Crop Rotations on Organic Farms

Organic farmers have reported manybeneficial effects from crop rotations onphosphorous relationships in the soil.According to these farmers, crop plantsraised in rotations generally have betterroot function, and so are better able to takeup phosphorous from the soil. The farmerspoint out that rotations employing greenmanures have a tendency to increase soilmicrobial activity, as well as plant-availablephosphorous. Finally, the enhancednutrient cycling that results from croprotations increases the amount of twoplant-available forms of phosphorous in thesoil: biomass phosphorous and labileorganic phosphorous.<strong>Crop</strong> <strong>Rotations</strong> Boost N, C, P, and K in SoilsA team of researchers reported in a 1998 issueof Agronomy Journal that the use of rotationalorganic farming practices over an eight-yearperiod increased soil organic carbon, solublephosphorous, exchangeable potassium, andsoil pH (acidity measurements).At the conclusion of the eight-year trial, soilorganic carbon was 2 percent higher in a fieldwhere organic rotational practices were usedthan in a baseline field where conventionalpractices and a two-year rotation scheme wasused. Likewise, total soil nitrogen was 22percent higher in the organic field than in thebaseline conventional field.Source: Clark et al. (1998)Light and Heavy FeedersOrganic farmers often base their croprotations on whether various plants in therotational lineup are light or heavy feeders.<strong>Crop</strong>s differ in their ability to extract waterand nutrients from the soil. Some plantswith shallow roots feed near the surface;others have root systems that explore thesoil at lower depths (Table 1). Following ashallow-rooted crop like onions or carrots,organic farmers may plant deeper-rootedcrops like corn to recover nutrients thatwere unused by the shallow feeders andmay have leached by irrigation or rainfallto lower depths in the soil profile.Conversely, these farmers sometimes followdeep-rooted heavy feeders with shallowrootedlight feeders to scavenge nutrientsthat may remain after heavy applications ofnutrients.Examples of Light and Heavy FeedersSome crops are heavyfeeders that deplete soils,while other crops are lightfeeders that build soils.Soil Depleting <strong>Crop</strong>sRow crops — corn, soybeans, vegetables,potatoesSoil Neutral or Soil Conserving <strong>Crop</strong>sCereal crops — wheat, barley, oatsSoil Building <strong>Crop</strong>sLegume sods — alfalfa, cloverGrass sods — prairie grass, meadows, pasturesCover <strong>Crop</strong>sWarm-season legumes, grown as covercrops, figure prominently in every rotationon an organic farm. This is because they area primary source of nitrogen for other cropsin the organic rotation. In most years,growing a winter legume like hairy vetch orcrimson clover will provide all thebiological nitrogen necessary for a summercash crop. Warm-season legumes likecowpeas, sunnhemp or soybeans also offeropportunities for biological nitrogenfixation during the summer season.Legumes often follow spring crops orOrganic Production—<strong>Crop</strong> <strong>Rotations</strong> on Organic Farms 6