University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln

U.S. Department of Agriculture: Agricultural Publications from USDA-ARS / UNL Faculty Research Service, Lincoln, Nebraska

2011

The Use of Cover Crops to Manage Soil

T. C. Kaspar USDA-ARS, [email protected]

J. W. Singer USDA-ARS, [email protected]

Follow this and additional works at: https://digitalcommons.unl.edu/usdaarsfacpub

Kaspar, T. C. and Singer, J. W., "The Use of Cover Crops to Manage Soil" (2011). Publications from USDA- ARS / UNL Faculty. 1382. https://digitalcommons.unl.edu/usdaarsfacpub/1382

This Article is brought to you for free and open access by the U.S. Department of Agriculture: Agricultural Research Service, Lincoln, Nebraska at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Publications from USDA-ARS / UNL Faculty by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 21

The Use of Cover Crops to Manage Soil T.C. Kaspar and J.W. Singer

over crops are used to manage soils for many diff erent reasons and are known by many Cdiff erent names. Cover crops are literally “crops that cover the soil” and one of their fi rst uses was to reduce soil erosion during fallow periods in annual cropping systems. Cover crops are also known as “green manures,” “catch crops,” or “living mulch.” Green manure cover crops are usually legumes that fi x N and are grown to provide N to the following cash crop. Catch crops are cover crops that are grown during fallow periods in cropping systems to take up nutrients, especially N, that would be lost if plants are not present. Lastly, living mulches are cover crops that are grown both during and aft er the cash crop growing season and are suppressed or managed to reduce their competition with the cash crop when it is growing. Aft er the cash crop has matured and before it begins growing again, the living mulch is allowed to grow unhindered. One way to manage living mulches is to restrict them to the “fallow” spaces between crop rows. Orchards or vineyards are sometimes managed with liv- ing mulches, but it is also possible to incorporate living mulches into annual cropping systems. Thus, as can be seen from their many names and descriptions, cover crops can fulfi ll many soil management functions. In terms of soil management, the basic premise for using cover crops is to reduce fallow periods and spaces in cropping systems. Natural ecosystems typically have some plants growing, covering the soil, transpiring water, taking up nutrients, fi xing carbon, and sup- porting soil fauna for most of the time that the ground is not frozen. Agricultural cropping systems producing grain, oilseed, and fi ber crops in temperate regions typically only have living plants for four to six months of the year and are fallow for the remaining six to eight months. Current planting and tilling practices oft en leave soil bare and exposed during fall, winter, and early spring. Some perennial cropping systems for nut or fruit crops (e.g., almonds and grapes) keep the spaces between rows fallow and tilled for extended periods. As a result of these fallow periods and fallow spaces in annual and perennial cropping sys- tems, soil is left unprotected from erosive forces, nutrients and organic matt er are lost or not replenished, runoff increases, soil fauna are stressed, and soil productivity diminishes. Thus, inserting cover crops into fallow periods or fallow spaces in cropping systems can accom- plish multiple soil management goals. This discussion is not intended to be a comprehensive review and will focus on the general principles and evidence for using cover crops to man- age soil erosion, runoff , soil nutrients, soil physical properties, soil water, soil organic carbon, soil chemical properties, and soil biology.

T.C. Kaspar ([email protected]) and J.W. Singer ([email protected]), USDA-ARS, Na- tional Laboratory for Agriculture and the Environment, 2110 University Boulevard, Ames, IA 50011. doi:10.2136/2011.soilmanagement.c21 Copyright © 2011. American Society of Agronomy and Soil Science Society of America, 5585 Guilford Road, Madison, WI 53711, USA. Soil Management: Building a Stable Base for Agriculture. Jerry L. Hatfi eld and Thomas J. Sauer (ed.)

321 was much greater in tilled systems than in Erosion and Runoff no-till systems and much greater in no-till Reducing water erosion is one of the main following a soybean crop than following reasons for growing cover crops (Langdale a soybean–wheat double crop. Kaspar et et al., 1991). Soils are generally more suscep- al. (2001), however, showed that oat (Avena tible to erosion when they are not covered sativa L.) or rye (Secale cereale L.) cover crops with the canopies of living plants or their in no-till soybean reduced interrill erosion plant residues. Annual crop plants, such in two of three rainfall simulator trials even as corn (Zea mays L.) and soybean [Glycine though residue cover did not increase sig- max (L.) Merr.], only provide signifi cant can- nifi cantly with cover crops and was already opy cover for four months or less each year. greater than 75% without cover crops. They Additionally, crops such as soybean, cott on hypothesized that the decreases in inter- (Gossypium hirsutum L.), or corn harvested rill erosion with cover crops was caused for silage oft en do not leave enough residues by reduced interrill transport of sediments. to fully protect the soil between harvest They observed that the anchoring of cover and development of the next crop canopy. crop plants or residues to the soil by roots Cover crops and their residues reduce ero- resulted in the formation of microdams, sion through the same mechanisms as the which probably resulted in sediment depo- cash crops. However, when cover crops are sition. Latt anzi et al. (1974) also observed grown in the fallow intervals between cash that increasing amounts of surface residues crops they extend the time the soil is cov- reduced interrill erosion by intercepting ered with living plants and also supplement splash transport of sediment, slowing inter- and anchor residues left by annual crops. rill fl ow velocity, and increasing water fi lm The impact of cover crops on erosion pro- depth behind residue microdams. cesses depends on how much they reduce Whereas interrill erosion is largely the forces of soil detachment and transport. dependent on raindrop impact to detach Cover crops reduce interrill erosion pri- soil particles, rill erosion relies on the shear marily because they increase the amount force of water fl owing in concentrated fl ow and duration of soil cover either with liv- paths to both detach and transport soil ing plants or plant residues. Soil cover is particles (Flanagan, 2002). Cover crops can the principal characteristic of cropping reduce rill erosion by reducing the shear systems that aff ects the amount of interrill force of fl owing water or by increasing the erosion. Because interrill erosion results resistance of soil particles to detachment. from the detachment of soil particles by One way cover crops reduce the shear force raindrop impact, living or dead plant mate- of runoff water is by reducing its volume rial that intercepts raindrops and dissipates through increased infi ltration. This occurs impact energy will reduce interrill erosion. because cover crops prevent surface seal- Ram et al. (1960) observed that soil detach- ing, increase storage capacity, and improve ment from raindrop impact was reduced as soil structure (Dabney, 1998). Additionally, cover crop canopy increased either because cover crops or surface residues (Brown and of plant density or plant growth. Later, Laf- Norton, 1994) can slow fl ow velocity at the len et al. (1985) showed that the relationship surface by increasing hydraulic resistance. between surface cover and erosion reduc- Lastly, because cover crop plants or resi- tion is exponential with smaller decreases dues are anchored to the surface by roots in erosion as surface cover approaches 100%. and because they hold other unanchored This explains why the relative benefi t of surface residues in place (Kaspar et al., 2001), incorporating cover crops into a cropping fl owing water cannot easily move residues system depends to some extent on the quan- and expose the soil surface to shear forces of tity, duration, and distribution of residues water (Foster et al., 1982). and plant canopies that are normally pres- Cover crops also reduce both rill and ent in the cropping system throughout the interrill erosion by increasing soil resis- year (Mutchler and McDowell, 1990). For tance to detachment. Cover crops are example, Mutchler and McDowell (1990) known to increase soil organic matt er near found that the reduction of erosion by a the soil surface (Wander et al., 1994), which hairy vetch (Vicia villosa Roth)–wheat (Triti- in turn should result in larger, more sta- cum aestivum L.) winter cover crop mixture ble aggregates that are less susceptible

