Cover crops are the cornerstone of any organic vegetable production system. Learn how they are used on an organic farm by and experienced grower, Daniel Parson, and the science behind why they work from a soil science expert, Julia Gaskin.

1. Getting the Most from Cover
Crops
Daniel Parson
Farmer/Educator
Oxford College Farm
Emory University
Julia W. Gaskin
Sustainable Agriculture Coordinator
Crop and Soil Science Department
College of Agricultural and Environmental Sciences
University of Georgia

2. What Are Cover Crops?
Crops grown primarily for soil or ecosystem
improvement rather than cash
Cereal Rye
Winter Summer
Oats/Austrian
Winter Peas
Sunn hemp
Buckwheat
Sorghum/Cowpeas
Rye/Crimson
Clover

3. Cover Crop Benefits
• Prevents soil erosion
• Increases soil quality
• Prevents nutrient loss
• Provides weed control
• Creates habitat for beneficial insects
• Suppresses diseases and nematodes
• Provides nitrogen from legumes

4. Farming with Cover Crops
• Establish planned crop rotations
• Fit cover crops into winter, spring,
summer, fall
• Establish your goals
– Soil fertility (N)
– Organic matter
– Beneficial insects
– Weed suppression

5. Field Crop Season
Rye Aisles Winter
1 Potatoes Spring
Sudex/Soybeans Summer
Garlic Fall
Winter
2 Spring
Late Squash and Beans Summer
Wheat/Crimson Clover Fall
Winter
3 Cabbage and Kale Spring
Buckwheat Summer
Cabbage and Kale Fall
Rye and Clover Winter
4 Spring
Peppers/Eggplant Summer
Oats/Winter Peas Fall
Winter
5 Beets Spring
Soybeans/Buckwheat Summer
Beets Fall
Rye/Crimson Clover Winter
6 Cucumbers/Squash Spring
Summer
Oats/Winter Peas Fall
Winter
7 Arugula and Lettuce Spring
Soybeans/Buckwheat Summer
Arugula and Lettuce Fall
Rye/Clover Winter
8 Sweet Potatoes and Okra Spring
Summer
Rye and Clover Fall
Winter
9 Early Beans, Basil, and Flowers Spring
Summer
Rye/Hairy Vetch Fall
Winter
10 Broccoli Spring
Buckwheat Summer
Broccoli Fall
Rye and Clover Winter
11 Spring
Melons and Winter Squash Summer
Oats and Winter Peas Fall
Winter
12 Tomatoes and Late Lettuce Spring
Oats and Winter Peas Summer
Fall
Winter
13 Carrots Spring
Soybeans/Buckwheat Summer
Carrots Fall
Field Rotation Plan 2013

6. Farming with Cover Crops
• Goal: Soil Fertility (N)
– Legume cover crops
– Reduce outside fertility needs
– Grow a good crop!
Crimson Clover, Hairy Vetch, Austrian
Winter Pea, Cowpea, Soybean

7. Fertility Management

8. Farming with Cover Crops
• Goal: Organic Matter
– Grasses and everything else
– Improve tilth and soil health
Abruzzi rye and Sudex: sorghum/sudan grass

10. Farming with Cover Crops
• Goal: Beneficial Insects
– Buckwheat and other flowering plants
– Relay-cropping of beneficial builders
– Grasses over winter for alternative food
Buckwheat, clover, vetch, rye, oats, wheat
and more

11. Buckwheat Blooming

12. Syrphid Fly on Buckwheat

13. Farming with Cover Crops
• Goal: Weed suppression
– Grow a good crop
– Crop timing: before peak weed seed
emergence
– Densely growing crop
Velvetbean, cowpea, rye, clover, oats

15. Growing Good Cover Crops
• Timing of planting
• Prepare the ground
– Minimal tillage
– Enough tillage
• Spread the seed according to
recommendations
• Cover if needed: final tillage pass

16. Crimson Clover and Oats
• When:
– Plant early-September
– Plow in when needed or April
• How:
– Cover oats, don’t cover clover
– Plant before rain or irrigate
• Why:
– Nitrogen, spring blooms
– Early spring planting: oats can winter-kill

