Basic Concepts in Nutrient Management

for Organic FarmingJonathan Deenik

Assistant Specialist, Soil FertilityDepartment of Tropical Plant and Soil Sciences

Outline

• Feed the soil• Diagnosis of Nutrient

Deficiencies• Soil tests

Feed the Soil• Feed the bugs!

- Microorganisms- Bacteria- Fungi- Actinomycetes- Protozoa- Algae

- Nematodes- Macrofauna

- Earthworms

Organic Approaches

Cover Crops and Compost“Feed the Soil”

SupplementalHigh N

Fertilizers

Courtesy E. Brennan

Incorporating cover crops adds plant nutrients and conditions soil

Courtesy E. Brennan

Sunn hemp is a good source of N

Sudan grass adds a lot of OM

http://www.wiu.edu/AltCrops/Othercrops_files/sunn-hemp.gif J.L. Deenik

Why are Cover Crops Important?

Cover Crops

Soil Fertility

Soil Biology

Arthropod Pests

Weeds

Soil Organic Matter

Diseases

Rotations

Nutrient Leaching

Soil Erosion

Courtesy E. Brennan

Compost conditions the soil

http://compost.tamu.edu/demos/palopinto/compost.jpg

Compost• Require large inputs over time• Not necessarily a source of plant nutrients in

the short-term• Low N content (≈1.0%), only 15% available in

the first year• Improves physical, chemical, and biological

properties• Maintenance/enhancement of soil quality.

Courtesy E. Brennan

Manures as FertilizersTwo Important Questions?

1. What is the N content of the manure?

3. How long does it take for the N to go from organic form to inorganic form?

Norg NH4+ NO3

-

Plant Available

Manures as Fertilizers

Chicken manure addedat ≈16 tons per acre

Dairy Manure addedat ≈40 tons per acre

Chicken Manure

N M

iner

aliz

ed (m

g kg

-1)

0

50

100

150

200

250

Wahiawa

Dairy Manure

N M

iner

aliz

ed (m

g kg

-1)

0

50

100

150

200

250Wahiawa

Swine Manure

0 5 10 15 20 25 30 35

N M

iner

aliz

ed (m

g kg

-1)

0

50

100

150

200

250

Wahiawa

Waialua

Waialua

Weeks

Waialua

0 5 10 15 20 25 30 35

Other N Rich AmendmentsWaialua Soil Wahiawa Soil

Weeks

0 2 4 6 8 10 12 14 16 18 20 22 24 26

N M

iner

aliz

ed (m

g kg

-1)

0

50

100

150

200

250

300

350

400

Fish/Blood MealKukui NutSunn Hemp

Weeks

0 2 4 6 8 10 12 14 16 18 20 22 24 26

0 T/acre CM

2.5 T/acre CM

5 T/acre CM

10 T/acre CM 16-16-16

Adding Chicken Manure to a Fertile Soil

Field ExperimentsWahiawa Soil

lb N per Acre

0 100 200 300 400 500 600 700

Tota

l Fre

sh B

iom

ass

(lb/a

cre)

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

Chicken manureDairy manureSwine manure16-16-16 Control

How Much to Add?1. Information

- crop N requirement- N content of amendment- expected amount of N that will mineralize

- high N materials ≈80% (>6%)- chicken manure ≈40-60%- dairy manure ≈30-40%

2. Example calculationFBM DM200/(0.1*0.8)= 200/(0.01*.3)=2,500 lb per acre 66,700 lb per acre

Supplemental Fertilizers

Supplemental Fertilizers

• To raise pH- Reduce existing/potential toxicities- Increases P availability- pH range 5.5 - 7.0- Liming can be expensive because

soils are buffered (clay content and OM)

• To supply Ca- Highly weathered soils are almost

always deficient in Ca

Liming is Important

Sources of Soil Acidity

Correcting Soil Acidity

Al3+ + H2O Al(OH)2+ + H+

Liming Reactions:

CaCO3 + H2O Ca2+ + HCO3- + OH-

OH- + H+ H2O

HCO3- + H+ H2CO3

Al3+ + 3OH- Al(OH)3

Liming

Tons CaCO3 per Acre

Summary• Soil health and productivity are maintained by

regular inputs of organic matter• Cover crops are a key component of soil

management• High N inputs may be required in intensive

vegetable production systems• How much to add depends on crop requirement,

N content, and N availability from the material• In weathered tropical soils acidity can be a

problem, and it must be corrected with liming