mineral toxicities (Section

Implement high standards of generalcrop management, including selection ofsuitable, pest -resistant, high -yieldingvarieties; use of certified seed; optimal

land preparation and crop establishment;and efficient control of pests anddiseases (insects, rats, snails, birds,weeds) to minimize yield losses.

4).


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2.2 The Nutrient Input - Output Budgetin an Irrigated Rice Field

The nutrient budget for a rice field (Figure 2)can be estimated as follows (all componentsmeasured in kg elemental nutrient ha -1):

B = M + A + W + N 2 - C - PS - Gwhere

Inputs: M is the nutrient source added(inorganic and organic); A is the atmosphericdeposition (rainfall and dust); W is theirrigation, floodwater, and sediment (dissolvedand suspended nutrients); and N 2 is thebiological N 2 fixation (N only).

Outputs: C is the net crop removal with grainand straw (total uptake less nutrients in cropresidues returned); PS is the total loss due topercolation and seepage; and G is the totalgaseous loss due to denitrification and NH 3volatilization.

The overall nutrient budget at a particular sitevaries widely depending on the cropping

system, crop management, and climaticseason. The N input from biological N 2 fixationis smaller where soil N status is high (e.g., dueto mineral fertilizer N use) and soil P status islow.

Sediments (W) are a major nutrient input intraditional lowland rice systems, particularly inirrigated rice systems located in river deltasthat are regularly affected by natural flooding.The flood prevention structures and dams thatare installed to improve irrigation and drainage,however, have decreased the addition ofnutrients in sediment inflow.

In the past, organic nutrient sources such asfarmyard manure, legume green manure, andazolla were a major source of nutrient inputs,but their use has declined in many regionssince the introduction of the Green Revolutiontechnology.

Values shown are common ranges of inputs and outputs of N, P, and K for an irrigated rice field (kg ha -1 per crop).

Figure 2. Components of the input -output balance of nutrients in a typical irrigated rice field.


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Crop removal (C) is the largest cause ofnutrient removal from the field, but the actualamounts removed depend on the harvest andpostharvest technology used. Nutrientscontained in the grain are removed from thefield, and husks, separated from the grainduring milling, may be burned at the mill site(e.g., in Indonesia) or returned to the field.Straw contains almost all of the K and Si, anda large part of the N, P, and S, taken up by thecrop. Therefore, straw management markedlyaffects the field nutrient budget (Section 2.7).

Gaseous losses of N (G) through NH 3volatilization and denitrification often exceed50% of applied fertilizer N. NH 3 volatilizationappears to be the major N-loss process for N

applied as a topdressing, but nitrous oxide isalso emitted from irrigated fields during periodsof alternate wetting and drying of the soil. Inirrigated rice systems with proper watercontrol, however, nitrification losses, NO 3leaching losses, and N 2 O emissions areusually small.

Leaching losses (PS) depend on theconcentration in the soil solution and the waterpercolation rate, both of which vary

considerably and are affected by soil texture.In irrigated rice systems where water isproperly controlled and percolation is impededby a compact soil layer beneath the plow sole,leaching losses are usually small. In coarse-textured soils where the plow sole is thin andpermeable, however, the amount of nutrientslost due to leaching may be large.

Nitrate leaching losses are large in rice-vegetable systems, where large amounts ofN fertilizer are applied to the vegetable crop.In this case, nitrate tends to accumulate in thesoil during the vegetable cropping period, anda large amount of N may be leached out intosurface water and groundwater during cropirrigation, before the rice crop is sufficientlydeveloped to absorb the NO 3 -N.

At an average concentration of 1 mg of nutrientL-1 , 1,000 mm of water adds ~10 kg of nutrientha -1 to a rice field, but this does not necessarilyimply a net gain of nutrients to the system. In

many cases, the amount of nutrient inputs from

rainfall and irrigation is smaller than the amountlost from leaching. For a particular field, wecan assume that nutrient losses from seepageare similar to nutrient inputs from seepagecoming from neighboring fields.

