cover crops- potential impacts on soil fertility and water quality eileen kladivko and george van...
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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