erin silva, organic production specialist university of

Cover crops and soil health

Erin Silva, Organic Production Specialist

University of Wisconsin, Dept. of Plant Pathology

Purposes of cover crops

• Reduce erosion from water and wind

• Increase soil organic matter and soil biological activity

• Provide biological nitrogen fixation

• Capture and recycle or redistribute nutrients

• Improve soil physical conditions (aggregation, compaction, moisture mgmt.)

• Source of forage and feed

• Weed suppression and biodiversity enhancement

https://web.extension.illinois.edu/smallfarm/downloads/56089.ppt

How do cover crops contribute to fertility?

• Green manure crop—cover crop or forage grown to incorporate into soil while green or flowering, to improve soil • SOM

• Catch crop or trap crop—cover crops planted to reduce nutrient leaching following a main crop • Cover crops do not create nutrients in soil, but can

recycle and release

• Legumes• “fix” nitrogen


How much of a benefit are cover crops to soil fertility?

• Legumes—biological N fixation

• How much N fixed? and released? and when?

• Non-legumes

• How much N trapped? and released? and when?


Two good nitrogen/cover crop resources

• Estimating plant-available nitrogen release from cover crops - PANFromCoverCrops.pdf

• http://www.swcs.org/documents/filelibrary/advances_in_nitrogen_management_for_water_quality/ANM9_A41356AAD3B6A.pdf

• Grasses and brassicas only acquire N from the soil

• Legumes can acquire N from the atmosphere through N fixation

• Over time cover crops and forages (as green manure crops) add to the organic

matter content of the soil and build nutrient content

• Less nutrient and organic matter increase if cover crops and forages are

harvested

Species Characteristics: Nitrogen Contribution

NodulesForage Radish

Cereal Rye Austrian

Winter Pea

LaChance et al., courtesy of Murrell

Cover crops and nutrient sequestration

• Trap nutrients that would otherwise “leak out” during fallow periods • leaching through

soil • losses as eroded

soil or runoff • Release nutrients

later—ideally at the time needed by the next crop


Trap crops

• Cereal rye, annual ryegrass, wheat, barley, sorghum sudan, pearl millet

• Age/stage of plant when killed, determines N% and C:N ratio, and therefore decomposition rate


Legume Green Manure Crops

• Produce 40-200 lbs. N/acre• Species

• Amount of growth/biomass produced

• %N in plant

• Approx. 40-60% of N available to subsequent crop

• Incorporation of green manure increases N for 4-6 weeks

• Later N applications needed?


Average biomass yields and nitrogen yields

of several legumes

Cover Crop Biomass Nitrogen

Tons/acre lbs./acre

Sweet clover 1.75 120

Berseem clover 1.1 70

Crimson clover 1.4 100

Hairy vetch 1.75 110

ATTRA, 2003

Distribution of plant nitrogen in

legume tops and roots

Crop Tops Roots

%N %N

Soybeans 93 7

Vetch 89 11

Cowpeas 84 16

Red clover 68 32

Alfalfa 58 42

ATTRA, 2003

Residue Addition and N AvailabilityA

vail

. S

oil

N

Time

High carbon

residues added

Immobilization (tie-up) Mineralization

(release of N)https://web.extension.illinois.edu/smallfarm/downloads/56089.ppt

Residue Addition and N AvailabilityA

vail

. S

oil

N

Time

Low carbon

residues added

No Immobilization (tie-up) Mineralization

(release of N)https://web.extension.illinois.edu/smallfarm/downloads/56089.ppt

Sullivan and Andrews, 2012

Estimating Plant Available N

https://www.uvm.edu/vtvegandberry/NMP/PANFromCoverCrops.pdf

Which cover crops should I choose?

• Where is it fitting into your rotation?

• Planting window

• Interaction with subsequent crops – insects, diseases

• What functions do you want to fulfill?

• What tools do I have to manage them?

erin silva, organic production specialist university of

Documents