A simple partial K, P, or S budget can beestimated as:

Partial input - output budget = M - C

= (fertilizer input + straw retained) - total plant uptake

Table 1 shows an estimated average nutrientbudget for an irrigated rice crop in Asia. Dataused to calculate fertilizer nutrient inputs andcrop nutrient removal are based onmeasurements taken in farmers fields. In thisexample, organic manures were not used toreflect the general trend for their replacementby mineral fertilizer use. These calculationsunderline the importance of strawmanagement in the nutrient balance. This isparticularly important for K for which relativelysmall amounts of fertilizer nutrients are addedand large amounts of nutrient may be removedwith the straw.

Further reading

Abedin Mian MJ, Blume HP, Bhuiya ZH, EaqubM. 1991. Water and nutrient dynamics of apaddy soil of Bangladesh. Z. Pfl.-Ern. Bodenk.154:9399.

App A, Santiago T, Daez C, Menguito C,Ventura WB, Tirol A, Po J, Watanabe I, DeDatta SK, Roger PA. 1984. Estimation of thenitrogen balance for irrigated rice and thecontribution of phototrophic nitrogen fixation.

Field Crops Res. 9:1727.Cassman KG, Peng S, Olk DC, Ladha JK,Reichardt W, Dobermann A, Singh U. 1998.Opportunities for increased nitrogen useefficiency from improved resourcemanagement in irrigated rice systems. FieldCrops Res. 56:738.

De Datta SK, Buresh RJ, Obcemea WN,Castillo EG. 1990. Nitrogen-15 balances andnitrogen fertilizer use efficiency in upland rice.

Fert. Res. 26:179187.


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Table 1. Nutrient budget for an irrigated rice crop yielding 6 t ha -1.

ItemN K

Comments

1152

5

17.00.3

0.5

P

(kg ha -1)

Inputs

Fertilizer (M)Rainfall (A)

Irrigation (W)

155

20

0

No manure applied


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2.3 Site - Specific NutrientManagement Strategy

Site -specific nutrient management (SSNM)focuses on developing a nutrient managementprogram that takes the following aspects intoaccount:

} Indigenous nutrient supply at each site(site -specific).

} Temporal variability in plant N statusoccurring within one growing season(season -specific).

} Medium -term changes in soil P and Ksupply based on the cumulative nutrientbalance.

Management of nitrogen

To optimize N use efficiency for each season,a dynamic N management strategy is required,in which the adjustment of the quantity of Napplied in relation to the variation in indigenousN supply is as important as timing, placement,and source of applied N. Nitrogenmanagement should therefore include thefollowing measures:

An estimate of crop N demand, potentialN supply from indigenous sources (soil,biological N 2 fixation) and N recoveryfrom inorganic and organic sourcesapplied. These factors are used toestimate the total fertilizer N requirement.

An estimate of the need for a basal Napplication according to soil N releasepatterns, crop variety, and crop

establishment method.Plant N status monitoring to optimize the

timing of split applications of mineral Nfertilizer in relation to crop demand andsoil N supply.

Long -term soil and crop managementpractices to optimize the indigenousnitrogen supply (INS).

Management of phosphorus and potassium

P and K management requires a long -termmanagement strategy. It is more important topredict the need to apply P and K, and theamount required, than to attempt to maximizerecovery efficiency for fertilizer P and K. Thisis because these nutrients are not readily lostor added to the root zone by biological andchemical processes that affect N.Management must be geared towardmaintaining the available soil nutrient supply,to ensure that P and K do not limit crop growthand thus reduce N use efficiency. Changes inpotential indigenous P and K supply can bepredicted as a function of the overall nutrientbalance. To predict the P and K inputs requiredfor maintaining a targeted yield level, keycomponents of P and K management shouldinclude the following:

An estimate of crop P and K demand,

potential indigenous P and K supply, aswell as P and K recovery from appliedinorganic and organic sources.

depending on soil K bufferingcharacteristics and an understanding ofthe relationship between K nutrition andpest incidence.

Knowledge of the relationship betweenthe P and K budget, residual effects of Pand K fertilizers, and changes in soilsupply over time.