322 The Use of Cover Crops to Manage Soil | T.C. KasparChapter and J.W. | Authors Singer Soil Management Practices to detachment (Dabney, 1998). Addition- 90% compared with no-till without cover ally, cover crop roots can physically bind crops. Oat cover crops, which winter-kill in aggregates together, which makes them Iowa, reduced interrill and rill erosion by even more resistant to detachment by fl ow- 26% and 65%, respectively. Neither rye nor ing water or raindrop impact. Mamo and oat cover crops signifi cantly increased sur- Bubenzer (2001) confi rmed that plant roots face cover, which was already greater than substantially reduced soil detachment and 75% for the no-till soybean without cover rill erodibility. crops. Because of the high residue cover, the A number of fi eld studies have measured quantity of interrill erosion was relatively signifi cant erosion reductions with cover low even for no-till without cover crops. crops. In Missouri, Zhu et al. (1989) com- Additionally, the rye cover crop increased pared soil erosion of no-till soybean with infi ltration in only 1 of the 3 yr. In spite of winter cover crops with erosion of no-till this, there were substantial reductions in rill soybean without cover crops under natural erosion. Kaspar et al. (2001) observed that rainfall. Annual soil loss was decreased 87%, the cover crops prevented soybean residues 95%, and 96% by chickweed (Stellaria media from moving or dislodging with surface L.), Canada bluegrass (Poa compressa L.), and water fl ow. As a result, the soil surface was downy brome (Bromus tectorum L.) winter not exposed to the shear force of fl owing cover crops, respectively, compared with water and this was partly responsible for the no cover crops. Apparently, the no-till soy- cover crops’ success in reducing rill erosion. bean crop at this location did not produce enough residues to adequately protect the soil. In another Missouri study (Wendt and Burwell, 1985), a winter rye or winter wheat Phosphorus Losses cover crop reduced the annual soil loss of in Runoff no-till corn grown for silage from 22 Mg ha−1 Losses of P from agricultural systems to to 0.9 Mg ha−1 and annual runoff from 245 surface waters are largely dependent on to 122 mm. In this same study, however, no- the amount of surface runoff and sediment till corn without silage removal had less soil transport that occurs. Phosphorus is trans- erosion than no-till corn with silage removal ported in runoff as soluble P and particulate and a cover crop, but both of these treat- P (Sharpley and Smith, 1991). Particulate P ments had less erosion than corn grown consists of P bound to soil sediment and P with either moldboard plowing or fi eld contained in organic matt er. Sharpley and cultivating. In Mississippi, Mutchler and Smith (1991) summarized research on the McDowell (1990) found that a wheat or hairy eff ect of cover crops on P losses and found vetch cover crop following cott on reduced that reductions in total P losses, which con- annual soil loss of conventional-tilled cott on sist mostly of particulate P, ranged from 54% from 74.2 Mg ha−1 to 20.4 Mg ha−1 and that to 94% (Table 21|1). This is not surprising con- of no-till cott on from 19.2 Mg ha−1 to 2.3 Mg sidering that cover crops reduce runoff and ha−1 when cott on followed soybean. Lastly, a sediment detachment and transport. They 3-yr rainfall simulator study in Iowa (Kas- also pointed out, however, that the eff ects of par et al., 2001) showed that winter rye cover cover crops on soluble P in runoff were more crops overseeded into no-till soybean in late variable (Table 21|1). Including cover crops summer reduced interrill erosion the fol- in a cropping system sometimes resulted in lowing spring by 54% and rill erosion by higher concentrations of soluble P in runoff

Table 21|1. Literature summary of percent reduction (–) or increase (+) in total P, soluble P concentration, or soluble P in runoff due to winter cover crops (adapted from Sharpley and Smith, 1991). Change in soluble Change in total P Change in soluble Reference Location Cover crop P concentration losses in runoff P in runoff in runoff Angle et al. (1984) Maryland Barley −92% +460% −13% Langdale et al. (1985) Georgia Rye −66% +54% +8% Pesant et al. (1987) Quebec Alfalfa/timothy −94% −60% −12% Yoo et al. (1988) Alabama Wheat −54% 0% −50%

323 and did not always reduce the cumulative (Raphanus sativus L.) cover crops. Annual amount of soluble P in runoff compared ryegrass maintained greater wet aggre- with no cover crops. Several studies have gate stability than the other cover crops shown that soluble P can be lost in runoff at samplings from May through Septem- fl owing over plant residues (Timmons et al., ber presumably because of more persistent 1970; Bechmann et al., 2005). However, on aggregate binding that led to less aggregate an annual basis, plant water use and infi l- breakdown (Dapaah and Vyn, 1998). Tisdall tration would be expected to increase with and Oades (1979) reported that perennial cover crops, which should reduce the vol- ryegrass (Lolium perenne L.) was more effi - ume of runoff . This would off set to some cient than white clover (Trifolium repens L.) in extent the higher soluble P concentrations of stabilizing aggregates in a loam soil because runoff in the presence of cover crops. perennial ryegrass supported a larger popu- lation of vesicular-arbuscular mycorrhizal hyphae and had greater hyphal and root length. Results from their electron micro- Soil Physical Properties graphs revealed hyphae covered with an Soil structure in simple terms is the physical amorphous material, which they conjectured relationship between the solid, liquid, and was a polysaccharide capable of binding clay gaseous phases of soil. The arrangement of particles. These studies demonstrate that soil particles into peds or aggregates deter- cover crop impacts on soil aggregation vary mines the size and shape of soil voids or with cover crop species, quantity of roots, pores and this greatly infl uences the move- soil type, and cropping systems. ment of water and gases in soil. As a result, Patrick et al. (1957) reported that aft er 25 soil structure can have a substantial impact yr of continuous cott on with tillage on a loam on plant growth. Conversely, cover crops, soil, 21.3% of the aggregates had diameters like any plants, can alter soil physical and >0.21 mm when a hairy vetch cover crop was structural properties directly through for- included in the cropping system compared mation of pores and aggregates by roots or with the 11.8% for a common vetch (Vicia indirectly through the input and decompo- sativa L.) cover crop treatment and 9.5% for sition of shoot and root residues. Obviously, a control without a cover crop. The authors a cover crop’s impact on soil structure will concluded that hairy vetch improved aggre- vary depending on climate, soil type, soil gation be tt er than common vetch because texture, soil depth, tillage, cropping system, hairy vetch produced more biomass than cover crop biomass, cover crop species, and common vetch. The hairy vetch treatment cover crop frequency in the crop rotation. For also had a lower bulk density, greater poros- example, Ball-Coelho et al. (2000) reported ity, and greater water holding capacity than that in a 3-yr corn–soybean–winter wheat the no cover control in the surface 0.06 m rotation, cover crops aft er winter wheat and of soil. Benoit et al. (1962) reported greater corn had a small eff ect on stability of micro- aggregation and hydraulic conductivity at aggregates and no eff ect on dry-aggregate the surface of a sandy loam soil aft er 3 yr of size distribution or wet-aggregate stabil- using a rye cover crop in a sweet corn and ity of macroaggregates in the upper 0.075 green bean (Phaseolus vulgaris L.) rotation m of a sandy soil with conventional, chisel, with spring tillage. In one of their treat- and no-till tillage systems. They concluded ments they removed all the cover crop shoot that because microaggregates were more material before spring tillage, demonstrat- stable than macroaggregates with cover ing that much of the eff ect of the rye cover cropping, the binding mechanisms prob- crop on soil structure was due mostly to the ably involved humic materials or microbial cover crop roots. To further investigate the products, which likely were not as impor- eff ect of cover crop roots they made mea- tant for macroaggregate stability (Degens surements below the plow layer (0.30–0.37 et al., 1996). Alternately, Dapaah and Vyn m) in the sixth year of the study and showed (1998) reported an increase in wet aggregate that cover crop roots decreased bulk den- stability in the upper 0.07 m of a sandy loam sity and increased capillary porosity and and loam soils aft er only one cycle of annual hydraulic conductivity relative to the no ryegrass (Lolium multifl orum Lam.), red clo- cover control. Williams and Weil (2004) also ver (Trifolium pratense L.), and oilseed radish presented evidence that cover crop roots can

