Crop Rotations on Organic Farms

<strong>Crop</strong> <strong>Rotations</strong> on Organic Farmsby Keith R. BaldwinFarmers in ancient cultures as diverseas those of China, Greece, and Romeshared a common understandingabout crop rotations. They learned fromexperience that growing the same crop yearafter year on the same piece of landresulted in low yields, and that they coulddramatically increase productivity on theland by cultivating a sequence of crops overseveral seasons. TheyIf improved soilaggregation andtilth are afarmer’s goals,long-termrotations shouldbe part of afarm plan.came to understandhow crop rotations,combined with suchpractices as covercrops and greenmanures, enhancedsoil organic matter,fertility, and tilth.For a variety of otherreasons that we will explore in thispublication, crops can and should bemanaged in rotations. No one disputes thefact that rotations are beneficial. The use oftwo- and three-year rotations by theContentsA Historical Perspective—Page 2<strong>Crop</strong> <strong>Rotations</strong> versus Continuous <strong>Crop</strong>ping—Page 3Figure 1. Soybeans are often part of anorganic rotation schedule in the South.Photo courtesy of USDA.majority of the grain farmers in thiscountry shows they agree that yields aregenerally higher when crops are grown<strong>Crop</strong> <strong>Rotations</strong> and Fertility—Page 5<strong>Crop</strong> <strong>Rotations</strong> and Pest Management—Page 9Recommended Reading—Page 14

sequentially in rotations. In thispublication, we will discuss crop rotationsas key strategies that farmers can use tobuild the soil, manage pests, and increaseyields. Our discussion will be organizedaround the following topics:• A historical perspective. We willdescribe how crop rotations fit into thehistory of farming in America,particularly in the South, and howmodern conventional farming methodshave altered the way some farmerspractice crop rotations.• <strong>Crop</strong> rotations versus continuouscropping. We will summarize theresults of several scientific studies thathave compared the impacts of longandshort-term rotations andcontinuous cropping, or monoculture, onsoil properties. These studies indicatethat using crop rotations can lead todramatic increases in soil fertility, helpto optimize nutrient and water use bycrops, and improve our soil resources.• <strong>Crop</strong> rotations and soil fertility. Wewill describe the use of crop rotations toimprove fertility.• <strong>Crop</strong> rotations and pest management.We will describe the use of croprotations to manage pests, includingdiseases, weeds, and insects.Continuous <strong>Crop</strong>pingIn the South, continuous cropping thatincorporates organic matter is better thanfallow periods for the soil. During fallowperiods, biomass additions are not made tothe soil. However, mineralization of organicmatter in the soil continues during fallowperiods, thus leading to reductions in organicmatter.A HISTORICAL PERSPECTIVEThere was a time in America when the useof long-term crop rotations figuredprominently in every farmer’s plans toboost soil fertility and control crop pestsand diseases. But since the 1950s, farmersincreasingly have replaced crop rotationswith modern practices, such as usingsynthetic fertilizers to supply annual cropnutrients, applying agri-chemicals tocontrol pests and diseases, and selectingimproved crop varieties for increased yields.Many farmers still use crop rotations, but inmuch shorter cycles. For instance,approximately 80 percent of the U.S. corncrop is now grown in two-year rotationswith soybeans or three-year rotations withsoybeans and wheat. These short, two- tothree-year rotations rarely include pastures,cover crops, or green manures. Thesemodern cropping systems have allowedAmerican farmers to benefit fromeconomies of scale by specializing theiroperations and marketing crops in volume.And these systems require fewer pieces ofequipment because the crops grown are lessdiverse.Such modern farming practices have helpedU.S. farmers to produce remarkable cropyields. Nevertheless, the use of intensivecropping for the past 50 years has causedsome negative impacts on ourenvironment, especially on our farm soils.Today, the biggest risks for farmers aredeclining soil quality and increasingenvironmental degradation. As a result,researchers are once again focusing on croprotations as a primary way to attainsustainable crop production, improvedyields, and the economic returns thatsupport a diversified rural economy.Organic Production—<strong>Crop</strong> <strong>Rotations</strong> on Organic Farms 2