A schedule for timing K applications,

Management of other nutrientsPrevention, diagnosis, and treatment are thekey management tools for other nutrients (e.g.,Ca, Mg, and S), micronutrients, and mineraltoxicities. Over the longer term, preventionthrough general crop management (e.g., usingadopted cultivars), water management, andfertilizer management (e.g., choice of fertilizers


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PI = panicle initiation

F = flowering

H = harvest

INS = indigenous N supply

IPS = indigenous P supply

IKS = indigenous K supply

Figure 3. Strategy for site -specific nutrient management in irrigated rice.


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2.4 Estimating Indigenous N, P,and K Supplies

DefinitionThe indigenous nutrient supply is thecumulative amount of a nutrient originatingfrom all indigenous sources that circulatethrough the soil solution surrounding the entireroot system during one complete crop cycle.

For practical purposes, the potentialindigenous nutrient supply of N (INS), P (IPS),and K (IKS) is defined as the amount of eachnutrient taken up by the crop from indigenous

sources when sufficient amounts of all othernutrients are supplied, and other limitations togrowth are removed. It can be measured asthe total plant nutrient uptake in a nutrientomission plot (provided that no other factorssuch as water, other nutrients, or pests affectgrowth):

INS = total N uptake in N omission plots

(i.e., plots receiving P, K, and other nutrients,but no N).

IPS = total P uptake in P omission plots

(i.e., plots receiving N, K, and other nutrients,but no P).

IKS = total K uptake in K omission plots

(i.e., plots receiving N, P, and other nutrients,but no K).

The potential indigenous supply as definedearlier is a crop -based measure that integratesthe supply of nutrients from all indigenoussources under field conditions, including:

soil supply across the whole rooting

irrigation water,

atmospheric deposition (rainfall, dust),

biological N 2 fixation (only in the case of

crop residues.

Estimating INS, IPS, and IKS fromgrain yield

We will now describe a simple approach forestimating INS, IPS, and IKS based on grainyield only. Other data, such as nutrient uptakemeasured in omission plots and soil testresults, can also be used to arrive at anestimate of how much N, P, and K are availablefrom indigenous sources during one croppingseason (in kg ha -1 ), but the three methodsdescribed in the following paragraphs areusually most applicable in the field, where fieldexperiment and soil analysis data may not beavailable.

depth,

INS), and

Nutrient omission plots

This is the most suitable methodfor estimating indigenous N supply(INS). Note the pale green color inthe plot where N fertilizer was notapplied.


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1 In a cropping season where favorableweather conditions and good yields areexpected, two steps are required:

(i) In a farmers field, establish threesmall (5 x 5 m 2 ) nutrient omissionplots: no N, no P, and no K. In theremaining area, apply all threemacronutrients N, P, and K. Choose abalanced fertilizer ratio for N:P:K of3:1:3 (i.e., for 3 kg N, apply 1 kgfertilizer P and 3 kg fertilizer K).

Assuming fertilizer recovery fractionsof 0.50 kg N uptake kg -1 N applied,0.25 kg P uptake kg -1 P applied, and0.50 kg K uptake kg -1 K applied, thiswould result in the optimal uptake

ratio for plant N:P:K of 6:1:6 (basedon on -farm data collected in Asia).

omission plots (0 N, 0 P, and 0 K) andin the fertilized field (NPK). If possible,oven -dry grain at 70C for 48 hours(i.e., ~3% moisture content) andadjust GY to 14% moisture content:GY 14% = oven -dry GY x 0.97/0.86

Otherwise, sun -dry the grain and

assume a moisture content of 14%.If it is not feasible to establish nutrientomission plots, collect data on grain yieldin a farmers field and record theamounts of fertilizer N, P, and K applied

(ii) Measure grain yield (GY) in the

for a cropping season with favorableweather and good yield.