324 The Use of Cover Crops to Manage Soil | T.C. KasparChapter and J.W. | Authors Singer Soil Management Practices improve subsequent cash crop root growth and reduced corn grain yield on the Coastal in silt loam soils with compacted plow- Plain in South Carolina. Similarly, Ewing et pans by increasing macroporosity in the al. (1991) reported lower soil water contents compacted soil layers. In their study, they in the upper 0.15 m and lower corn yields fol- used the minirhizotron camera technique lowing a crimson clover (Trifolium incarnatum to observe soybean roots growing through L.) cover crop in North Carolina on a soil with the plowpans in root channels created by a root-restricting soil layer. In both of these decomposing cover crop roots of forage rad- studies, the negative impact of water use by ish (Raphanus sativus L.). the cover crop was exaggerated by periods of litt le or no precipitation and the limited water holding capacity of the rooted soil volume. In contrast to these studies, Moschler et al. (1967) Soil Water Status in Virginia observed greater corn yields and Cover crops decrease soil water content soil water contents when rye cover crop resi- through uptake and transpiration while dues were left on the soil surface compared living and can increase soil water content with no cover crop, removal of rye residues, through increased surface residue cover or burying rye residues with tillage. Lastly, and infi ltration aft er termination (Wagger Clark et al. (1997) in Maryland found that and Mengel, 1988; Unger and Vigil, 1998; rye, hairy vetch, and rye–hairy vetch mix- Qi and Helmers, 2010). Thus, the relative ture cover crops did not signifi cantly deplete impact of cover crops on soil water avail- soil water contents in the upper 0.20 m of the able to the following crop depends on cover soil. Aft er cover crop termination, residues crop management, timing and amount of left on the soil surface conserved soil water precipitation, and total water holding capac- later in the growing season and contributed ity of the root-accessible portion of the soil to corn yield increases. They also showed profi le (Frye et al., 1988). Cover crop tran- that the greater residue cover produced by spiration and soil drying may be benefi cial cover crops terminated closer to the time on heavy soils in wet springs because it may of corn planting conserved more soil water allow for earlier planting of the cash crop during corn growth than cover crops termi- (Wagger and Mengel, 1988). In fi elds with nated earlier. Obviously, the impact of cover subsurface drainage systems, cover crops crops on soil water content is complicated. can reduce drainage volume during the From a soil water management perspective, spring before cash crops are planted (Qi and the decision of when to terminate a cover Helmers, 2010), which can reduce losses of crop depends in part on availability of irri- nitrate (Kanwar et al., 2005). Alternately, in gation, probability of rainfall to replace the dry years on coarse-textured soils with low water used by the cover crop, and the need water holding capacity or shallow rooting for cover crop residues to reduce evaporation depth, water use by cover crops may reduce from the soil surface. Additionally, unless soil water available at planting and may irrigation is available, cover crops may not ultimately reduce yields if rainfall does not be suitable for cropping systems in semiarid replenish the water used by the cover crop regions where annual precipitation some- (Campbell et al., 1984). Once a cover crop times does not replace the water used by the has been terminated, however, cover crop previous cash crop or where the probability residues left on the soil surface can increase for replenishing water used by the cover crop surface residue cover, reduce evaporation, is low (Unger and Vigil, 1998). Obviously, and increase soil water contents (Wagger farmers will need location-based guidelines and Mengel, 1988). or decision-aide tools to assist them in man- Because cover crops increase transpiration aging soil water with cover crops. when living and decrease evaporation when dead, there have been confl icting reports on the eff ect of cover crops on available soil water and cash crop yields. Campbell et al. Nitrogen (1984) observed that a rye cover crop termi- Cover crops can be utilized to manage N in nated with herbicides aft er corn planting agricultural soils by altering N cycling and substantially reduced the soil water con- availability. Cover crops grown during fal- tent of the upper 0.60 m of the soil profi le low periods in cropping systems change

325 the annual patt erns of N uptake and min- living plants can dramatically reduce leach- eralization, reduce downward movement ing losses of NO3 in two ways: (i) by taking of NO3, retrieve NO3 from deep soil layers, it up, which reduces its concentration in the and fi x atmospheric N2, if the cover crops soil solution, and (ii) by taking up water, are legumes. Ultimately, successful manage- which reduces the amount of water mov- ment of N using cover crops requires that N ing through the soil profi le. Large leaching availability be synchronized so that inor- losses of NO3 occur in many cropping sys- ganic N is readily available during periods tems, in part because there are extended of active uptake by cash crops and mini- fallow periods during each year when liv- mally available during periods when cash ing plants are not removing NO3 and water crops are not growing to reduce losses of N from the soil, usually from cash crop matu- to air and water. Various aspects of the rela- rity in fall until crop canopy development tionship between cover crops and N have the following spring. Cover crops reduce been discussed in a number of previous annual leaching losses of NO3 because they reviews (Meisinger et al., 1991; Wagger et al., extend the period of active N and water 1998; Dabney et al., 2001; Thorup-Kristensen uptake to periods of the year when the cash et al., 2003). This discussion will examine crops are normally not present. For example, how cover crops reduce losses of N from soil reductions in NO3 leaching losses observed and aff ect N availability to cash crops. with winter cover crops range from 6 to 94%

Soil N, in the form of nitrate (NO3), is (Table 21|2). soluble in water and can be lost from agricul- The wide range in cover crop NO3 tural cropping systems with the downward leaching reductions reported resulted movement of water through the soil pro- from diff erences in cover crop species, the fi le. In many agricultural fi elds, percolating amount of cover crop growth, the amount water and NO3 are intercepted by agri- of N in the soil due to either fertilization cultural drainage systems, which rapidly or mineralization, and the amount of water transport water and NO3 off site to surface moving through the soil. For example, non- waters. In fi elds without drainage systems, legume cover crops usually reduce NO3 percolating water and NO3 can eventually leaching losses more than legumes (Tonitt o reach groundwater by continuing down- et al., 2006). McCracken et al. (1994) in ward or reach surface waters by following Kentucky observed that a rye cover crop subsurface fl ow pathways. The presence of reduced leaching losses by 94% whereas

