Crop Rotations on Organic Farms

precede fall vegetable crops in organicrotations.Based on calculations of how quickly aparticular cover crop will decompose whenincorporated into the soil, organic farmersmay choose to follow the crop with either alight or heavy feeding crop. If the expectedrate of decomposition of cover cropbiomass is rapid and biomass-nitrogen yieldis expected to be high (such as a succulentwinter legume killed in mid-May), a farmermay want to plant a heavy feeder. If theexpected rate of decomposition is slow(such as a mature cereal grain) or biomassproduction may be low (such as a legumekilled in late March), a farmer may want tofollow with a light feeder.Cover <strong>Crop</strong>s and Soil Moisture.Farmers must be aware of the impact ofcover crops on soil moisture, especially indry years. Cash crops can suffer fromdrought stress in the spring when apreviously grown cover crop has drawndown soil moisture. Germination may alsobe reduced or delayed, which has anadverse impact on early season growth andfinal yield. Alternatively, in wet years, covercrops can help to deplete soil moisture andallow earlier field operations. Duringdroughty years, decomposition ofincorporated legume biomass in dry soilsmay be slower than normal, so the releaseof biological nitrogen may be inhibited. Asa result, yields might be reduced by earlyseasonnitrogen deficiency. This can becountered with additions of nitrogenfertilizer.EXAMPLEHow Cover <strong>Crop</strong>s Are Included in <strong>Crop</strong><strong>Rotations</strong>A farmer decides to plant tomatoes, which arerelatively heavy feeders, in late spring after alldanger of frost has passed. In the late spring,in preparation for planting tomatoes, thefarmer incorporates a hairy vetch cover cropinto the soil to add organic matter andnitrogen to the soil.The farmer chose hairy vetch as a cover cropbecause he/she knew that tomato plants needan early-season shot of nitrogen. Hairy vetch,with its low carbon to nitrogen (C:N) ratio andrapid decomposition rate, can fulfill that need.What if the farmer had planted lettuceinstead? Lettuce must be planted much earlierin the season than tomatoes. And legumeslike hairy vetch make most of their biomass inthe spring after lettuce would normally beplanted.In this case, the farmer might want to plant asmall grain cover crop, such as cereal rye,during the preceding fall. Cereal rye willactively recover any leftover nitrogen from thepast summer crop. That nitrogen will beavailable to the lettuce as the rye, stillrelatively green, breaks down quickly in theearly spring.Organic Production—<strong>Crop</strong> <strong>Rotations</strong> on Organic Farms 7