Cover Crops for Organic Farms

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As with just about everything else when itcomes to farming, the practice of growingcover crops to produce nitrogen in the soilis complicated by many variables. Weather,differences in growing seasons, the types ofcover crops involved, and the timing ofcover crop desiccation to produce the optimumbenefit all come into play. Amassingexperience in cover crop production issimply the best way for a farmer to learnhow to deal with the interplay of all thesevariables.Legumes Versus Grasses. As we’veseen above, legume cover crops play a vitalrole in producing N for subsequent cashcrops. What part do nonleguminous covercrops, which do not produce nitrogen, havein the cropping scheme?Organic farmers often plant nonleguminouswinter cover crops to trap the soil Nthat is left over from summer cash cropsand to prevent this N from leaching out ofthe root zone or running off the field.Generally, grass cover crops are veryeffective—much more so than legumes—intrapping and recovering N from the soil.Grass and legume cover crop mixtures aremore efficient than legumes alone intrapping leftover N in the soil, but don’t doas effective a job as straight grass covercrops.There are many advantages, however, toplanting cover crops that are grass andlegume mixtures called bicultures.Researchers (Clark et al., 1994) reportedthat a cereal rye-hairy vetch biculturesuccessfully scavenged potentially leachableN from the soil, and also added fixed Nfor use by an ensuing corn crop. Additionally,the cover crop used excess water inthe soil, which also helped limit N leachinglosses.Grass species establish ground cover morequickly than legume monocultures, andtheir root growth remains active in thecooler temperatures of autumn (Ranells andWagger, 1997). Cover crop mixtures thatinclude grasses can, therefore, prevent soilerosion more effectively. Growing deeprootedand shallow-rooted cover cropstogether will also help a farmer to makebetter use of water and other resourcesthroughout the soil profile.Legumes or Grasses? How To Choose aCover CropGenerally, cover crop selection is based oneach farmer’s situation and production goals.For example, if the purpose of a cover is toprovide readily available, biologically fixednitrogen for cash crops, then the farmershould choose a legume, such as hairy vetchor cowpea.If the cover crop will be managed as a surfacemulch for weed suppression or incorporatedto improve soil quality, then the farmer shouldchoose a grass cover crop, such as cereal ryeor a sorghum-sudangrass mix. Both of thesegrass cover crops can produce large amountsof biomass with high C:N ratios at maturity,and both are reported to suppress someweeds.Farmers can also more effectivelymanipulate nitrogen cycling with mixedcover crop species. Combining maturecereals, which have high carbon to nitrogen(C:N) ratios and break down slowly,with legumes, which have low C:N ratiosand break down more quickly, can influencedecomposition of cover crop residues.The decomposition of such cover cropmixtures will occur more quickly than thatof cereal alone, releasing N more quickly foruptake by cash crops.Organic Production—Cover Crops for Organic Farms 8
Planting mixtures of cover crops can help afarmer to use the allelopathic potential ofthe cover crops to suppress weeds. Allelopathicsuppression of weeds depends onboth the cover crop and the weed. Therefore,a broader spectrum of weed controlmay be possible by growing a mixture ofcover crops, with each species contributingallelopathic activity towards specific weedspecies (Creamer and Bennett, 1997).Mixtures of cover crops can also be plantedto influence insect populations. Species thatmay not produce much biomass or biomassN may be included in mixtures to attractbeneficial insects into the cropping system.Measuring Cover Crop Nitrogen. Toensure that cash crops receive enoughnutrients, farmers must accomplish thesecalculations in this sequence:1. Determine the biomass produced.2. Determine the nutrient levels in thatbiomass.3. Predict how quickly the biomass willdecompose, releasing nutrients for cashcrops.4. Calculate whether additional nutrientsare required for the desired crop yields.Cover Crop TipCalculating the nutrient levels released bygreen manures for a subsequent cash cropnormally requires three measurements:• The amount of biomass (dry weight).• The nutrient composition of the covercrop.• The decomposition rate of the cover cropduring the cash cropping season.To estimate yield, take cuttings from severalareas in the field. Dry and weigh the samples.Use a yardstick or metal frame of knowndimensions and clip the plants at ground levelwithin the known area. Dry the samples in anoven at about 140°F for 24 to 48 hours untilthey are crunchy dry. Use the followingequation to determine per acre yield of drymatter:Yield (lb/acre) =Total weight of dried samples (lb) X43,560 sq ftArea (sq ft) sampled 1 acreFor example, two 3 feet by 3 feet (9 sq ft or 1 sqyd) samples weigh 2.5 pounds. The driedbiomass yield equals:Yield (lb/acre) =2.5 lbX43,560 sq ft = 6,050 lb/acre.18 sq ft 1 acreThough not as accurate, yield can be estimatedfrom the height of the cover crop and thepercentage of ground it covers. At 100 percentground coverage and a 6-inch height, mostnonwoody legumes contain roughly 2,000pounds per acre of dry matter. For each additionalinch, add 150 pounds.For example, a hairy vetch cover crop is 18inches tall and has 100 percent groundcoverage. The first 6 inches of dry biomassweighs roughly 2,000 pounds. The 12additional inches of growth weighs 150 poundsper inch. The additional weight is:12 X 150 = 1,800 lb,and the total weight of the cover crop drymatter is:2,000 + 1,800 = 3,800 lbIf the stand has less than 100 percent groundcoverage, multiply the total weight by thepercentage of ground covered, represented as adecimal number (the percentage divided by100). If the percentage of ground covered in theexample above is 60 percent, then the weightof the dry matter is:3,800 X 0.60 (60/100) = 2,280 pounds of drybiomass (Adapted from Sarrantonio, 1998)Organic Production—Cover Crops for Organic Farms 9
Page 2 and 3: • Cover crops for the Southeast.
Page 4 and 5: crops like rye (which has a C:N rat
Page 6 and 7: For instance, some green manure or
Page 10: These calculations normally require
Page 14 and 15: clover is seeded at 8 to 15 pounds
Page 16 and 17: nitrogen in 45 to 60 days. Seed is
Page 18 and 19: discrimination and identification o
Page 20 and 21: measurement. Australian Journal ofA
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Cover Crops for Organic Farms

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