Table 21|2. Literature summary of percent reduction in nitrate N leaching losses due to win- ter cover crops (adapted in part from Meisinger et al., 1991). Reduction in Reference Location Cover crop N leaching Jones, 1942 Alabama Oats 81% Jones, 1942 Alabama Hairy vetch 6% Chapman et al. 1949 California Mustard 80% Chapman et al. 1949 California Purple vetch 30% Martinez and Guirard, 1990 France Ryegrass 63% Staver and Brinsfi eld, 1990 Maryland Rye 77% Staver and Brinsfi eld, 1998 Maryland Rye 80% McCracken et al., 1994 Kentucky Rye 94% McCracken et al., 1994 Kentucky Hairy vetch 48% Wyland et al., 1996 California Rye 65–70% Brandi-Dohrn et al., 1997 Oregon Rye 32–42% Ritter et al., 1998 Delaware Rye 30% Rasse et al., 2000 Michigan Rye 28–68% Strock et al., 2004 Minnesota Rye 13% Kladivko et al., 2004 Indiana Winter wheat + less fertilizer 61% Kaspar et al., 2007 Iowa Rye 61%

326 The Use of Cover Crops to Manage Soil | T.C. KasparChapter and J.W. | Authors Singer Soil Management Practices a hairy vetch cover crop reduced leaching by 48%. In other studies, combi- nations of factors have aff ected the reductions in

NO3 leaching. In a Min- nesota study (Strock et al., 2004), which showed a 13% average reduction in NO3 leaching, the rye cover crop was planted only fol- lowing the corn phase in a corn–soybean rotation, produced on average 1.4 Mg ha−1 of shoot dry mat- ter, and in 1 of the 3 yr there was almost no drain- age or NO3 loss. In years of low winter precipitation, less water moves down- ward through soil and more NO3 remains in the soil profi le even without a Fig. 21|1. Spring soil nitrate N content versus winter pre- cover crop (Fig. 21|1; Tho- cipitation with and without cover crops from various rup-Kristensen et al., 2003). experiments in Denmark (adapted from Thorup-Kristensen In contrast to the Minne- et. al., 2003). sota study, Kaspar et al. (2007) in Iowa reported that over 4 yr a winter rye cover crop following both phases of a reduce soil NO3 concentrations, thus reduc- corn–soybean rotation reduced NO3 loads ing its conversion by soil microorganisms to in tile drainage water by an average of 61% N2O, NO, or N2 during denitrifi cation (Smith and produced an average shoot biomass and Tiedje, 1979; Haider et al., 1987). Nitrous of 1.7 Mg ha−1. In the Iowa study, cumula- oxide and NO can also be lost during the tive annual drainage was always greater aerobic nitrifi cation process (Davidson et than 138 mm and signifi cant NO3 losses al., 2000). Because plants can also take up occurred every year. Other studies have NH4, they might also be expected to reduce shown that combining a winter cover crop gaseous losses by this aerobic process, but with other N management practices can this has not been confi rmed. In a controlled also eff ectively reduce NO3 losses. A study environment study, in which swine manure in Indiana (Kladivko et al., 2004) reduced was injected into soil with or without a

NO3 loads by 61% with a reduction in fer- growing rye cover crop, Parkin et al. (2006) tilizer N rates and a winter wheat cover showed that N2O emissions were signifi - crop following corn. Thus, when a winter cantly lower with a rye cover crop present. cover crop produces moderate growth and Thus, cover crops, like any living plants, substantial water percolation occurs, cover have the potential to reduce gaseous losses crops can substantially reduce NO3 losses of N through microbial reactions by reduc- to drainage water or deep percolation in ing soil N availability (Davidson et al., 2000). annual cropping systems. Aft er cover crops are dead, however, their Living cover crops also have the poten- residues can lose N through emissions of tial to reduce gaseous losses of N as N2, NO, NH3, N2O, NO, or N2. Quemada and Cabrera and N2O, from soil (Davidson et al., 2000). (1997) applied crimson clover residues to the

Of these, N2O is also important as a green- surface of soil cores and measured maxi- house gas. A number of studies have shown mum losses of NH3 and N2O of 6.0 and 2.6% that plant roots can eff ectively compete with of total residue N, respectively under very soil microorganisms for available NO3 and wet and warm conditions. Emissions of N2O,

327 NO, or N2 resulting from denitrifi cation or 21|1). When winter precipitation was less nitrifi cation of N mineralized from cover than 400 mm, soil nitrate contents were crop residues would be controlled by the much higher without a cover crop. Cover same factors that control gaseous losses of crop species also infl uences the amount of fertilizer N: NO3 or NH4 availability, carbon soil N taken up by the cover crop. In general, availability, and the aeration status (David- nonlegume cover crops take up more soil N son et al., 2000; Rosecrance et al., 2000). For than legume cover crops. Ranells and Wag- cover crop shoot residues, anaerobic con- ger (1997a) observed that a rye cover crop ditions would be more likely to occur if recovered more 15N (39%) than a crimson residues are incorporated with tillage rather clover cover crop (4%). Shipley et al. (1992) than left on the surface. Incorporation of reported that cereal rye and annual rye- legume cover crop residues with a low C to grass took up 45 and 27%, respectively, of N ratio likely would increase net N miner- fall-applied 15N, whereas hairy vetch and alization, increase availability of both NH4 crimson clover only recovered 8%. and NO3, and increase gaseous losses of Cover crops can also have a positive N. The relative importance of denitrifi ca- impact on N availability by increasing the tion or nitrifi cation to the gaseous losses input of N into cropping systems through would depend on the aeration status of N2 fi xation by leguminous cover crops. the soil (Davidson et al., 2000). Alternately, Legumes will also take up N from the incorporation of grass cover crop shoot resi- soil, but their growth is not limited by low dues with a high C to N ratio likely would soil N levels as are nonleguminous cover decrease net N mineralization, decrease crops. Unfortunately, legumes generally availability of both NH4 and NO3, and do not grow rapidly in cool fall or winter decrease gaseous losses of N. For example, weather and grow much bett er aft er the Rosecrance et al. (2000) observed that incor- weather warms in the spring. Power and porated hairy vetch cover crop residues Zachariassen (1993) reported that there resulted in greater net N mineralization were considerable diff erences among eight and greater N2O losses than incorporated legumes in N2 fi xation at soil temperatures rye residues. On the other hand, soil incor- of 10, 20, and 30°C and that for half of the ° poration of cover crop residues probably legumes N2 fi xation at 10 C was much lower would make NH3 emissions from residues than that at the other temperatures. There is less likely because NH3 is soluble in water also considerable variation in legume cover and NH4 binds with soil particles. crop growth and N2 fi xation among loca- Cover crops aff ect N availability to cash tions and cropping systems depending on crops through uptake of inorganic N, fi xa- climate, soils, planting date, and termina- tion of N2 by leguminous cover crops, and tion date. For example, Decker et al. (1994) decomposition of cover crop residues. Inter- evaluated three legume winter cover crops seeded cover crops or living mulches can over seven environments in Maryland and directly reduce N availability for cash crops on average the shoots of hairy vetch, Aus- because they are growing and taking up trian winter pea (Pisum sativum L.), and N at the same time as the cash crop. Win- crimson clover contained 152, 138, and 121 ter or off -season cover crops can also reduce kg N ha−1, respectively. In this same study, a soil N availability to the cash crop, if the winter wheat cover crop accumulated 39 kg N taken up by the cover crops still would N ha−1, so we can probably assume that accu- have been present in the soil when the mulation of N greater than 39 kg N ha−1 was cash crop needed N. Nitrogen taken up by probably the result of N2 fi xation. Hively winter cover crops would not aff ect N avail- and Cox (2001) in the colder environment of ability for the cash crops, if the N taken up central New York, however, observed that would have been lost through leaching or four legume cover crop species produced gaseous emissions or if it were replaced by only 2 to 35 kg N ha−1 over 2 yr. Whereas, mineralization or fertilization. For exam- Ebelhar et al. (1984) in Kentucky reported a ple, Thorup-Kristensen et al. (2003) used 2-yr average of 209 kg N ha−1 in shoot bio- data from a number of experiments to show mass of a hairy vetch cover crop compared −1 that NO3–N remaining in the soil with or with 36 kg N ha in a rye cover crop. Thus, without cover crops was not diff erent when in the appropriate cropping system and winter precipitation exceeded 400 mm (Fig.

