Cover crops-

Potential impacts on soil fertility and water quality

Eileen Kladivko and George Van Scoyoc Agronomy Dept., Purdue University

Sources of information

 ATTRA (Appropriate Technology Transfer for Rural Areas) – Preston Sullivan, 2003, “Overview of cover crops and green manures”, available at http://www.attra.org/attra-pub/covercrop.html

 Sustainable Agriculture Network (SAN) Marianne Sarrantonio, 1998, “Building soil fertility and tilth with cover crops,” in Managing Cover Crops Profitably, 2 nd ed., http://www.sare.org/publications/covercrops/covercrops.pdf

Purposes of cover crops

 Reduce erosion from water and wind  Increase soil organic matter  Capture and recycle or redistribute nutrients  Provide biological nitrogen fixation  Improve soil physical conditions (aggregation, compaction, moisture mgmt.)  Weed suppression, biodiversity, soil biological activity, extra forage

Cover crop as part of a system

     Choose cover crops to best fit desired purpose(s) and niche (window) in system.

“Green manure” crop—cover crop or forage grown to incorporate into soil while green or flowering, to improve soil “Catch” crop or “trap” crop—cover crops planted to reduce nutrient leaching following a main crop “Living mulch”—cover crop interplanted with cash crop Forage crop —can serve as green manure crop after grazing or haying is finished.

Cover crops and nutrient cycling

      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 Fix N from atmosphere (legumes) Translocate nutrients from deeper in subsoil, to near surface after cover crop death Increase soil biological activity in topsoil, potentially releasing nutrients from soil minerals Cover crops do not “create” nutrients in soil, but can recycle and release; except legumes can add N

Cover crops and N cycling

 Legumes —biological N fixation How much N fixed? and released? and when?

 Non-legumes —How much N trapped? and released? and when?

Key points:



Non-legumes do not contribute N through bacterial 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

Legume Green Manure Crops

 Produce 40-200 lbs.N/acre, depending on species, biomass produced, %N in plant  Approx. 40-60% of N available to subsequent crop  Incorporation of green manure into soil results in increased N for 4-6 weeks; after this supplemental N may be required

Table 1. Average biomass yields and nitrogen yields of several legumes (4).

Cover Crop Sweet clover Berseem clover Crimson clover Hairy vetch

ATTRA, 2003

Biomass Tons/acre 1.75

1.1

1.4

1.75

Nitrogen lbs./acre 120 70 100 110

Example N yields from legume cover crops in upper Midwest

Where Iowa Mich.

Mich.

Ontario Cover Red clover Crimson clover Hairy vetch Red Clover lbs N/acre ~80 lbs 38-68 lbs 48-100 ~50 (after wheat crop, for next corn crop)

Table 2. Distribution of plant nitrogen in legume tops and roots (6).

Soybeans Vetch Cowpeas Red clover Alfalfa Crop

ATTRA, 2003

Tops %N 93 89 84 68 58 Roots %N 7 11 16 32 42

Challenges

 Long enough growth period to produce enough biomass to fix sufficient N, in cooler climates (for full season corn in field crop system)  Good results in south (longer winter growth)  Earlier work at Purdue showed limited N fixation in central Indiana for corn crop  Usually need manure, composts, or other organic materials in addition to legume covers, to provide sufficient N to main crop

Red Clover seeded into corn just prior to canopy closing in mid-June.

It provides a cover crop protecting soil and capturing nutrients over the winter. Red Clover flowering the following June after the corn has been harvested in the fall.

It will now be plowed down as a green manure crop to provide some nitrogen for the next crop.

Field Rotation-Hardwick Farm - England Stockfree

(Iain Tolhurst- 20 yrs of organic farming on 17 acres)

 Year 1 + 2: Red clover . Cut and mulched.

 Year 3: Potato O/w green manure, clover/vetch if sown by mid Sept,

Cereal rye

for later sowing.

 Year 4 Brassicas. Winter/spring cropping. Possible

under-sow cereal rye

late September.

 Year 5 Allium. Onion+leek. Onion Leek

u/s cereal rye/oats

.

intercropped clover .

 Year 6 Carrot after leek. Parsnip after onion. Possible B. Beans sow Oct-March  Year 7 Sweet corn. Squash. Both u/s red clover/Lucerne

.

Nutrient Key: Legume /

Catch Crop

Iain Tolhurst. Jan 2000

.

Red Clover

Hairy Vetch and Rye

Onion interseeded with Red Clover

Potato

Trap crops

 Amount of biomass produced is key to nutrient uptake —good stand, rapid growth  Age/stage of plant when killed, determines N%, C:N, plant composition, and therefore decomposition rate (along with weather!) Huge challenge!  Cereal rye, annual ryegrass, wheat, oats, barley

2002 cover crop demonstrations— Indiana

Putnam Co. an. ryegrass w manure Sampled April 3 May 3 % N

3.54

2.24

lbs bio mass/A

900

biomass lb N/A

31.9

3635 81.4

May 1

1.94

660 12.8

Fountain Co.

Jennings Co.

an. ryegrass an. ryegrass wheat May 22 May 22

0.90

0.95

2865 1109 25.7

10.5

Table 3. Common C:N ratios of cover crops.

Organic Material C:N Ratio Reference

Young rye plants Rye at flowering Hairy Vetch Crimson clover Corn stalks Sawdust ATTRA, 2003 14:1 20:1 10:1 to 15:1 15:1 60:1 250:1 4 4 8 6 4 9

Wide Carbon to N Ratio > 25/1

Residue Addition and N Availability

High carbon residues added Time

Immobilization (tie-up) Mineralization (release of N)

Residue Addition and N Availability

Low carbon residues added Time No Immobilization (tie-up)

Mineralization (release of N)

Table 4. Biomass yield and nutrient accruement by selected cover crops (10). Crop Biomass

* lbs/ac

Nitrogen

lbs/ac

Potassium

lbs/ac

Phosphorus

lbs/ac

Magnesium

lbs/ac

Calcium

lbs/ac Hairy Vetch Crimson clover Austrian W.P. Rye 3,260 4,243 4,114 5,608 141 115 144 89 *Dry weight of aboveground plant material. ATTRA, 2003 133 143 159 108 18 16 19 17 18 11 13 8 52 62 45 22

Water quality impacts

 In Indiana, Ohio, and Illinois, much of the N leaching losses occur in fallow period (fall, winter, early spring), because most of the tile drainage flows occur during this time  Winter cover crops can reduce N losses significantly, by trapping N that would otherwise leach to tile drains, and possibly by reducing total water flow to tiles   Effectiveness depends on good stand and rapid biomass production Trap crops cannot absorb indefinitely —excessive nutrient applications for ex. with excessive manure, will still result in high nitrate concentrations and losses

Annual flow-weighted mean concentrations

5 m 10 m 20 m 40.0

35.0

30.0

25.0

20.0

15.0

10.0

5.0

0.0

1984 1986 1988 1990 1992 1994 1996 1998 2000

20 10 0 50 40 30

SEPAC - Drain spacing and nitrate-N loads (lb/A/yr)

16 ft 33 ft 66 ft 1986-88 1997-99

Summary

 Legume cover crops can “fix” N from atmos.

 Amount depends on amount of growth, species, soil, etc.

  Often need additional N sources such as compost or manure Non legume cover crops effective to “trap” N that would leach through soil  Amount depends on amount of growth  Release depends on C:N ratio, weather, etc.

 Cover crops as part of a system for nutrients