2 If nutrient omission plots wereestablished, follow the decision tree inFigure 4 and calculate the indigenous

nutrient supply for each nutrient. Thefactor by which the grain yield in therespective nutrient omission plot ismultiplied refers to the average amountof a nutrient (kg ha) taken up by theplant to produce one ton of grain in fields,according to whether the nutrient islimiting or not (based on on -farm datacollected in Asia). We make the followingassumptions:

A full supply of nutrients other than theelement missing in the omission plots,e.g., a full supply of N and K in a 0 Pplot.

The harvest index is approximately0.5 (modern rice variety with nosevere yield -reducing factors).If the grain yield in a plot (field) with a

full NPK supply is less than 70% ofthe potential yield (Y max ), factors otherthan NPK are limiting. Improve cropmanagement first before estimatingINS, IPS, and IKS.

3 If nutrient omission plots were notestablished but fertilizers were applied in

GY(NPK) is the grain yield in t ha -1 (GY, t ha -1 , 14% moisture content) in a farmers field receiving N, P, and Kfertilizer. Y max is the maximum potential yield (Section 2.1).

Figure 4. Estimation of indigenous nutrient supplies of N, P, and K (INS, IPS, and IKS) from

grain yield in nutrient omission plots (0 N, 0 P, and 0 K plots).


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a balanced NPK ratio as suggestedearlier, calculate the indigenous nutrientsupplies according to Equations (N1),(P1), and (K1):

INS (kg N ha -1 ) (GYx 17) - (RE N x FN) (N1)

IPS (kg P ha-1 ) (GY x 3) - (RE P x FP) (P1)

IKS (kg K ha -1 ) (GY x 17) - (RE K x FK) (K1)

where GY is the grain yield in t ha -1 (14%moisture content); the factors 17, 3, and 17are the average amounts of N, P, and K (kgha -1) taken up by the plant to produce 1 t ofgrain in fields that received NPK fertilizer(based on on -farm data collected in Asia); RE N,RE P , and RE K are the apparent recoveryefficiencies of applied N (0.40.6 kg kg -1,

Section 3.1), P (0.20.3 kg kg-1

, Section 3.2),and K (0.4 - 0.6 kg kg -1, Section 3.3); and FN,FP, and FK are the amounts of fertilizer N, P,and K that were added (kg ha -1).

These equations are mentioned in Sections3.1, 3.2, and 3.3 in Boxes 1, 4, and 6,respectively.

NOTES:

Calculations based on nutrient omission

plots are more accurate than the indirectestimates using Equations (N1), (P1),and (K1) mentioned above. Equation(N1) should only be used as a last resortbecause estimates of RE N are not veryreliable.

nutrient supply are affected by:

cultivar differences in harvest index,

Plant -based measures of indigenous

rooting patterns, and nutrient uptake/

nutrient use efficiency,crop establishment method,

seasonal variability in climate, and

pests and other unquantified yield -limiting factors such as lodging.

The use of omission plots is only valid formodern high -yielding varieties with aharvest index of about 0.50. If large yieldlosses were detected (e.g., due tolodging), grain yield should be corrected

(e.g., by calculating straw yield/2,assuming a harvest index of 0.50).

For a particular soil, INS, IPS, and IKSmeasured in wet broadcast -seeded riceare smaller than in transplanted rice.

The indigenous nutrient supply is the potential supply of a nutrient fromindigenous sources. Thus, it can only bemeasured accurately in a season withfavorable climatic conditions and propercrop management and assuming thatfactors such as the supply of othernutrients, water supply, and pests anddiseases do not limit plant growth. Forrice grown in a tropical climate, theindigenous nutrient supply is best

measured in the dry season.In some countries, there is a continuoustransfer of soil fertility from border areasto the center of the field (or vice versa ),where threshing is done and where strawand chaff are later burned. Misleadinginformation on indigenous nutrient supplymay therefore result from the use ofsmall omission plots.

The same principles can be used for

estimating the indigenous supply of othernutrients.

Further readingDobermann A, Adviento MAA, Pampolino MF,Nagarajan R, Stalin P, Skogley EO. 1998.Opportunities for in situ soil testing in irrigatedrice. In: Proc. 16th World Congr. Soil Sci. ISSS,CIRAD, Montpellier. p Symposium 13A, 106.