328 The Use of Cover Crops to Manage Soil | T.C. KasparChapter and J.W. | Authors Singer Soil Management Practices climate, legume winter cover crops can fi x eventually eliminated the yield diff erence signifi cant amounts of N. between the rye and the no cover crop treat- In general, when fertilizer N is applied ment, but not between the rye and hairy at planting it is more readily available and vetch treatments. They concluded that fac- a greater proportion is recovered by annual tors other than N availability were involved. crops than N contained in plant residues. Ranells and Wagger (1997b) have suggested For example, Harris et al. (1994) using 15N planting grass and legume cover crop spe- showed that over 2 yr corn and barley crops cies in bicultures or mixtures to reduce the took up 40% and 17% of the fertilizer and C to N ratio of the residues and increase red clover 15N, respectively. However, 47% the rate of residue decomposition and N of the red clover 15N remained in the soil, release. Another approach to managing the whereas only 17% of the fertilizer 15N was C to N ratio of cover crops is to terminate found in the soil. Availability to the cash the cover crop earlier in its life cycle. Wag- crop of N taken up or fi xed by a cover crop ger (1989a) found that terminating a rye, depends on the decomposition rate of cover hairy vetch, or crimson clover cover crop crop residues. Residue decomposition is 14 d earlier resulted in lower C to N ratios, aff ected by many soil, plant, and environ- lower concentrations of cellulose, hemicellu- mental factors (Parr and Papendick, 1978). lose, and lignin, faster decomposition, and One of the main factors aff ecting decompo- more released N. In addition to changing sition of cover crop residues and release of the composition of cover crop residues, ter- N is the C to N ratio of the residues. Resi- minating a cover crop signifi cantly before dues from grass or cereal cover crops have planting the cash crop allows more time for relatively high C to N ratios, decompose decomposition of cover crop residues and slowly, and in some cases immobilize NO3 mineralization of residue N. Optimal syn- already present in the soil (Wagger, 1989b; chronization of availability of mineralized Rosecrance et al., 2000). Legumes have lower residue N with uptake of N by the cash crop, C to N ratios than nonlegumes, decompose however, is diffi cult to achieve. more quickly, and release or mineralize Residue incorporation with tillage is more N (Wagger, 1989a; Ruff o and Bollero, another way to increase the rate of resi- 2003). Using surface-applied 15N-labeled due decomposition and N release. Tillage cover crop residues Ranells and Wagger breaks or cuts cover crop residues into (1997a) showed that averaged over 2 yr corn smaller pieces, covers residues with soil, recovered 14% of crimson clover N com- and improves soil-to-residue contact. This pared with only 4% of a rye cover crop N. The greater N availability of legume cover crops is also refl ected in the N fertilizer response of corn following a rye or hairy vetch winter cover crop (Fig. 21|2; Miguez and Bollero, 2006). Miguez and Bollero (2006) showed that at the 0 kg N ha−1 fertilizer rate, the hairy vetch cover crop had greater corn yield than either the rye or no cover crop treatment indicat- ing greater N availability. In contrast, the rye cover crop treatment had a lower corn yield than the no cover crop treatment at the 0 kg N ha−1 fertilizer rate, indicating that Fig. 21|2. Corn grain yield N fertilizer response curves the rye cover crop probably for corn following no cover crop, a rye cover crop, or a immobilized soil N. In their hairy vetch cover crop in Illinois (adapted from Miguez study, increasing N fertilizer and Bollero, 2006).

329 keeps residues wett er and more accessible cover crops in a corn silage system. Simi- to soil microorganisms. Varco et al. (1989) larly, Sainju and Singh (2008) found that a found that in the fi rst year, corn recov- hairy vetch cover crop increased total soil ered 32% of the N in 15N-labeled hairy N over 3 yr in the 0.0- to 0.3-m soil layer. vetch residues when they were incorpo- In many soils with high background lev- rated with tillage compared with only 20% els of organic matt er and total N, it is when residues were left on the surface in diffi cult to measure changes in total soil N no-till. Power et al. (1991) observed that due to changes in management practices. with no added N fertilizer corn yielded McCracken et al. (1989) was not able to substantially more when a hairy vetch measure a signifi cant increase in total soil cover crop was incorporated with tillage N, but did measure an increase in poten- than when it was terminated with herbi- tial N mineralization and soil inorganic N cide and remained on the surface. Some levels in late May aft er 10 yr of hairy vetch questions remain as to the eventual fate of cover crops. Hansen et al. (2000) observed all of the N in cover crop shoot residues that spring wheat required 27 kg N ha−1 when they are allowed to remain on the less fertilizer to obtain the same yield aft er soil surface in no-till systems, but incorpo- a perennial ryegrass winter cover crop was ration with tillage to increase availability discontinued aft er 24 yr. When they termi- also would negate some of the erosion nated their study, this indirect evidence of control and water conservation benefi ts of increased N availability had persisted for cover crop residues. 4 yr aft er the cover crop was discontinued. Cover crops also impact long-term soil In some cases, however, improved soil N N availability by increasing total soil N retention by long-term cover crops may not through additions of fi xed N or prevention result in positive changes in soil N status, of N losses. Few studies have looked at the but instead may increase cash crop yield cumulative eff ect of winter cover crops on or grain N content, which would increase total soil N when they have been planted N outputs from the system (Ball Coelho et every year for multiple years. Because only al., 2005). Thus, long-term cover crop use a portion of cover crop residue N is miner- can change the N balance of cropping sys- alized and taken up by the following cash tems by reducing losses, by supplying fi xed crop, the remaining cover crop N may be N, and by increasing total soil N. Eventu- susceptible to loss through leaching or ally, as the system comes to equilibrium, N gaseous emissions when it is mineralized availability to the cash crop may increase. (Harris et al., 1994). However, if cover crops Further research examining N budgets, are planted every year and fallow periods gaseous emissions, and changes in total soil are avoided, it is likely that most of the N of cropping systems with cover crops are cover crop N will continue to be taken up needed to bett er understand the cycling of in later years either by cash crops or cover N in these systems, which should improve crops and recycled in the soil–plant sys- soil productivity and management. tem. Then, if inputs of N exceed outputs, this should gradually increase the total N content and N availability of the soil until a new equilibrium is reached. Garwood et Soil Organic Carbon al. (1999) calculated an estimated N bal- Soil productivity is closely linked to soil ance for cropping systems over 8 yr that organic matt er (SOM) and its primary indicated that the treatments with winter component soil organic carbon (SOC). rye cover crops had accumulated on aver- Sequestration of C in SOM is also an age an additional 160 kg N ha−1 over that important approach for reducing the con- time. They hypothesized that the major- centration of CO2 in the atmosphere (Lal et ity of this N had come from a reduction in al., 1999). Soil organic matt er, which includes

