Cover cropsPotential 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
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).
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 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 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.
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
lbs bio- biomass
mass/A lb N/A
Sampled
%N
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
Young rye plants
Rye at flowering
Hairy Vetch
Crimson clover
Corn stalks
Sawdust
C:N Ratio
14:1
20:1
10:1 to 15:1
15:1
60:1
250:1
ATTRA, 2003
Wide Carbon to N Ratio
> 25/1
Reference
4
4
8
6
4
9
Residue Addition and N Availability
Avail. Soil N
High carbon
residues added
Time
Immobilization (tie-up)
Mineralization
(release of N)
Residue Addition and N Availability
Avail. Soil N
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).
Biomass* Nitrogen Potassium Phosphorus Magnesium Calcium
lbs/ac
lbs/ac
lbs/ac
lbs/ac
lbs/ac
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.
Crop
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
5m
10 m
20 m
40.0
Nitrate-N (mg/L)
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
SEPAC - Drain spacing and nitrate-N loads (lb/A/yr)
50
16 ft
33 ft
66 ft
40
30
20
10
0
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