Dobermann A, White PF. 1999. Strategies for

nutrient management in irrigated and rainfedlowland rice systems. Nutr. Cycl. Agroecosyst.53:l18.

Witt C, Dobermann A, Abdulrachman S, GinesHC, Wang GH, Nagarajan R,Satawathananont S, Son TT, Tan PS, Tiem LV,Simbahan GC, Olk DC. 1999. Internal nutrientefficiencies of irrigated lowland rice in tropicaland subtropical Asia. Field Crops Res. 63: 113138.


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2.5 Crop Nutrient Requirements -The Nutritional Balance Concept

lnternal nutrient use efficiencyand nutrient removal

In a situation where crop growth is not limitedby water supply, weed problems, or pestinfestations, biomass production is mainlydriven by N supply, the most limiting nutrientin irrigated rice. Thus, the rice plants demandfor other macronutrients mainly depends onthe N supply. Considerable uncertainties aboutcrop N, P, and K requirements may arise,however, because the internal nutrient useefficiency (kg grain produced per kg nutrientin aboveground plant dry matter) varies widelydepending on nutrient supply, cropmanagement, and climatic conditions.

[NB: Nutrient removal (kg nutrient t -1 grain) = 1,000/internal efficiency (kg grain kg -1 nutrient).]

Based on a large number of field observations,we have estimated the total nutrient removalper ton of grain (Table 2). These values includeextreme situations in which nutrients are either

under maximum dilution or accumulation in theplant, i.e., where nutrients are either limitingor available in surplus.

Nutrient accumulation and dilution

The relationship between grain yield andnutrient accumulation in total aboveground drymatter at physiological maturity of irrigatedlowland rice can be investigated for particulartarget yields using a modeling approach basedon QUEFTS (Quantitative Evaluation of theFertility of Tropical Soils) developed by BertJanssen and his colleagues in Wageningen,The Netherlands. The model requires theempirical determination of two boundary linesdescribing the maximum accumulation (YNuA)and dilution (YNuD) of nutrients in the plant(Figure 5). YNu represents the optimumnutrient uptake requirement to achieve aparticular grain yield target for the givenboundary lines.

This concept assumes a linear relationbetween grain yield and nutrient uptake atlower uptake levels when nutrient uptake is atits maximum under conditions of limited

nutrient supply. The actual plant nutrientaccumulation under such conditions shouldtheoretically be close to the line of maximumdilution of the respective nutrient in the plant(YNuD in Figure 5), but it is unlikely that all

Nutritional balance

Response to N and P fertilizersmay be small because of Kdeficiency. Balanced fertilizationrequires that a// nutrientdeficiencies be eliminated by

proper nutrient management.


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Table 3. Optimal internal use efficiency for N, p, and Kin irrigated rice.

Nitrogen

(kg grain kg -1 N)

68

Phosphorus

(kg grain kg -1 P)

385

Potassium

(kg grain kg -1 K)

69

of the climate -adjusted potential yield (i.e., Y= Y a = Y max x 0.80). Optimal internal nutrientuse efficiency values are shown in Table 3.

Irrespective of the selected potential yield, theN:P:K ratio in plant dry matter (asrecommended by the QUEFTS model) isabout 5.7:1:5.6. This is similar to the averageplant N:P:K ratio of 5.3:1:5.4 calculated froma large data set on grain yield and plant nutrientaccumulation collected in six Asian countries.

Using these nutrient requirements for a site -specific nutrient management approach inirrigated rice, no other site -specific or season -specific information is required other than theclimate -adjusted potential yield (which can beobtained from crop simulation models, long -term experiments, or local experts). Thus, thisapproach allows the estimation of nutrientrequirements to achieve a particular yieldtarget and provides a useful tool for identifying

economical yield targets such as the attainableyield, Y a (Section 2.1).

NOTES:The internal nutrient use efficiency ascalculated by QUEFTS is based on theaverage values of numerous modern,high -yielding indica varieties grown atvarious experimental sites in Asia.