In the South, farmers historically have usedcrop rotations in the production of cotton,tobacco, and peanuts. For example, thesewere traditional rotations for tobacco in theearly 20th century:• tobacco—wheat—clover,• tobacco—wheat—cowpea, or• tobacco—wheat—red clover—mixedforages—corn.The cowpea was generally included whenlegumes were used as a green manure tomaintain soil humus.Cotton rotations were similar to those fortobacco. Other crops in the rotation wereselected to maintain soil humus content,for their potential as livestock feed, or both.On livestock farms, a rotation of corn—oats—wheat—clover—pasture was oftenemployed to produce livestock feed.<strong>Crop</strong> <strong>Rotations</strong>: The Benefits• Build soil fertility.• Preserve the environment.• Boost economic returns.• Aid control of weeds, diseases, andharmful insects.• Add to crop and market diversity.CROP ROTATIONS VERSUSCONTINUOUS CROPPINGAs discussed in other publications withinthis series, organic farmers work toward akey goal: to improve soil quality andstructure. This goal isn’t accomplishedovernight. It takes years of concerted effortto feed the soil and build a friable soilstructure with these characteristics:• Nutrients and water reside in a soilnutrient reservoir or pool and becomeavailable to plants over both the shortand long term.• A healthy microbial pool of livingmicroorganisms exists to facilitatenutrient cycling from the nutrientreservoir to plant roots.• A natural ecosystem is established thatserves as an environmental filter. Thisfilter helps to protect the agroecosystemfrom potentially adversefarming practices and environmentalcalamities.Soil Organic MatterOrganic farmers often judge and monitorsoil health based on the amount of organicmatter in each farm field. Active soilorganic matter refers to a diverse mix ofliving and dead organic materials near thesoil surface that turn over or recycle everyone to two years. Active organic matterserves as a biological pool of the major plantnutrients. The balance between the decayand renewal processes in this biologicalpool is very complex and sensitive. Thepopulations of microorganisms that makeup the biological pool are the driving forcesin soil nutrient dynamics. Together theyalso play a key role in building a soilstructure that both retains and freelyexchanges nutrients and water—a soilwhere plant roots thrive.<strong>Crop</strong> <strong>Rotations</strong> and Soil Organic MatterWhich crop rotation factors affect soil organicmatter?• Rotation length• Loss of organic matter from tillageoperations• Interactions with fertilization practicesSource: Karlen et al., 1994Organic Production—<strong>Crop</strong> <strong>Rotations</strong> on Organic Farms 3

Conventional TillageIt’s easy to see how this delicate worldbeneath the soil surface can be affected soreadily one way or the other by variouscropping systems. Imagine the impact onsoil fertility, structure, and nutrientprocesses when the soil is turned up andmixed regularly. Many studies have shownthat in most conventionally tilledagricultural soils, both soil organic matterand microbial activity decrease. Althoughbulk organic matter may still exist undersome high-tillage systems, active soilorganic matter is lost.Extensive tillage stimulates microbialactivity by providing soil microorganismswith the oxygen they need to break downor consume organic matter. Populations ofmicroorganisms build quickly under thesecircumstances and actively decompose anyamendments, green manures, or cropresidues that are turned in with tillage.Organic matter doesn’t normallyaccumulate in the soil under theseconditions.Because they do not generally include greenmanure or forage crops, continuouscropping and short-term rotational systems(systems that use two- or three-yearrotations) deplete soil organic matter levels.As a result, soil structure—as measured bysoil aggregate stability, soil bulk density,water infiltration, and soil erosion—candegrade.Effective <strong>Crop</strong> <strong>Rotations</strong><strong>Rotations</strong> are most effective whencombined with such practices as manuring,composting, cover cropping, greenmanuring, and short pasturing cycles.Together, these practices create soil qualityimprovements such as increased soilaggregate stability, decreased crusting ofsoil surfaces, and increased granularstructure and friable consistence. <strong>Rotations</strong>that include sod, pasture, or hay crops alsohelp to decrease bulk soil density, whichcan greatly impede root growth andnutrient flow. Simply put, managementsystems that maintain or increase soilorganic matter have the potential forincreasing soil productivity for all croppingsystems, including organic systems.<strong>Crop</strong> <strong>Rotations</strong> and Soil ErosionFarmers who practice long-term crop rotationcan reduce soil erosion on their land. A studyconducted in 1988 found 6 inches moretopsoil on an organic farm than on anadjacent conventional farm in the Palouseregion of Washington state.The 7,700-acre organic farm had beenmanaged without the use of commercialfertilizers and with limited use of approvedpesticides since its soil was first plowed in1909. Researchers compared the organicfarm’s topsoil to that of a nearby conventionalfarm. The 1,400-acre conventional farm, firstcultivated in 1908, began receivingrecommended rates of commercial fertilizersin 1948 and pesticides in the early 1950s.The difference in topsoil depth between thetwo farms was attributed to significantlygreater erosion on the conventional farmbetween 1948 and 1985. Researchersattributed the difference in erosion rates tocrop rotation because the organic farmerincluded green manure crops within therotation plan while the conventional farmerdid not.Source: Reganold et al, 1988Organic Production—<strong>Crop</strong> <strong>Rotations</strong> on Organic Farms 4