17. Grain Rye and Austrian Pea
• When: September-December and
beyond
• How:
– Cover both at planting for better results
– Cut back rye rate by 25-50%
• Why:
– Nutrients (N) and organic matter
– Beneficial insect habitat: prey

18. Buckwheat
• When: last frost to weeks before first
frost
– Quick cover for fields
– Relay plant for greatest effect
• Why: beneficial insect attractor
• How: lightly cover seed, allow to reseed

19. Cowpea/Soybean and Sudex
• When: summer for long-term cover
• How: cover lightly at planting
– Mow Sudex at 40 inches
– Add buckwheat for early cover—will
reseed!
• Why: Nitrogen, OM, and soil tilth

20. How Do you Maximize Benefits?
• Maintain healthy soil – maintain or build soil
organic matter
• Keep soil covered
• Pick cover crop to meet your goals
• Maximize biomass!
– Timely planting and establishing a good stand

21. Biomass Needed
• Maintain soil organic matter
– 6,000 to 8,000 lbs/ac/yr
• Control weeds in current growing season
– At least 30% cover by 4 weeks after planting
• Control weeds in following cash crop season
– 7,000 to 8,000 lbs/ac

22. Examples of Biomass
Cereal rye
7,000 lbs/ac
Sunn hemp
11,100 lbs/ac
Cowpeas
4,420 lbs/ac

23. Percent Cover
Oats/Austrian Winter
Peas planted Oct 1, 2013
Oats/Austrian Winter
Pea planted Sept 1, 2013
Photos
taken
Feb 2,
2014

24. Cover Crop Residues (lbs/ac)
Winter
• Cereal rye- 3,000 to 10,000
• Crimson clover – 3,500 to 5,500
• Tillage radish – 4,000 to 7,000
Summer
• Sudan sorghum – 8,000 to 10,000
• Millet (German/Japanese) - ≈4,000
• Buckwheat – 2,000 to 4,000 -6 to 8 wks
• Sunnhemp – 5,000 to 11,000
• Cowpeas – 2,500 to 4,500

25. Cover Crop Mixtures
Grain is tall; legume spreads
underneath for weed suppression
Recommended rates
2/3 legume to 1/3 grain
½ legume to ½ grain
Growing conditions
influence which species
dominate.
High N favor grains
Low N favors legumes

26. Problematic for Farmers
• Cover crops are an important nitrogen (N)
source
• How much N can I expect?
• When will it be released?
• Is it profitable?

27. Nitrogen in Cover Crop Residues
lb N / Acre
Alfalfa 100-250
Crimson Clover 50-160
Austrian Winter Pea 40-175
White Clover 75-140
Vetch 45-200
Summer Legumes
Cowpeas 40-60
Soybean 35-45
Sunn Hemp 20-280
Reeves, 1994; Mansoer et al, 1997; Schomberg et
al. 2007 Gaskin unpublished data

28. Nitrogen Release
• All nitrogen in cover crop not available to next
crop
• Example: Cowpeas followed by broccoli
– Total N in cowpeas = 168 lbs/ac
– Estimated N supplied by cover crop 78 lbs/ac (46%)
– Rule of thumb: Divide by 2

29. Nitrogen Release
• Nitrogen not released till cover crop
decomposed
• Soil temperature and moisture
– Need warmer temperatures for microbes to work
– Need soil moisture for microbes to work
• Too dry – no decomposition
• Too wet – right microbes inhibited

30. Nitrogen Release
• Microbes get N first!!
• Carbon to N ratio:
< 25 net mineralization (N released)
Think green materials!
> 25 net immobilization (N not available)
Think brown materials!
• Cover crop quality
– lignin, tannins and polyphenols slow the rate of
decomposition

31. Growth Stage and C:N

32. Timing of N Release
Crimson Clover
0
20
40
60
80
100
0 50 100 150
Days After Placing in the Field
%ofOriginalNRemaining
NT 1992
NT 1993
NT 1985
CT 1985
Quemada et al., 1997. Wilson & Hargrove, 1986
Nitrogen uptake
by corn
Incorporated
On Surface