NO3 leaching by the cover crop. Gaseous soil humus and all the plant, animal, and losses of N, however, were not measured in microbial residues in the soil, is generally this study and the change in soil organic assumed to be 50 to 58% C by mass (Nel- matt er was not signifi cant. Kuo and Jellum son and Sommers, 1996). In general, SOC (2000) measured an increase in soil organic increases when inputs of plant residue C to N aft er 8 yr of rye, ryegrass, or hairy vetch the soil are greater than C losses through

330 The Use of Cover Crops to Manage Soil | T.C. KasparChapter and J.W. | Authors Singer Soil Management Practices decomposition, erosion, and leaching (Hug- mentioned earlier, however, soil incorpora- gins and Fuchs, 1997; Paustian et al., 1997). tion or more extensive tillage can increase Because cover crops are normally grown the relative rate of cover crop decomposi- during fallow periods of cropping systems, tion and reduce the retention of cover crop the addition of cover crops to a cropping C in soil. Beale et al. (1955) reported that system can increase total residue C inputs SOM was 28% higher aft er 10 yr of a vetch to soil and has the potential to increase SOC and rye cover crop with mulch tillage com- (Karlen and Cambardella, 1996; Lal et al., pared with cover crops and moldboard 1999; Jarecki and Lal, 2003). Similarly, the plow tillage. In spite of this evidence for rate at which cover crop residues decom- increases in soil C and residue C inputs pose also aff ects the balance between with cover crops, it is oft en diffi cult to mea- losses and inputs of C into soil. Cover crop sure a change in SOC in cropping systems residue decomposition depends primarily to which cover crops have been added. This on temperature, water content, biochemical is partly because it is diffi cult to measure constituents, residue quantity, C to N ratio, small changes in SOC in fi eld soils with and soil contact. Kuo et al. (1997) reported relatively high background SOC levels and that SOC half-lives for rye, hairy vetch, and large variations in SOC with depth and annual ryegrass were similar and aver- terrain (Kaspar et al., 2006). Additionally, aged 31 d and 57 d in 2 yr when residues cover crops may not produce large amounts were buried 15 cm below the soil surface of biomass in some locations or climates in mesh bags, before planting corn. They and cover crop biomass may be a rela- att ributed the slower decomposition in 1 of tively small percentage of the total biomass the 2 yr to wett er soils and lower tempera- produced in some cropping systems like ture. Although hairy vetch had lower shoot continuous corn. For example, Eckert (1991) C to N ratios than ryegrass or rye, this did in Ohio was not able to detect an increase not aff ect the observed decay rate, which in soil C with a rye cover crop in no-till probably indicates that N was not limit- continuous corn or corn–soybean rotations. ing and that other factors such as lignin Duiker and Hartwig (2004) reported simi- concentration of residues or environmen- lar SOC levels in a crownvetch (Coronilla tal conditions limited the decomposition varia L.) living mulch treatment and the rate. In contrast, Ruff o and Bollero (2003) control aft er 13 yr. They concluded that reported that hairy vetch decomposed severe suppression of the crownvetch liv- more rapidly than rye and that the decom- ing mulch to reduce competition with position rate of both responded to water the corn crop had also reduced the soil content and temperature. C inputs and the SOC bene fi ts. Similarly, Cover crops have been used successfully Utomo et al. (1990) observed no change in to increase soil C (Table 21|3) especially in soil C with a rye cover crop in either no-till locations with mild winters that allow sub- or conventional tillage, but measured an stantial cover crop growth (Beale et al., increase with a hairy vetch cover crop in 1955; Patrick et al., 1957; Utomo et al., 1990; no-till, which produced more biomass than Kuo et al., 1997; Nyakatawa et al., 2001; rye in a 0 N treatment. Mendes et al. (1999) Sainju et al., 2002). In some of these stud- found that red clover or triticale (×Triticose- ies, cover crop residues were incorporated cale Witt mack) winter cover crops did not with tillage (Beale et al., 1955; Patrick et al., increase soil C in a tilled vegetable produc- 1957; Kuo et al., 1997; Sainju et al., 2002). As tion system.

Table 21|3. Cover crop increases in soil organic matter (SOM) or soil organic carbon (SOC). Increase in SOM Reference Location Cover crop Depth of sample or SOC† Beale et al. (1955) South Carolina Vetch and rye 0.13 m 31% Patrick et al. (1957) Louisiana Hairy vetch 0.07 m 85% Kuo et al. (1997) Washington Rye 0.30 m 7%† Sainju et al. (2002) Georgia Rye 0.20 m 12%† Villamil et al. (2006) Illinois Rye and hairy vetch 0.30 m 9% † Soil organic carbon.

331 artifi cially altered earthworm populations Soil Chemical Properties and determined that earthworms increased Aside from the impacts of cover crops on C stability of aggregates >1.0 mm diameter fol- and N cycling in soil, litt le is known about lowing a cereal rye–hairy vetch cover crop. the eff ect of cover crops on other nutrients They also reported that earthworms exhib- or soil pH. Nyakatawa et al. (2001) reported ited a preference for organic materials high no eff ect of a winter rye cover crop in a in N, increased the storage of C and N in continuous cott on system on soil pH aft er aggregates, and facilitated the decomposi- two cycles compared with a winter fallow tion of coarse organic material deposited on treatment. Eckert (1991) observed that a rye the soil surface. winter cover crop increased soil pH by a Cover crops can also increase the poten- small amount in two of three cropping sys- tial for microfaunal activity. Reddy et al. tems where corn followed rye and concluded (2003) reported that aft er 3 yr with crim- that this was due to rye’s assimilation of son clover or cereal rye cover crops soil had

NH4 and not a relocation of Ca. Eckert (1991) greater total bacterial and fungal propagule also reported that rye increased exchange- density and fl uorescein diacetate hydro- able K concentrations in the surface 0.05 m, lytic activity (FDA) than the soil without a presumably by accumulating K from the cover crop. The FDA assay is a measure of soil and depositing the K-containing shoot the soil enzyme esterase and is used as an and root residues on the soil surface and in indicator of microbial activity and biomass. the upper 0.05 m. The rye cover crop did not In their study, the crimson clover cover signifi cantly aff ect soil concentrations of C, crop had a greater stimulatory eff ect on soil P, Ca, or Mg in these studies. biology than cereal rye. The authors specu- lated that the legume cover crop had more readily available amino acids and carbohy- drates than the grass cover crops because of Soil Biology a lower C to N ratio. Lundquist et al. (1999) Cover crops increase the potential for macro- reported on the short-term (42 d) eff ects of and microfaunal activity in soils because cereal rye incorporation in contrasting veg- they increase the total inputs of organic etable management systems. Their results material to soils, they increase the length of indicated that following rye incorporation, time each year that plants are growing and counts of active bacteria increased 24 to 52% inputt ing C into the soil, and they moder- in the fi rst 7 d and populations of bacterial- ate changes in soil temperature and water feeding nematodes increased 400 to 600% content by increasing surface cover. For between 7 and 14 d. Active fungal hyphal example, Mele and Carter (1999) concluded lengths and fungal-feeding nematodes were that crop residues on the soil surface and no less responsive to rye incorporation during soil disturbance support higher densities the 42-d period. and weight of some species of earthworms Considerable evidence exists regarding because of increased water content of sur- the control of weeds, plant pathogens, and face layers, food sources at the surface, and nematodes by chemical substances released retention of burrows. Reeleder et al. (2006) from cover crop plants or from their resi- reported higher densities of earthworms, dues into the soil (Inderjit and Keating, predominately Aporrectodea turgida, in 1 of 1999). Chemicals released during decom- 2 yr of their study aft er 8 yr of a rye cover position of cover crop residues may be crop. Averaged across tillage, the rye treat- released in their unaltered form or may be ment had 33.3 worms m−2 compared with altered or transformed by soil microorgan- 12.8 worms m−2 in the no rye treatment. They isms. As with many of the eff ects of cover postulated that earthworm populations crops on soil properties that have been dis- may have been higher in the rye treatment cussed, these chemicals released into the because of increased availability of water. soil are common to many plant species Also, in this same study populations of and not just cover crops. These substances microarthropods were generally higher are known as allelochemicals and they can with a rye cover crop than without, but have positive or negative eff ects on plants or the total population of soil fungi was unaf- soil organisms (Inderjit and Keating, 1999). fected by cover crops. Kett erings et al. (1997) These compounds are primarily secondary