CROP ROTATIONS AND SOILFERTILITYU.S. farmers soon may have no choice as toadopting such practices as crop rotations.Alarming increases in nitrogenconcentrations in surface and groundwaterhave been attributed to the use of nitrogenand phosphorous fertilizers on farms. Suchproblems may result in laws mandating bestnutrient management practices on farms.<strong>Crop</strong> <strong>Rotations</strong>:Match Profits with PracticesThe challenge for farmers practicing croprotation is this: to define systems thatmaintain farm profits with practices thatimprove soil quality and preventenvironmental degradation.Farmers may want to consider such options asalternative crops, double and triple cropping,value-added enterprises (such as producingcover crop seed or forages and green manuresfor composting), or a combination of all ofthese.runoff or leach. Researchers estimate thatfrom 40 to 75 percent of the total nitrogencontained in a legume cover crop isavailable in the soil for subsequent crops,depending on environmental conditions.In addition, trap crops like small grains canbe used to capture leftover nitrogen fromfarm fields after a harvest of cash crops.Small grains have extensive, fibrous rootsystems that can effectively mine the soilfor available nitrogen. By capturing andstoring residual soil nitrogen, these trapcrops prevent this nitrogen from leachingor running off farm fields.Phosphorus and PotassiumThe effect of crop rotations on soil nitrogen(N), phosphorous (P), potassium (K) andcarbon (C) is very complex. Southeasternorganic farmers report that including deeprootedcover crops in rotations helps tobetter distribute phosphorous andpotassium from deep within the soil profileto the soil surface, where plant roots havebetter access to them.NitrogenFarmers use a best nutrient managementpractice when they use legumes in croprotations to supply biologically fixed,atmospheric nitrogen as a replacement orsupplement for inorganic nitrogenfertilizer. The amount of nitrogen inlegume cover crops varies among species,but legumes generally contribute 50 to 200pounds of nitrogen per acre. This nitrogenis mineralized over an extended period oftime, so that any surpluses of it do notreadily run off into streams andunderground water supplies. The nitrogenin conventional fertilizers, however, isavailable immediately and surpluses canSoil Organic Carbon Reflects Soil QualitySoil organic carbon (SOC) is an importantindicator of soil quality because it influencessoil structure. Soil structure affects soilstability, as well as its capacity to hold water,and it is a driving force in nutrient cycling.Equilibrium concentrations of SOC in agroecosystemsare the direct result of the farmingpractices implemented on the land.Organic farmers can attempt to build higherSOC with sustainable farming practices.However, they should realize that following achange in land management, SOC changesslowly.Organic Production—<strong>Crop</strong> <strong>Rotations</strong> on Organic Farms 5