If the internal nutrient use efficiency of a

particular variety is greater than that ofother varieties, the boundary linesdescribing the 'envelope' must beadjusted.

If new varieties with a greater potentialyield are released, internal nutrient useefficiency may be greater even at loweryield levels, resulting in an upward shiftof the 'envelope'.

Further readingJanssen BH. 1998. Efficient use of nutrients:an art of balancing. Field Crops Res. 56: 197 -201.

Janssen BH, Guiking FCT, van der Eijk D,Smaling EMA, Wolf J, van Reuler H. 1990. Asystem for quantitative evaluation of the fertilityof tropical soils (QUEFTS). Geoderma 46:299-31 8.

Witt C, Dobermann A, Abdulrachman S, GinesHC, Wang GH, Nagarajan R,Satawathananont S, Son TT, Tan PS, Tiem LV,Simbahan GC, Olk DC. 1999. Internal nutrientefficiencies of irrigated lowland rice in tropicaland subtropical Asia. Field Crops Res. 63: 11 3-138.


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2.6 Recovery Efficienciesof Applied Nutrients

DefinitionThe recovery efficiency (RE) of appliedfertilizer is defined as the amount of fertilizernutrient taken up by one crop, divided by theamount of fertilizer applied. The first -seasonRE (or recovery fraction) can be estimated infertilizer trials using the difference method:

RE (kg kg -1 ) = (U 2 - U 1 )/(F 2 - F 1 )

where RE is the recovery efficiency (kg ofnutrient uptake per kg of nutrient applied); Uis the total nutrient uptake with grain and straw(kg ha -1), and F 1 and F 2 are the amounts offertilizer nutrient added (kg ha -1 ) in two differenttreatments.

Treatment 2 receives a higherfertilizer nutrientrate than Treatment 1. A zero -fertilizer controlcan be used as the reference (Treatment 1).

An estimate of recovery efficiency is necessaryto calculate the amount of fertilizer nutrient

required to meet plant nutrient demand for agrain yield target using the general formula:

F (kg ha -1 ) = U - IS/RE

where F is the amount of fertilizer nutrientrequired to meet nutrient uptake demand (U)to support the grain yield target (Section 2.5);U is the total nutrient uptake with grain andstraw (kg ha -1 ); IS (kg ha -1 ) is the indigenoussupply of the respective nutrient (Section 2.4);and RE is the recovery efficiency (kg of nutrient

uptake per kg of nutrient applied).Examples of the application of this formula aregiven in Sections 3.1, 3.2, and 3.3.

Determinants of recoveryefficiencies

If other factors do not limit crop growth, theRE of a nutrient is related to the following:

} the indigenous nutrient supply,

the amount of fertilizer nutrient applied,

plant uptake or sink potential, which inturn depends on the availability of othernutrients and the climate -adjustedpotential yield of a particular rice cultivar.

and

This is explained in greater detail using therelationship between the actual plant Paccumulation at maturity and the potential Psupply to the crop as an example (Section 2.5).

The potential nutrient supply is defined as theamount of a nutrient that originates fromindigenous and/or fertilizer sources andpasses through the soil solution during theentire growing season. This theoretical nutrientpool cannot be directly measured, but therelationship between nutrient accumulationand potential supply can be modeled using thefollowing assumptions:

The measured indigenous supply isequal to the uptake of a nutrient up tomaturity under optimal conditions in anutrient omission plot (Section 2.4). Theactual (potential) indigenous nutrientsupply, however, is usually greater thanthe nutrient uptake in such an omissionplot because optimal conditions arerarely achieved under field conditions.Furthermore, a certain amount ofnutrients will never be taken up by thecrop and will remain in the soil solution,

especially when nutrient concentrationsare large.

potentially available to the crop. Thisassumption holds for many irrigated soilscropped to rice, in which fixation of P andK is substantially smaller than in aeratedsoil. The assumption is not valid for N,where recovery efficiency depends notonly on the total amount of appliedfertilizer N but also on the number of

All applied fertilizer nutrients are


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splits and the timing of applications.Fertilizer N that is not immediately takenup by the plant is prone to losses due tovolatilization.