332 The Use of Cover Crops to Manage Soil | T.C. KasparChapter and J.W. | Authors Singer Soil Management Practices metabolites and include phenolics, terpe- eff ective than root residues at inhibiting let- noids, alkaloids, steroids, polyacetylenes, tuce (Lactuca sativa L.) germination. Liebl et and essential oils (Inderjit and Keating, al. (1992) reported that cereal rye provided 1999). Inderjit and Keating (1999), however, excellent weed control, with or without also reported that amino acids and organic herbicides, and concluded that the weed acids have been shown to possess allelo- suppression of the rye cover crop could pathic potential, although clear eff ects on have been caused by an allelopathic eff ect of biological activity have not always been decaying rye residues or the physical pres- proven. In any case, cover crops off er unique ence of the rye mulch on the soil surface or opportunities for using allelochemicals to both. Similar to the range of allelochemical control weeds, pathogens, and nematodes in concentrations found among Brassica spp., agricultural cropping systems because they Burgos et al. (1999) found that concentra- can be grown during the fallow periods in tions of the allelochemicals in rye DIBOA these systems. [2,4-dihydroxy-1,4-(2H)-benzoxazine-3- Cover crops that are members of the Poa- one] and BOA [2-(3H)-benzoxazolinone], ceae and Cruciferae families have the potential both hydroxamic acids, ranged from 137 to be used as biofumigants to control plant to 1469 μg g−1 dry tissue among eight culti- parasitic nematodes and soil fungi. Brassicas vars grown in the fi eld. They also reported contain glucosinolates that undergo hydro- data from a parallel greenhouse study lysis to produce compounds with broad that demonstrated that concentrations of biocidal activity (Brown and Morra, 1997). hydroxamic acid peaked 60 d aft er planting Kirkegaard and Sarwar (1998) quantifi ed rye. Although the allelochemicals produced levels of glucosinolates in 65 brassica spe- by rye and other cover crops may provide cies and reported that total glucosinolates benefi cial suppression of weeds, these same on a per unit area basis ranged from 0.8 to chemicals may also be responsible for yield 45.3 mmol m−2 when sampled at the mid- decreases of cash crops following cover fl owering growth stage. They concluded crops. For example, both Johnson et al. (1998) that the variation in biomass and glucosin- and Raimbault et al. (1990) reported reduc- olate compounds and their concentrations tions in either grain or silage yields when in various plant parts provide opportuni- corn was planted immediately aft er termi- ties for enhanced selection to increase their nating a rye cover crop with herbicide. In biofumigation potential. Sarwar et al. (1998) later studies, Raimbault et al. (1991) showed showed that the compounds released from that corn planted 14 d aft er rye cover crop Brassicas suppressed fi ve cereal root patho- termination produced 9% higher silage gens in laboratory tests. Similarly, Pott er et yields than corn planted immediately aft er al. (1998) found that leaf tissue of a variety of rye was terminated. They hypothesized Brassica species incorporated into soil was that one of many possible explanations highly nematicidal. McBride et al. (2000) also could be that the extra time allowed any found that incorporated cereal rye signifi - allelochemicals produced by the rye to be cantly suppressed nematode activity. They dissipated or denatured. had hypothesized that low molecular weight organic acids were responsible for the nem- atode-suppressing qualities of rye and they detected organic acids in soil solution shortly Summary aft er incorporation of fresh rye foliage. The Inserting cover crops into fallow periods rapid degradation of the organic acids in the and spaces in cropping systems is a benefi - soil, however, led them to conclude that these cial soil management practice. Cover crops acids probably were not solely responsible can protect the soil from erosion, reduce for suppressing the nematodes. losses of N and P, increase soil C, reduce Understanding the chemical mecha- runoff , inhibit pests, and support benefi - nisms for weed suppression by cover crops cial soil fauna. Although cover crops have is challenging. Barnes and Putnam (1986) in the potential to maintain and enhance soil a greenhouse study reported that rye resi- productivity and to reduce off site impacts dues and their aqueous extracts lowered of N, P, sediment, and greenhouse gases emergence and radicle elongation for several (CO2 and N2O), they are not widely used species and that shoot residues were more in most agricultural systems (Singer et al.,