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

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

Table 1. Effective root zone depth of key crops calculated in inchesField <strong>Crop</strong>sBarley, 24Corn (Field), 24Cotton, 24Flax, 24Oats, 24Peanuts, 24Rye, 24Sorghum, 24Soybeans, 24Sunflower, 24Tobacco, 18Wheat, 24Forage <strong>Crop</strong>sAlfalfa, 24Bluegrass, 18Bromegrass, 24Ladino Clover, 18Orchardgrass, 24Red & Sweet Clovers,24Sudan Grass, 24Ryegrass, 24Bermuda Grass, 18Tall Fescue, 18Vegetable <strong>Crop</strong>sAsparagus, 24Beets, 12Broccoli, 12Cabbage, 12Cantaloupes, 18Carrots, 12Cauliflower, 12Celery, 12Corn (sweet), 24Cucumbers, 18Kale, 18Lettuce, 6Lima Beans, 18Onions (bunch), 6Onions (dry),12Peas, 18Peppers, 18Potatoes, 18Radish, 6Snap Beans, 18Spinach, 6Squash, 18Tomatoes, 18Watermelons, 24Fruit <strong>Crop</strong>sApples, 24Blueberries, 18Cane Fruits & Grapes,18Peaches, 18Pears, 18Strawberries, 6TurfAthletic Fields (in activeuse), 6Athletic Field (not inactive use), 12Golf Greens/Fairways, 6Grass Sod (beingestablished or preparedfor immediate sale), 6Grass Sod (lawn andsod being heldfor sale), 12FlowersAnnual Flowers, 6Ericaceous OrnamentalPlants (Azalea, etc.), 12Gladioli/Peonies/Irises,12Other Bulb or CormPlants, 12Nursery PlantsBedded Plants (afterpropagation). 6Finished LandscapePlants,(ready for sale),18 to 24Ground Cover Plants(vinca, ivy, etc.), 6Lining-out Plants, 12Perennial Ornamentals,24Trees, Shrubs (conifersand flowering shrubs),24Root depth was based on these factors:1. The depth of soil to which most of the total root system has developed when the marketable part ofthe crop is being produced or when the loss of water from turf and ornamental plants is greatest.2. Research and experience regarding the overall water needs of each crop for maximum quality as wellas yield or growth.3. The kind of soil in which some crops are grown. The depth of irrigation while the crop is developing itsroot system should be determined by the actual root depth at the time of irrigation.Data adapted from: Soil Moisture Sensors for Irrigation Management, Bulletin 312, University of MarylandCooperative Extension Service, 1984; Evapotranspiration and Irrigation Water Requirements, ASCE Manualon Engineering Practice, No. 70. Disclaimer: Commercial products are named in this publication forinformational purposes only. The authors, Virginia Cooperative Extension, and Virginia PolytechnicInstitute and State University do not endorse these products specifically and do not intend discriminationagainst other products that are not mentioned but which might also be suitable.Organic Production—<strong>Crop</strong> <strong>Rotations</strong> on Organic Farms 8

Versatility is a Plus for Organic Farmers• Organic vegetable farmers have ampleopportunities to change their rotationplans even in mid-season, for example,as a response to insufficient nitrogenfrom green manures. They can choosefrom a diversity of vegetable crops thathave widely ranging nutrientrequirements. So, rotations can bequickly altered to fit the situation.• Many vegetable crops, such as lettuce,remain in the field for a relatively shortperiod, thus allowing for multiplecroppings.• Producing two or three crops in oneseason may offset the costs associatedwith leaving a field out of productionevery third or fourth year for“rebuilding.”• The patchwork nature of many small- tomedium-sized market vegetable farms,containing many small fields, allowsfarmers to give individual attention tothe particular fertility or physical needsof each field.Source: Sarrantonio, 1992CROP ROTATIONS AND PESTMANAGEMENTConsider the sheer abundance of insects,pathogens, weeds, and plant diseases, andyou will realize the critical role of croprotations in reducing damage by these pestson organic farms. Farmers who implementa good crop sequence must consider twothings at once:• How one crop can benefit from the cropthat precedes it.• How any pest problems these cropsshare can be addressed.Because organic farmers cannot useconventional agricultural chemicals tomanage crop pests and must rely largely oncultural strategies, they must have a betterunderstanding than most farmers abouthow crop pests live and function. Basically,they must outwit these pests as theyemploy many kinds of strategies tocomplement crop rotations.Learning Pest HistoriesUnderstanding the natural history of a pestis extremely important for determining thesequence of crops in a rotation. Manyinsects and diseases attack more than onefamily of plants, and rotating into adifferent family may do little to reducepathogen potential or insect pressure if thesubsequent crop is also a host plant. Forexample, southern blight, Sclerotium rolfsii,is a pathogen that attacks most vegetablecrops, regardless of family, genus, orspecies. If a field has a history of problemswith this pathogen, managers may have toinclude a row crop in the rotation—forexample, corn or some other grass, hay, ora pasture crop for two or three years.Controlling Soilborne DiseasesOrganic farmers also must know about thesoilborne pathogens that build up when asoil is sown with the same crop or family ofcrops every year.<strong>Crop</strong> Families. Generally, crops in thesame family should not follow one anotherin the field. For instance, cantaloupesshould not follow cucumbers. A cucumbermelon-squashrotation obviously invitesdisease problems. At a minimum, cropsfrom a particular family should beseparated by at least two years of crops fromother families. For example, a rotation ofOrganic Production—<strong>Crop</strong> <strong>Rotations</strong> on Organic Farms 9

