SOIL – PLANT – WATERRELATIONSHIPS,

Eto & Crop water

requirements

M. Sachin DuttAsst. Director of Agriculture

Faculty, WALAMTARI

WHY Soil – Plant – Water Relations ?For designing, operating, managing efficiently any irrigation system, so as to suit different crops

SOIL:Soil is a natural body in which plants grow.

Soil is a three phase system. The interfaces between phases exhibit phenomena such as adsorption, surface tension, adhesion, swelling & shrinking, capillarity etc.

Soil is a storehouse for plant nutrients, an anchorage for plants and a reservoir that holds water needed by plants.

HOW SOIL HOLDS WATER ?

Soil holds water in two ways, as a thin film on individual soil particles, and as water stored in the pores of the soil.

Water stored as a thin film on individual soil particles is said to be in adsorption. Adsorption in simple terms is a thin film of water adhering to the outside layers of soil particles.

Water stored in the pores of thesoil is said to be in capillary storage.

SOIL MOISTURE TENSION (SMT):

•The force or tenacity with which water is retained or held in the soil is SMT / soil moisture potential / soil moisture suction. It indicates the force per unit area that must be exerted to remove water from the soil. •SMT is a result of surface tension (capillarity), adhesion & cohesion.

SOIL MOISTURE CONSTANTSFIELD CAPACITY (FC):FC is upper limit of available moisture range in soil to plants. The Soil moisture tension at FC varies from -0.1 to -0.3 atmospheres (atms).

PERMANENT WILTING POINT (PWP):It is the soil moisture content at which plants can no longer obtain enough moisture to meet the transpiration requirements and remain wilted under water is supplied to the soil. At PWP the plant growth stops. PWP is lower limit of available moisture range in soil to plants. The moisture tension at PWP is -15 atms.

HYGROSCOPIC COEFFICIENT (HC):It is the amount of water held very tightly around soil particles. Much of the water is in non-liquid phase and can move only in vapour form. Water potential at HC is about -28 atms.

Gravitational water

Available water

Permanent Wilting Point

Field Capacity

Soil

Moi

stur

e (%

)

Soil Moisture Potential (Atms)-0.01 -0.1 -1 -15 -100 -1000

0

1

0

2

0

3

0

4

0

5

0

60

SOIL MOISTURE CURVE OF A LOAMY SOIL

Unavailable Water

Permanent Wilting Point

Field Capacity

Moi

stur

e %

0

5

10

2

0

25

3

0

Sand Sandy Loam Silt Clay Loam Loam

Increase in fineness of Soil Texture

RELATION OF SOIL TEXTURE AND MOISTURE CONSTANTS

PLANT WATER RELATIONS:

1. Water Constitutes 80-90% of plant cells and tissues where there is active metabolism.

2. It is in the form of a continuous liquid phase from root hairs to leaf epidermis (Outer layer).

3. Plant roots take up water and transfer through conducting vessels (Xylem) and then to mesophyll cells of leaves and reach Stomata (Transpiration site) in the liquid state and finally to the atmosphere through Stomata as vapour.

4. Water acts as a solvent for carrying minerals, gases etc in the plant body and hence essential for overall growth of plants.

5. Plant Roots develop when there is enough moisture in the soil, Mineral availability, Soil air, presence of impervious soil layer etc.

LEAF STRUCTURE & STOMA

EFECTIVE ROOT ZONE DEPTH:

When soil and other growing conditions are favorable, crops develop well branched root system that penetrates the soil to the depth unique to that crop. This depth is referred to as Effective Root Zone Depth. Crops absorb Moisture and Nutrients from this depth.

Depending on the Root Zone , Crops are classified as follows:1. Shallow Rooted (60 cm Depth), Eg: Rice, Onion, Cabbage,

Cauliflower, Potato etc.2. Moderately deep rooted(90 cm), Eg: Groundnut, Castor,

Tobacco, Wheat, Chillies etc.3. Deep rooted (120 cm), Eg: Cotton, Maize, Jowar, Bajra.4. Very Deep rooted (180 cm), Eg: Sugarcane, Citrus, Safflower,

Coffee, Grapevine etc.

The Quantity of water available in the root zone determines the irrigation interval and the quantity of water to be given at each irrigation.

MOISTURE EXTRACTION PATTERN:

Most of the feeder roots of the crops are in the upper part of the root zone (top 45 cm) and near the base of plant. Plants do not draw moisture equally from the entire root zone.

Most active absorption of water occurs in plants through root hairs and zones of root elongation behind growing root tips.

