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, 2nd 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 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

Example N yields from legume cover crops in upper Midwest

Where Cover lbs N/acre

Iowa Red clover ~80 lbs

Mich. Crimson clover 38-68 lbs

Mich. Hairy vetch 48-100

Ontario Red Clover ~50

(after wheat crop, for next corn crop)

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

Crop Tops Roots

%N %N

Soybeans 93 7

Vetch 89 11

Cowpeas 84 16

Red clover 68 32

Alfalfa 58 42

ATTRA, 2003

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 seededinto 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 cropto provide some nitrogen for the next crop.

Field Rotation-Hardwick Farm - EnglandStockfree (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 intercropped clover. Leek u/s cereal rye/oats.

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.

Iain Tolhurst. Jan 2000.Nutrient Key: Legume / Catch Crop

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

Sampled % N

lbs bio-

mass/A

biomass

lb N/A

Putnam Co. an. ryegrass w manure

April 3 3.54 900 31.9

May 3 2.24 3635 81.4

Fountain Co. an. ryegrass

May 1 1.94 660 12.8

Jennings Co. an. ryegrass

May 22 0.90 2865 25.7

wheat May 22 0.95 1109 10.5

Table 3. Common C:N ratios of cover crops. Organic Material C:N Ratio Reference Young rye plants 14:1 4 Rye at flowering 20:1 4 Hairy Vetch 10:1 to 15:1 8 Crimson clover 15:1 6 Corn stalks 60:1 4 Sawdust 250:1 9

Wide Carbon to N Ratio > 25/1

ATTRA, 2003

Residue Addition and N AvailabilityA

vail.

Soi

l N

Time

High carbon residues added

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

Residue Addition and N AvailabilityA

vail.

Soi

l N

Time

Low carbon residues added

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

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

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

1984 1986 1988 1990 1992 1994 1996 1998 2000

Nitr

ate-

N (m

g/L)

5 m 10 m 20 m

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

0

10

20

30

40

50

1986-88 1997-99

16 ft33 ft66 ft

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