33. Predicting N from Cover Crop
Minimum data set:
• Biomass
• N concentration in cover crop
Better data set:
• Cover crop quality
• Soil moisture and temperature
• Initial soil conditions

34. Biomass Is Critical!
Crimson clover:
3% Nitrogen and 1,000 lbs/ac
30 Lbs Nitrogen/acre
Crimson clover:
3% Nitrogen and 4,000 lbs/ac
120 Lbs Nitrogen/acre
Crimson clover:
3% Nitrogen and 6,000 lbs/ac
180 Lbs Nitrogen/acre

35. Planting Date is Critical to High
Biomass!!
Oats/Austrian Winter
Peas planted Oct 1, 2013
Oats/Austrian Winter
Pea planted Sept 1, 2013
This cover crop NEVER caught
up. Only got 20 lbs nitrogen/ac 
Photos
taken
Feb 2,
2014

36. Cover Crop Sampling
• Need aboveground biomass sample
– Use quadrat to sample known area
• Clovers, smaller cover crops – 1 to 2ft2 quadrats, 3 to 4
samples
• Larger or mixed species – 2 to 3 ft2 quadrats, 3 to 4
samples
– Cut cover crop to soil surface
– Record wet weight

37. Quadrats
4 square feet
1 square feet

38. Need Dry Biomass
• Spread cover crop out on tarp in the sun
• Dry till “crispy”
• Weigh again

39. Current Resources
– Measure biomass
– Legume rule of thumb:
• 3.5 to 4% N before flowering
• 3 to 3.5% at flowering
• Reduce by 1% for woody legumes
– Cereal grains rule of thumb:
• 2 to 3 % before flowering
• 1.5 to 2.5% after flowering

40. Current Resources
• Availability during growing season,
divide N by 2

41. Cover Crop Sample Submission
• Fresh biomass sample submission
– Put fresh quadrat samples on clean tarp
– Mix THOROUGHLY
• Tearing up large plants parts if need be
Photo Dr. Dan Sullivan, OSU

42. Cover Crop Sample Submission
• Mound sample up into cone
• Divide into 4
• Keep 2 subsamples
• Repeat if necessary
• Put representative sample in 1 gallon plastic bag
– Ship with blue ice to keep it cool
• Request N and percent moisture analysis
Keep
Keep

43. Current Resources
Oregon State – Estimating PAN release from cover crops & OSU
Organic Fertilizer and Cover Crop Calculator

44. Biomass – 6,372 lbs/ac
Cover crop N – 2.64%
N Prediction Model
Total N = 168 lbs/ac
30% available
Yields 28% greater than
county average
Rec N = 120 lbs/ac
CC Cred = 90 lbs/ac
Applied = 30 lbs/ac

45. Caution – High biomass cereal grain
cover crops can immobilize N
Biomass – 15,859 lbs/ac
Cover crop N – 1.06%
Yields only 38%
county average
Total N = 168 lbs/ac
0% available
Rec N = 90 lbs/ac
CC Cred = 45 lbs/ac
Applied = 54 lbs/ac

46. Taking It to the Farm!
Average Yield Cover Crop 0 N = 5,179 lbs/ac
Average Yield Cover Crop 1x N = 6,498 lbs/ac
Average Yield No Cover 1x N = 5,467 lbs/ac
Funding by: NRCS National
Conservation Innovation Grant
Cowpeas followed by broccoli
One time harvest

47. Taking It to the Farm 2!
Cover crop 0 N = 1,803 lbs/ac
Cover Crop 1N = 2,850 lbs/ac
No cover 1 N = 3,458 lbs/ac

48. Cover Crop Resources
• www.SustainAgGA.org. Click on Resources on lefthand side
• Managing Cover Crops Profitably, 2nd ed. Sustainable
Agriculture Network. www.sare.org/publications/covercrops/
covercrops.pdf
• Sustainable Practices for Vegetable Production in the South
www.cals.ncsu.edu/sustainable/peet/ index.html
• National Sustainable Agriculture Information Service (ATTRA)
www.attra.org

49. The University of Georgia
Crop & Soil Science Dept.
College of Agricultural and Environmental
Sciences
Sustainable Agriculture Program
www.SustainAgGA.org