MOISTURE SENSITIVE PERIODS OF CROP GROWTH:

The Visible stages of crop growth are:1. Germination & Emergence2. Vegetative growth3. Flowering4. Grain formation

During certain stages plants are most sensitive or critical to shortage of water which are known as Moisture Sensitive periods. Moisture stress during critical stages will reduce yield and adequate provision of water and fertilizer at other stages will not help in recovering yield lost.

• Water shortage during Germination and early seedling stage has harmful effects because of the tenderness of leaves and roots.

• In the Vegetative stage, Water shortage reduces leaf area and hence subsequent growth.

• Moisture stress during Flowering reduces grain formation and results in shriveled grain.

MOISTURE SENSITIVE STAGES OF CERTAIN CROPS (CRITICAL PERIODS)Rice : Primordial development, heading and floweringSorghum : Booting, earing, milking Maize : Knee height, Tasseling-silking, Early grain formationWheat : Crown root initiation, shooting, earingGroundnut : Flowering, peg penetration, seed developmentSunflower : Budding, Head formation, Flowering, seed fillingSafflower : Rosette, floweringCotton : Flowering to boll formation

CROP WATER REQUIREMENT:

Estimation of Crop water requirement is one of the basic needs for crop planning and for planning any irrigation project.

The Quantity of water required by a crop in a given period of time for its normal growth under field conditions.

CWR = E.T/C.U + Application losses + special needs (land preparation, transplanting, leaching etc)

IR = CWR – (ER + S)

Irrigation requirement = Water requirement – (Effective rainfall + Groundwater contribution)

EVAPOTRANSPIRATION (E.T.):

Transformation of water from Liquid to Vapour phase from Soil surface, Water surface, Plant surface.

Evaporation from plant surface is referred as Transpiration.

Consumptive Use (CU): The quantity of water used by plants for Evapotranspiration and for its metabolic activities. Since the water required for metabolic activities is negligible, the term CU is taken equivalent to E.T.

Potential Evapotranspiration (P.E.T) or Reference E.T (ET0): The Concept of ET0 is to characterize the microclimate of the field in terms of the Evaporative Demand i.e., the maximum evaporation rate which the atmosphere is capable of extracting from a field.

CROP EVAPOTRANSPIRATION (ETC):

The amount of Water required for the crop for the purpose of meeting Evaporation & Transpiration demands. It is related to ETO as follows:

ETC = ETO X KC

Wherein Kc is referred as Crop-Coefficient. It is dependent on the stage of growth of crop.

The growth period of any crop is divided into 4 stages:1. Initial stage: Germination period and early growth period of the

crop (<10% crop coverage)2. Crop development stage: Soil cover by the crop is about 70-80%.3. Midseason stage: Flowering, Start of maturity indicated by

discoloring of leaves.4. Late season stage: Full maturity or harvesting.

Kc Values of some important crops:

Crop

Crop development stagesTotal

growing periodInitial Crop

development Midseason Late season At harvest

Cotton 0.4-0.5 0.7-0.8 1.05-1.25 0.8-0.9 0.65-0.7 0.8-0.9Groundnut 0.4-0.5 0.7-0.8 0.95-1 0.75-0.85 0.55-0.6 0.75-0.8

Maize 0.3-0.5 0.7-0.85 1.05-1.2 0.8-0.95 0.55-0.6 0.75-0.9Beans 0.3-0.4 0.65-0.75 0.95-1.05 0.9-0.95 0.85-0.95 0.85-0.95

Cabbage 0.4-0.5 0.7-0.8 0.95-1.1 0.9-1 0.8-0.95 0.7-0.8Potato 0.4-0.5 0.7-0.8 1.05-1.2 0.85-0.95 0.7-0.75 0.75-0.9

Rice 1.1-1.15 1.1-1.5 1.1-1.3 0.95-1.05 0.95-1.05 1.05-1.2Safflower 0.3-0.4 0.7-0.8 1.05-1.2 0.65-0.7 0.2-0.25 0.65-0.7Sorghum 0.3-0.4 0.7-0.75 1.0-1.15 0.75-0.8 0.5-0.55 0.75-0.85Soybean 0.3-0.4 0.7-0.8 1.0-1.15 0.7-0.8 0.4-0.5 0.75-0.9

Sugarcane 0.4-0.5 0.7-1 1.0-1.3 0.75-0.8 0.5-0.6 0.85-1.05Sunflower 0.3-0.4 0.7-0.8 1.05-1.2 0.7-0.8 0.35-0.45 0.75-0.85Tobacco 0.3-0.4 0.7-0.8 1.0-1.2 0.9-1.0 0.75-0.85 0.85-0.95Tomato 0.4-0.5 0.7-0.8 1.05-1.25 0.8-0.95 0.6-0.65 0.75-0.9

ESTIMATION OF POTENTIAL EVAPOTRANSPIRATION OR ETO

Different methodologies have been developed to predict the amount of water for optimal crop production based on Climatological data. The FAO scientists have recommended the following 4 empirical formulae for estimation of ETo.