families might include Brassicaceae (colecrops), followed by Asteraceae (lettuce, cutflowers), followed by Solanaceae (tomatoes,potatoes, peppers, eggplants), followed byCurbitaceae (squashes, cucumbers, melons).Length of Rotation. The length of timesoilborne pathogens remain viable in thefield is critical to any decision about thelength of the rotation before replanting thesame vegetable crop. Although there aresome exceptions, a four-year rotation thatincludes a succession of crops not susceptibleto the same pathogens will generallyminimize problems from soilborne diseases.For this strategy to be effective, however, itis important that pathogen-susceptibleweeds and volunteer plants be excludedfrom the field, too.Table 2. Rotation periods to reduce soilbornediseasesYears without aVegetable Disease Susceptible <strong>Crop</strong>Asparagus Fusarium rot 8Cabbage Clubroot 7Cabbage Blackleg 3 - 4Cabbage Black rot 2 - 3Muskmelon Fusarium wilt 5Parsnip Root canker 2Peas Root rots 3 - 4Peas Fusarium wilt 5Pumpkin Black rot 2Radish Clubroot 7Source: S.A. Johnson & P.J. Nitzche, USDASome exceptions to this rule include clubroot of crucifers, sclerotinia white mold onlettuce, and fusarium wilt (a diseaseaffecting many vegetables). Long rotationsof four years and more are desirable foravoiding these diseases. Where there is ahistory of problems with long-livedpathogens, these practices have provenbeneficial:• soil solarization (covering a field areawith clear plastic so the sun can raisesoil temperatures enough to destroypathogens),• compost additions,• use of resistant varieties, and• long rotations.As a general rule, a two-year rotation willreduce the incidence of foliar (leaf) diseasesbecause a primary source of inoculum forinfection is often the infected tissue fromprevious crops. Generally, that tissue will bewell-decomposed in two years, and anyinoculum will disappear along with cropresidues. For example, most of theinoculum for early blight in tomatoes,cercospora of cucurbits, and most foliarbacterial diseases can be eliminated bydestruction and incorporation of residuesand a one-year waiting period beforereplanting.Tillage Practices. Various tillagepractices can make an impact on diseasecontrol or crop rotations. For instance,southern blight pathogen survival from oneyear to the next is generally restricted tothe upper 2 or 3 inches of soil, and burialbelow these depths is an effective diseasecontrol strategy. Farmers must also considerhow different types of tillage systems caninfluence a rotation. For example, ifmanagement plans call for a period of notill,incorporation of crop residues will bedelayed and the selection of crops for arotation in this instance may be morelimited. Tillage practices that enhance soildrainage generally reduce the incidence ofseedling diseases. Including rotational cropsthat are planted on high ridges or in plantbeds in the rotation often reduces theincidence of damping-off disease.Organic Production—<strong>Crop</strong> <strong>Rotations</strong> on Organic Farms 10

Hard-case Pathogens. Someparticularly troublesome pathogens in thesoil can be controlled by relatively shortrotations of specific plants that areunsuitable hosts for the disease. Otherpathogens require longer rotations tocontrol.An example of a disease that requires a longrotation is Granville wilt or southernbacterial wilt. In fact, it is not possible tomanage this pathogen successfully wherethe infestation level is moderate to highwithout an appropriate crop rotation.<strong>Rotations</strong> are effective against southernbacterial wilt because these bacteria do notmultiply in the soil without susceptibleplant tissue. Thus, populations decline if asuitable plant, such as the tomato, is absentfor even one year. Planting a nonhost crop,such as soybeans, fescue, corn, cotton, andsorghum, for just one year will significantlyreduce the losses to this disease in thefollowing tomato crop.Some Pathogens Require Long-term <strong>Crop</strong><strong>Rotations</strong>An example of a pathogen that requireslonger crop rotations for control isStreptomyces soil rot or pox of sweet potato. Atypical rotation plan to control this pathogenmight look like this:• Year 1. Check soil pH, apply lime ifneeded, and plant beans.• Years 2 and 3. Plant corn and small grain.• Year 4. Plant tobacco.• Year 5. Check soil pH. If at pH 5.2 orunder, solarize soil and plant sweetpotatoes. When developing a rotationprogram, sweet potatoes should notfollow a crop requiring a high soil pH.Source: Averre and Ristaino, 1991As is true with any other soilbornepathogen, the longer the rotation, the moreefficient the control. Other managementpractices also help to control this pathogen,such as drainage improvements, avoidanceof late or deep cultivation (or both), stalkand root destruction, and soil solarization(Melton and Shew, 1998).Nematode ManagementSusceptibility to parasitic nematodes, whichare common plant pests, is anotherconsideration in planning a crop rotation.This is a complex issue because manydifferent pest nematode species occur, andtheir ability to infect vegetable crops varieswith the nematode species and the crop.Generally, rotations should separate a cropsensitive to a particular nematode, say rootknot nematode, with crops that are notsensitive or easily infected by thatnematode. Nematode populations thendecline in intervening years to levels belowthresholds for economic injury tosusceptible crops. When nematodepopulations become lower than thesethresholds, sensitive crops can be replanted.Green manure crops that do not serve ashosts to problem nematodes are sometimesused as intervention crops. There is a greatdeal of ongoing research in matchingvegetable crops and green manures tosuppress adversarial nematodes. Rapeseedand mustard have shown insensitivity toinfection by a wide range of parasiticnematodes and are commonly planted byorganic farmers to “clean up” soil duringwinter months. Some cover crops have alsobeen shown to suppress nematodepopulations, such as velvetbean, sorghumsudangrass,and sunnhemp.Organic Production—<strong>Crop</strong> <strong>Rotations</strong> on Organic Farms 11