The potential P supply is the sum of the

indigenous P supply (IPS) and fertilizer P (FP).For practical reasons, P uptake in a P omissionplot (UP OP ) is often assumed to be equal tothe actual IPS (Section 2.4).

In general, the relationship between nutrientaccumulation at maturity and potential supplycan be expected to follow a curved line with atransition from source to sink limitation ofnutrient uptake, as illustrated in Figure 6. PlantP uptake would be source -limited in a situation

of low potential P supply (e.g., IPS + FP 1).Hence, available P will be most efficiently usedby the plant so that UP 1 is relatively close tothe 1:1 line, representing the situation whereall supplied P would be taken up by the crop.With increasing potential P supply (e.g., if FP2 was chosen instead of FP 1), the recoveryefficiency decreases because plant P uptakebecomes increasingly restricted by thegenetically determined seasonal yieldpotential, until plant P uptake (or plant growth)

does not increase further with increasing Psupply. In this situation of nutrient uptake orsink limitation, plant P uptake has reached itsmaximum (UP max ).

The greatest recovery efficiencies of fertilizernutrients can be expected in situations where

sink potential (UP max ) is large (i.e., favorableclimatic conditions, sufficient water supply, lowpest pressure, etc.) and indigenous nutrientsupplies and fertilizer rates are small. A largesink potential would force the line describingthe relationship between plant nutrientaccumulation and potential nutrient supply tofollow a steeper curve, bringing it closer to the1:1 line. Increasing the fertilizer rate, however,will eventually lead to a decrease in nutrientrecovery efficiency.

The lowest recovery efficiencies can beexpected at low sink potentials (flat curvedescribing the relationship between plantnutrient accumulation and potential nutrientsupply) and high levels of indigenous supplies.

In this situation, recovery efficiencies may below even at small fertilizer application rates.

Estimates of recovery efficiencies

Clearly, a large variation in recoveryefficiencies can be expected among farmers'fields and in the same field over time,depending on differences in general soilproperties, cropping history, current cropmanagement, and climatic conditions.

Additional variation may be introducedbecause of problems in water supply, weeds,and pests. This makes it difficult to specify arepresentative RE, which is needed forestimating fertilizer rates as described inSections 3.1, 3.2, and 3.3 for N, P, and K.

Figure 6. Schematicrelationship between actual

plant P accumulation (UP)with grain and straw atmaturity of rice and

potential P supply for a particular maximum Puptake potential (UP max ).IPS = indigenous P supply,FP = fertilizer P.


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In irrigated lowland rice fields with good cropmanagement and grain yields of 5 - 7 t ha -1,typical fertilizer recovery efficiency ranges are0.30 - 0.60 kg kg - 1 for N (median: 0.40), 0.10 -0.35 kg kg - 1 for P (median: 0.20), and 0.15 -0.65 kg kg - 1 for K (median: 0.35).

[NB: lnterquartile ranges and medians of about 320 on -farm trials with site -specific fertilizer managementconducted in 1997 - 98 in six countries of Asia.]For practical decision making, we suggestusing these average recovery efficiencieswhen calculating fertilizer requirements of Pand K (Sections 3.2 and 3.3). For nitrogen,we recommend using an RE of about 0.50 kgkg - 1 when calculating fertilizer N rates, wheremanagement options are used to increase N

efficiency (Section 3.1), particularly in caseswhere tools for dynamic, real -time Nmanagement are available (Section 5.6).

NOTES:

Estimates of RE of fertilizer N topdressedat different growth stages are given inSection 3.1 and information on practicalmeasures to improve RE, is given inSection 5.6.

Particularly for P and K, it may not beadvisable to calculate fertilizerrequirements according to the generalequation F = U - IS/RE when the gapbetween plant nutrient demand for a yieldtarget and indigenous nutrient supply issmall. A maintenance dose of P and Kwould be sufficient in this situation toreplenish the soil nutrient pool and avoidnutrient mining.