1. Modified Penman method.2. Radiation method.3. Blaney – Cridle method.4. Pan Evaporation method.

The modified Penman method is probably the most accurate and it requires data on Temperature, Humidity, Wind and Solar Radiation. Other components are derived from these parameters.

The Equation is as follows:ETO = C [ W . Rn + (1-W) . f(U) . (ea – ed)]

Radiation term Aerodynamic termWherein

C = Adjustment factor to compensate the effect of day and night weather condition (Wind speed in day and night, Relative Humidity (Max), Incoming shortwave radiation (Rs).

W = Temperature and altitude related weightage factor for effect of radiation.

Rn = Net Radiation in equivalent evaporation (mm/day)Rn = 0.75 Rs – RnlRs = Incoming Short wave radiationRs = (0.25 + 0.50 n/N) Ra

Ra = Extra terrestrial radiation in equivalent evaporation (mm/day)n = Actual measured bright Sunshine hours.N = Maximum possible Sunshine hours.

Rnl = Net Long wave radiation (mm/day) which is a function of Temperature, Vapour pressure, Sunshine duration.

Rnl = f(T) . f(ed) . f(n/N)

f(U) = Wind related function (Wind velocity in Km/day at 2 mt height).

f(U) = 0.27 (1+U/100), U = Mean Wind velocity in Km/day

ea = Saturated vapour pressure at mean temperature (mbars)ed = Mean actual Vapour pressure of air (mbars)ed = ea x RH min/100

Calculate the ETO based on the following data:

Latitude = 15 degree North in June.Altitude = 200 mTemp mean = 30 degree CDay Wind velocity = 15 km/hrNight Wind velocity = 12 km/hrMean Sunshine hours (n) = 8 hr/dayRH max = 60%RH min = 40%

S. NoItem from

ETo

formulaCalculation Value from

Table Final Value

1 W - 0.78 W = 0.782 ea - 42.4 mb ea = 42.4 mb3 ed ed = ea x RH min/100 - ed = 42.4 x 40/100 = 16.96 mb - ed = 16.96 mb4 Rn Rn = 0.75 Rs - Rnl - - Rs = [0.25+0.5(n/N)] Ra n = 8 (given), N =

13, Ra = 15.8 Rs = 8.81 mm/day

Rnl = f(T) . f(ed) . f(n/N) f(T) = 16.7, f(ed) = 0.16, f(n/N) = 0.66

Rnl = 16.7x0.16x0.66 = 1.76 mm/day

Rn = 0.75x8.81 - 1.76 = 4.85 mm/day - Rn = 4.85 mm/day

5 f(U) f(U) = 0.27(1+U/100) - - U mean = (15+12/2)x24 = 324 km/day - -

f(U) = 0.27(1+324/100) = 1.15 - f(U) = 1.156 ea-ed 42.4-16.96 = 25.44 - ea-ed = 25.44 mb7 C for RH max 60%, Uday/Unight = 15/12=1, Rs

= 8.81, U day in m/s = 4.16 m/s (15 km/hr)- C = 0.94

8 ETo ETo = 0.94[0.78x4.85 + (1-0.78) x 1.15 x 25.44] = 0.94 [3.783 + 6.436] = 0.94 x 10.22 = 9.606

- ETo = 9.6 mm/ day or 9.6 x 30 = 288 mm/month of June

Calculate the ETO based on the following data:

Latitude = 13 degree North in July.Altitude = 410 mTemp mean = 29 degree CDay Wind velocity = 20 km/hrNight Wind velocity = 10 km/hrMean Sunshine hours (n) = 9 hr/dayRH max = 80%RH min = 55%

EFFECTIVE RAINFALL (ER):

It is the utilizable rainfall. It is that part of the total amount of rainfall which is directly or indirectly useful for crop production. Ineffective rainfall is that part of the rainfall which is lost by surface runoff, deep percolation losses etc.

EMPERICAL ESTIMATION OF EFFECTIVE RAINFALL (Pe):

Pe = 0.8P – 25 (When P i.e.,Rainfall is more than 75mm/month)

Pe = 0.6P – 10 (when P is less than 75mm/month)

In general 40-50% rainfall is considered Effective.

IRRIGATION REQUIREMENT:

When to Irrigate?

Only after knowing the quantity of water available for the plants in the root zone and the Evapotranspiration demand, one can judge the time to irrigate.