Major Plant Parasitic Nematode Genera inthe U.S. and How They Damage Plants♦ Root-knot nematodes form galls oninjured plant tissue. The galls block waterand nutrient flow to the plant, stuntinggrowth, impairing fruit production, andcausing foliage to yellow and wilt. Rootsbecome rough and pimpled andsusceptible to cracking.♦ Cyst nematodes give plants an unthrifty ormalnourished appearance, and cause themto produce smaller-than-normal tops.Foliage is liable to wilt and curl, while rootsbecome thick and tough and take on a redor brown coloring.♦ Sting nematodes are found mainly in theSouth, especially in sandy soils with meagerorganic matter content. Areas of stuntedplants are an early indicator. As these areasgrow larger and finally meet, the plantsthat were first affected will start to die atthe margins of older leaves.♦ Root-lesion or meadow nematodes causeinternal browning in potato tubers and inthe roots of corn, lettuce, peas, carrots,tomatoes, and brassicas.Source: Roger B. Yepsen, 1984Weed Management<strong>Crop</strong> rotations should be designed thatmake it difficult for weeds to grow andreproduce. Disturbing the soil with somesort of timed tillage is a good way to createan inhospitable or unstable setting for weedgrowth. Certain crops can suppress weedsby out-competing them for water andnutrients, or by shading them so theycannot receive adequate sunshine.Allelopathic Properties. Many cropplants can also create what is calledallelopathic interference. These plants releasechemicals either while they are growing ordecomposing that prevent the germinationand growth of other plants. Plants differ intheir allelopathic properties and in theirsusceptibility to allelopathic chemicalsproduced by other crops. Thus, broadleafweed germination may be inhibited in thespring following plowdown of a wintercereal-rye cover crop, but sweet corn sowninto that stubble may not be influenced inthe least. Researchers have effectively usedcover crops of wheat, barley, oats, rye,sorghum, and sudangrass to suppress weedsthrough allelopathy, competition, andshading. Weed suppression has also beenreported from residues and leachates ofcrimson clover, hairy vetch, and otherlegumes. When killed and left on thesurface as mulch, cover crops continue tosuppress weeds, primarily by blocking outlight.EXAMPLEA Rotation To Suppress WeedsIn a field planted to continuous spring lettuce,a farmer could follow the crop with a fastgrowing, vigorous summer cover crop, suchas soybean or cowpea and Japanese millet,and then plant fall broccoli. Little or no weedseed will be spilled, and subsequent lettucecrops will have less weed competition.Insect ManagementManaging insect pests in crops withoutusing pesticides is no easy task for organicfarmers. Organic farmers rely largely ongood management practices, such as cropOrganic Production—<strong>Crop</strong> <strong>Rotations</strong> on Organic Farms 12