The given typical recovery efficienciesfor P and K are only valid for irrigatedlowland rice systems in Asia, i.e., forgiven fertilizer rates, yield levels, andcrop management practices (includingmethod of fertilizer application andcultivars used). In soils with high P -fixation potential (e.g., Ultisols, Oxisols)or K - fixation potential (e.g., K -depletedVertisols), the RE of P or K may be muchsmaller, particularly under upland

conditions or rainfed lowland conditionswhere prolonged flooding does not occur.

As P and K demand is largely driven bypotential yield and the availability of N,recovery efficiencies of P and K may be

improved through improved Nmanagement and a more balancednutrient use.

The recovery efficiencies of P and K alsodepend on the method of fertilizerapplication. In rice, the recoveryefficiencies of P and K are probablysmaller for fertilizer incorporated into thesoil (typical for TPR) than for topdressedapplication of P or K (typical for DSR inthe tropics).

Further reading

Barrow NJ. 1980. Differences amongst a wide -ranging collection of soils in the rate of reactionwith phosphate. Aust. J. Soil Res. 18:215 - 224.Chakravorti SP, Biddappa CC, Patnaik S.1982. Recovery of phosphorus applied to soilas influenced by soil -water conditions and timeof application. J. Indian S OC . Soil Sci. 30:384 -386.

Dongale JH, Kadrekar SB. 1991. Direct,residual and cumulative influence ofphosphorus on crop yield, recovery andtransformation of phosphorus in greengram(Phaseolus radiafus )-rice (Oryza sativa)sequence on lateritic soil. Indian J. Agric. Sci.611736-740.

Janssen BH, Guiking FCT, van der Eijk D,Smaling EMA, Wolf J, van Reuler H. 1990. Asystem for quantitative evaluation of the fertilityof tropical soils (QUEFTS). Geoderma 46:299 -31 8.

Janssen BH, Lathwell DJ, Wolf J. 1987.Modeling long - term crop response to fertilizerphosphorus. II. Comparison with field results.

Agron. J. 79:452 - 458.Janssen BH, Wolf J. 1988. A simple equationfor calculating the residual effect of phosphorusfertilizers. Fert. Res. 15:79 - 87.


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Morel C, Fardeau JC. 1990. Uptake ofphosphate from soils and fertilizers as affectedby soil P availability and solubility ofphosphorus fertilizers. Plant Soil 121:217-224.

Smaling EMA, Janssen BH. 1993. Calibration

of QUEFTS, a model predicting nutrient uptakeand yields from chemical soil fertility indices.Geoderma 59:21 -44.

Wolf J, de Wit CT, Janssen BH, Lathwell DJ.1987. Modeling long -term crop response tofertilizer phosphorus. I. The model. Agron. J.79:445 -451.


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2.7 Managing Organic Manures, Straw,and Green Manure

Prior to the introduction of mineral fertilizers,all the N and other nutrients used to grow ricein flooded soils were provided by irrigationwater, sediments, biological N 2 fixation, andanimal manure. N fertilizer use becamewidespread following the invention of theHaber Bosch process, which is used to convertatmospheric N 2 into mineral fertilizer.Traditional rice varieties, however, did notrespond well to added fertilizer N. It was onlywith the introduction of modern, N -responsive,

high -yielding varieties that the demand for Nand other nutrients became greater than thatwhich could be supplied from indigenous andcycled sources. Over the past 30 years,governments have subsidized N fertilizermanufacture and distribution to safeguard foodsupplies during periods of rapid economic andpopulation growth. This and the greaterconvenience of mineral N fertilizer use haveled to a decline in the use of most organicnutrient sources.

Where possible, nutrient sources such asfarmyard manure, straw, and green manureshould be used in combination with mineralfertilizers to satisfy part of the rice cropsrequirement for nutrients and to sustain soilquality in the long run. In many areas, however,the supply is not sufficient, and using organicmanure is more costly than applying equivalentamounts of nutrients as mineral fertilizer.Organic rice farming is practiced in small areasbut depends on

rice nutrient disorders nutrient management volume 1

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