Water availability can be quantified by many methods viz., Gravimetric method, Tensiometers, Electrical resistance, Pan method etc.

For the purpose of understanding and demonstration the use of Tensiometers is dealt hereunder.

TENSIOMETERS:Tensiometers are simple, reliable instruments which provide a measure of the moisture status of the soil. It reads from 0 to -85 centibars (cb).Tensiometers help answer the irrigators' questions of when and how much irrigation water to apply. Maintaining proper moisture conditions is necessary for achieving optimal plant growth and quality. Monitoring the moisture status of the soil allows for timely and efficient irrigations, and for avoiding unnecessary irrigations.

•At a typical tensiometer station, two tensiometers are installed. One, located in the upper root zone, monitors the active root area and is used to determine when an irrigation is needed. •A second tensiometer, installed near the bottom of the root zone, is used to adjust the irrigation amount or system run-time in order to ensure that sufficient water is being applied, and to avoid over-irrigation and loss of water and chemical amendments due to drainage beyond the root zone. •The moisture status of the soil is monitored by reading the tensiometers periodically. •When the tension readings in the upper tensiometer reach a certain level, an irrigation is needed. This tension level is determined by the irrigator, and depends upon such factors as crop type, soil condition, and root depth. •The lower tensiometer is monitored to ensure that enough water has been applied to refill the root zone.•When the upper unit indicates high soil suction values, irrigation is started. •Irrigation is continued until the reading on the lower unit drops, indicating that the irrigation water has penetrated to that depth and the whole active root zone has been re-wetted.

CALCULATE THE CROP WATER REQUIREMENT OF GROUNDNUT CROP WITH THE FOLLOWING DATA:

Area = 2000 HaDuration = 105 daysDate of sowing = 15th OctoberETO values for October 5.23 mm/day with RF of 69mmETO values for November 4.7 mm/day with RF of 24mmETO values for December 4.05 mm/day with RF of 2mmETO values for January 4.58 mm/day with RF of 0mm

Kc values for Groundnut crop:1. Establishment stage – 10 days – Kc = 0.42. Vegetative phase – 25 days – Kc = 0.73. Flowering phase – 30 days – Kc = 1.04. Yield formation – 30 days – Kc = 0.755. Ripening – 10 days – Kc = 0.6

S. No Item October November December January TOTAL

I II I II I II I II

(1day) (16 days) (15 days) (15 days) (15 days) (16 days) (15 days) (12 days) 105 days

1 ETo 5.23 5.23 4.7 4.7 4.05 4.05 4.58 4.58

2 Kc 0.4 0.4 0.7 0.7 1 1 0.75 0.75 (for 1 day) (for 9 days) (for 15 days) (for 3 days) (for 15 days) (for 3 days) (for 15 days) (for 2 days) 63 days

0.7 1 0.75 0.6 (for 7 days) (for 12 days) (for 13 days) (for 10 days) 42 days

3ETc

(mm/day) 2.09 2.09 3.29 3.29 4.05 4.05 3.44 3.44

3.66 4.70 3.04 2.75

4ETc

(mm/fortnt) 2.09 18.83 49.35 9.87 60.75 12.15 51.53 6.87

25.63 56.40 39.49 27.48

5 ETc Total 2.09 44.46 49.35 66.27 60.75 51.64 51.53 34.35

510.436Special needs 100 50

7 Effective RF 31.4 4.4

8NIR

(mm/fortnt) 102.09 13.06 44.95 66.27 60.75 51.64 51.53 84.35 474.6

CALCULATE THE CROP WATER REQUIREMENT OF TOMATO CROP WITH THE FOLLOWING DATA:

Month Jan Feb Mar Apr May June July

ETo(mm/day)

4 5.0 5.8 6.3 6.8 7.1 6.5

Humidity medium (60%)

Wind speed

medium 3 m/sec

Duration of growing period (from sowing): 150 days

Planting date: 1 February (direct sowing)

Initial stage 35 days (Kc = 0.45) Dev. Stage 40 days (Kc = 0.70) Mid-season stage 50 days (Kc = 0.95) Late season stage 25 days (Kc = 1.15) Harvest 0 days (Kc = 0.85)

WATER REQUIREMENT OF VARIOUS CROPS:CROP Water Required

Rice: 1240 mmGroundnut: 550 mmSugarcane: 2800 mmRagi: 400-450 mmBajra: 400-500 mmMaize: 400-550 mmTobacco: 300-400 mmSesame: 350-400 mmCotton: 650-850 mmCastor: 500 mmSunflower: 350-500 mmSoybean: 450 mmChillies: 500 mmBanana: 1600-2250 mm