otations, to keep pests in check. They musthave a good working knowledge of insect,disease, and weed life cycles, along withcultural controls that affect pestpopulations. A farmer must also know apest’s feeding habits and preferences, aswell as plant crops that are unappetizing topests prevalent in the region.Needless to say, rotations will not controlall insect pests. <strong>Rotations</strong> have little impacton highly mobile insects because theseinsects have the ability to invade fromadjacent fields or other areas. A good rule ofthumb is to sequence crops in a rotationthat are hosts to entirely different sets ofpests, have different growth habits, and aredissimilar in other respects.All of this helps to interfere with thenormal needs of a pest during its life cycle,such as an insect’s need to find food, apathogen’s need for a suitable host toinfect, or, in the case of weeds, the need fora crop architecture or tillage regime toexploit.In a California study conducted in 1988,researchers M.L. Flint and P.A. Robertsfound that crop pests that can be controlledby rotations had some commoncharacteristics:• The host range of the pest needs to befairly narrow or at least must notinclude plants that are reasonablycommon in a given area.• The pest source should be the fielditself.• The pest must be incapable of survivinglong periods without a living host. Thatis, pest populations must decreasesubstantially within a year or two ofremoving a living host plant. The pestshould be as immobile as possible, suchas soil and root-dwelling nematodesand soilborne pathogens (if they do notproduce airborne spores, such asRalstonia solanacearum).Carefully Plot Your Tactics in the War onInsect PestsInsect pests that are less mobile and that feedon specific plants in limited areas are theeasiest to control. Employing rotations in thebug battle boils down to making it as hard aspossible for insects to find the host plants theylove.Farmers try to disrupt insect growth andbreeding schedules by including nonsusceptiblecrops in the rotation. Whenfarmers expect problems from particularinsects, they may want to physically separatesusceptible species with non-susceptiblespecies and thus make it harder for insects tocross over to crops they favor.Another tactic is to include cash or covercrops in the rotation that will attract thebeneficial insects that help keep pestpopulations in check. Researchers in Georgiareported high densities of big-eyed bugs,ladybugs, and other beneficial insects invetches and clovers planted as cover crops(Bugg and Waddington, 1994). There isanecdotal evidence that beneficial insectshave destroyed Colorado potato beetlesfeeding on eggplant planted into strip-tilledcrimson clover.Organic Production—<strong>Crop</strong> <strong>Rotations</strong> on Organic Farms 13

RECOMMENDED READINGSources CitedAverre, C.W., and J.B. Ristaino. 1991.Streptomyces soil rot (pox) of sweetpotato.Vegetable Disease Information Note 3.North Carolina Cooperative ExtensionService. NC State University, Raleigh.Bugg, R.L., and C. Waddington. 1994. Usingcover crops to manage arthropod pests oforchards. Agriculture, Ecosystems andEnvironment. 50:11-28.Clark, M.S., W.R. Horwath, C. Shennan, andK.M. Scow. 1998. Changes in soilchemical properties resulting fromorganic and low-input farming practices.Agronomy Journal. 90:662-671.Flint, M.L. and P.A. Roberts. 1988. Using cropdiversity to manage pest problems: SomeCalifornia examples. American Journal ofAlternative Agriculture. 3:163-167.Karlen, D.L., G.E. Varvel, D.G. Bullock, andR.M. Cruse. 1994. <strong>Crop</strong> rotations for the21st century. Advances in Agronomy.53:1-45.Melton, T.A., and H.D. Shew. 1998. GranvilleWilt. Tobacco Disease Information Note2. North Carolina Cooperative ExtensionService. NC State University, Raleigh.Mitchell, C.C., and J.A. Entry. 1998. Soil C, Nand crop yields in Alabama's long-term“Old Rotation” cotton experiment. SoilTillage Research. 47:331-338.Perucci, P. U., Bonciarelli, R. Santilocchi, andA.A. Bianchi. 1997. Effect of rotation,nitrogen fertilization, and management ofcrop residues on some chemical,microbiological, and biochemicalproperties of soil. Biology and Fertility ofSoils. 24:311-316.Reganold, J.P. 1988. Comparison of soilproperties as influenced by organic andconventional farming systems. AmericanJournal of Alternative Agriculture. 3:144-145.Sarrantonio, M. 1992. Opportunities andchallenges for the inclusion of soilimprovingcrops in vegetable productionsystems. HortScience. 27754-758.Yepsen, Roger B. Jr. (ed.) 1984. TheEncyclopedia of Natural Insect & DiseaseControl (pp. 267-271). Rev. ed. RodalePress, Emmaus, PA.Additional ReadingBullock, D.G. 1992. <strong>Crop</strong> rotation. CriticalReviews in Plant Sciences. 11:309-326.Creamer, N.G., M.A. Bennet, and B.R. Stinner.1996. Mechanisms of weed suppression incover crop-based production systems.HortScience. 31:410-413.Dick, W.A. 1992. A review: Long-term effectsof agricultural systems on soilbiochemical and microbial parameters.Agriculture, Ecosystems andEnvironment. 40:25-36.Francis, C.C., and M.D. Clegg. 1990. <strong>Crop</strong>rotations in sustainable productionsystems. In C.A. Edwards, R. Lal, P.Madden, R.H. Miller, and G. House (Eds.)Sustainable Agricultural Systems (pp. 107-122). Soil and Water ConservationSociety, Ankeny, IA.Fraser, D.G., J.W. Doran, W.W. Sahs, andG.W. Lesoing. 1988. Soil microbialpopulations and activities underconventional and organic management.Journal of Environmental-Quality. 17:585-590.Gerhardt, R.A. 1997. A comparative analysisof the effects of organic and conventionalfarming systems on soil structure.Biological Agriculture and Horticulture.14:139-157.Helenius, J. 1997. Spatial scales in ecologicalpest management (EPM): Importance ofregional crop rotations. BiologicalAgriculture and Horticulture. 15:163-170.Janzen, H.H., C.A. Campbell, and S.A. Brandt.1992. Light fraction organic matter insoils from long-term crop rotations. SoilScience Society of America Journal.56:1799-1806.Organic Production—<strong>Crop</strong> <strong>Rotations</strong> on Organic Farms 14

Johnston, S.A., and P.J. Nitzche. No date.Rotation periods suggested to help controlvegetable diseases. New Jersey ExtensionService.Karlen, D.L., E.C. Berry, T.S. Colvin, and R.S.Kanwar. 1991.Twelve-year tillage and croprotation effects on yields and soilchemical properties in northeast Iowa.Communications in Soil Science andPlant Analysis. 22:1985-2003.Liebman, M., and E. Dyck. 1993. <strong>Crop</strong>rotation and intercropping strategies forweed management. EcologicalApplications. 3: 92-122.Lopez-Bellido, L., F.J. Lopez-Garrido, M.Fuentes, J.E. Castillo, and E.J. Fernandez.1997. Influence of tillage, crop rotationand nitrogen fertilization on soil organicmatter and nitrogen under rain-fedMediterranean conditions. Soil & TillageResearch. 43:277-293.Prince, A.L., S.J. Toth, A.W. Blair, and F.E.Bear. 1941. Forty-year studies of nitrogenfertilizers. Soil Science. 52:247.Robertson, F.A., and W.C. Morgan. 1996.Effects of management history andlegume green manure on soilmicroorganisms under organic vegetableproduction. Australian Journal of SoilResources. 34:427-220.Wander, M.M., S.J. Traina, B.R. Stinner, andS.E. Peters. 1994. Organic andconventional management effects onbiologically active soil organic matterpools. Soil Science Society of AmericaJournal. 58:1130-1139.Wyland L.J., L.E. Jackson, and W.E. Chaney.1996. Winter cover crops in a vegetablecropping system: Impacts on nitrateleaching, soil water, crop yield, pests andmanagement costs. Agriculture,Ecosystems and Environment. 59:1-17.Organic Production—<strong>Crop</strong> <strong>Rotations</strong> on Organic Farms 15

The Organic Production publication series was developedby the Center for Environmental Farming Systems,a cooperative effort betweenNorth Carolina State University,North Carolina A&T State University, and theNorth Carolina Department of Agriculture and Consumer Services.The USDA Southern Region Sustainable Agriculture Research and Education Programand the USDA Initiative for Future Agriculture and Food Systems Programprovided funding in support of the Organic Production publication series.David Zodrow and Karen Ven Epen of ATTRAcontributed to the technical writing, editing, and formatting of these publications.Prepared byKeith R. BaldwinProgram Leader, ANR/CRDExtension Specialist—HorticultureNorth Carolina A&T State UniversityPublished byNORTH CAROLINA COOPERATIVE EXTENSION SERVICEAG-659W-0506/2006—BSE06-45788Distributed in furtherance of the acts of Congress of May 8 and June 30, 1914. North Carolina State University and North CarolinaA&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, nationalorigin, religion, sex, age, or disability. In addition, the two Universities welcome all persons without regard to sexual orientation.North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governmentscooperating.