333 Brown, P.D., and M.J. Morra. 1997. Control of soil-borne 2007). Incorporating cover crops into crop- plant pests using glucosinolate-containing plants. ping systems requires time, money, inputs, Adv. Agron. 61:167–231. machinery, labor, and modifi cations of cur- Burgos, N.R., R.E. Talbert, and J.D. Matt ice. 1999. Cultivar and age diff erences in the production of allelochemi- rent practices without immediate fi nancial cals by Secale cereale. Weed Sci. 47:481–485. return to the farmer. Improvements in soil Campbell, R.B., D.L. Karlen, and R.E. Sojka. 1984. Con- productivity resulting from cover crops servation tillage for maize production in the U.S. may require many years before benefi ts Southeastern Coastal Plain. Soil Tillage Res. 4:511–529. Chapman, H.D., G.F. Liebig, and D.S. Rayner. 1949. A are detectable. Additionally, availability of lysimeter investigation of nitrogen gains and losses cover crop management information, cover under various systems of covercropping and fertil- ization and a discussion of error sources. Hilgardia crop seed, and custom planting and spray- 19:57–95. ing services need to be improved to facilitate Clark, A.J., A.M. Decker, J.J. Meisinger, and M.S. McIntosh. adoption of cover crops in grain, oilseed, 1997. Kill date of vetch, rye, and a vetch-rye mixture: and fi ber cropping systems. However, if II. Soil moisture and corn yield. Agron. J. 89:434–441. Dabney, S.M. 1998. Cover crop impacts on watershed farmers are given fi nancial and productiv- hydrology. J. Soil Water Conserv. 53:207–213. ity incentives to grow cover crops, farmers, Dabney, S.M., J.A. Delgado, and D.W. Reeves. 2001. Using crop consultants, and scientists should be winter cover crops to improve soil and water quality. Commun. Soil Sci. Plant Anal. 32:1221–1250. able to overcome these management, sup- Dapaah, H.K., and T.J. Vyn. 1998. Nitrogen fertilization ply, and service problems relatively quickly. and cover crop eff ects on soil structural stability and Therefore, scientists need to demonstrate to corn performance. Commun. Soil Sci. Plant Anal. 29:2557–2569. farmers with long-term integrated studies Davidson, E.A., M. Keller, H.E. Erickson, L.V. Verchot, that maintaining or enhancing soil produc- and E. Veldkamp. 2000. Testing a conceptual model tivity with cover crops provides long-term of soil emissions of nitrous and nitric oxides. Biosci- ence 50:667–680. fi nancial benefi ts. Additionally, scientists Decker, A.M., A.J. Clark, J.J. Meisinger, F.R. Mulford, need to demonstrate to nonfarmers that pro- and M.S. McIntosh. 1994. Legume cover crop con- viding incentives for widespread adoption tributions to no-tillage corn production. Agron. J. 86:126–135. of cover crops could provide valuable eco- Degens, B.P., G.P. Sparling, and L.K. Abbott . 1996. Increas- system services and improvement of water ing the length of hyphae in a sandy soil increases the quality, while still maintaining current lev- amount of water-stable aggregates. Appl. Soil Ecol. 3:149–159. els of food, biomass, and fi ber production. Duiker, S.W., and N.L. Hartwig. 2004. Living mulches of legumes in imidazolinone-resistant corn. Agron. J. 96:1021–1028. Ebelhar, S.A., W.W. Frye, and R.L. Blevins. 1984. Nitrogen References from legume cover crops for no-tillage corn. Agron. Angle, J.S., G. Mc Clung, M.S. Mc Intosh, P.M. Thomas, J. 76:51–55. and D.C. Wolf. 1984. Nutrient losses in runoff from Eckert, D.J. 1991. Chemical att ributes of soils subjected conventional and no-till corn watersheds. J. Environ. to no-till cropping with rye cover crops. Soil Sci. Soc. Qual. 13:431–435. Am. J. 55:405–409. Ball-Coelho, B.R., R.C. Roy, and C.J. Swanton. 2000. Till- Ewing, R.P., M.G. Wagger, and H.P. Denton. 1991. Tillage age and cover crop impacts on aggregation of a sandy and cover crop management eff ects on soil water and soil. Can. J. Soil Sci. 80:363–366. corn yield. Soil Sci. Soc. Am. J. 55:1081–1085. Ball Coelho, B.R., R.C. Roy, and A.J. Bruin. 2005. Long- Flanagan, D.C. 2002. Erosion. In R. Lal (ed.) Encyclopedia term eff ects of late-summer overseeding of winter of soil science. Marcel Dekker, Inc., New York. rye on corn grain yield and nitrogen balance. Can. J. Foster, G.R., C.B. Johnson, and W.C. Moldenhauer. 1982. Plant Sci. 85:543–554. Critical slope lengths for unanchored cornstalk and Barnes, J.P., and A.R. Putnam. 1986. Evidence for allelop- wheat straw residue. Trans. ASAE 25:935–939, 947. athy by residues and aqueous extracts of rye (Secale Frye, W.W., R.L. Blevins, M.S. Smith, S.J. Corak, and J.J. cereale). Weed Sci. 34:384–390. Varco. 1988. Role of annual legume cover crops in Beale, O.W., G.B. Nutt , and T.C. Peele. 1955. The eff ects of effi cient use of water and nitrogen. p. 129–154. In W.L. mulch tillage on runoff , erosion, soil properties, and Hargrove (ed.) Cropping strategies for the effi cient crop yields. Soil Sci. Soc. Am. Proc. 19:244–247. use of water and nitrogen. ASA, CSSA, and SSSA, Bechmann, M.E., P.J.A. Kleinman, A.N. Sharpley, and L.S. Madison, WI. Saporito. 2005. Freeze-thaw eff ects on phosphorus Garwood, T.W.D., D.B. Davies, and A.R. Hartley. 1999. loss in runoff from manured and catch-cropped soils. The eff ect of winter cover crops on yield of the follow- J. Environ. Qual. 34:2301–2309. ing spring crops and nitrogen balance in a calcareous Benoit, R.E., N.A. Willits, and W.J. Hanna. 1962. Eff ect loam. J. Agric. Sci. 132:1–11. of rye winter cover crop on soil structure. Agron. J. Haider, K., A. Mosier, and O. Heinemeyer. 1987. The 54:419–420. eff ect of growing plants on denitrifi cation at high soil Brandi-Dohrn, F.M., M. Hess, J.S. Selker, R.P. Dick, S.M. nitrate concentrations. Soil Sci. Soc. Am. J. 51:97–102. Kauff man, and D.D. Hemphill, Jr. 1997. Nitrate leach- Hansen, E.M., J. Djurhuus, and K. Kristensen. 2000. ing under a cereal rye cover crop. J. Environ. Qual. Nitrate leaching as aff ected by introduction or dis- 26:181–188. continuation of cover crop use. J. Environ. Qual. Brown, L.C., and L.D. Norton. 1994. Surface residue 29:1110–1116. eff ects on soil erosion from ridges of diff erent soils Harris, G.H., O.B. Hesterman, E.A. Paul, S.E. Peters, and and formation. Trans. ASAE 37:1515–1524. R.R. Janke. 1994. Fate of legume and fertilizer nitro-

334 The Use of Cover Crops to Manage Soil | T.C. KasparChapter and J.W. | Authors Singer Soil Management Practices

gen-15 in a long-term cropping systems experiment. Langdale, G.W., R.A. Leonard, and A.W. Thomas. 1985. Agron. J. 86:910–915. Conservation practice eff ects on phosphorus losses Hively, W.D., and W.J. Cox. 2001. Interseeding cover crops from southern piedmont watersheds. J. Soil Water into soybean and subsequent corn yields. Agron. J. Conserv. 40:157–161. 93:308–313. Langdale, G.W., and R.L. Blevins. D.L. Karlen, D.K. Huggins, D.R., and D.J. Fuchs. 1997. Long-term N man- McCool, M.A. Nearing, E.L. Skidmore, A.W. Thomas, agement eff ects on corn yield and soil C of an Aquic D.D, Tyler, and J.R. Williams. 1991. Cover crop eff ects Haplustoll in Minnesota. p. 121–128. In E.A. Paul et on soil erosion by wind and water. p. 15–22. In W.L. al. (ed.) Soil organic matt er in temperate ecosystems: Hargrove (ed.) Cover crops for clean water. Proc. Int. Long-term experiments in North America. CRC Press, Conf., Jackson, TN. 9–11 April 1991. Soil and Water Boca Raton, FL. Conserv. Soc., Ankeny, IA. Inderjit, and K.I. Keating. 1999. Allelopathy: Principles, Latt anzi, A.R., L.D. Meyer, and M.F. Baumgardner. 1974. procedures, processes, and promises for biological Infl uences of mulch rate and slope steepness on inter- control. Adv. Agron. 67:141–231. rill erosion. Soil Sci. Soc. Am. J. 38:946–950. Jarecki, M.K., and R. Lal. 2003. Crop management for soil Liebl, R., F.W. Simmons, L.M. Wax, and E.W. Stoller. 1992. carbon sequestration. Crit. Rev. Plant Sci. 22:471–502. Eff ect of rye (Secale cereale) mulch on weed-control Johnson, T.J., T.C. Kaspar, K.A. Kohler, S.J. Corak, and S.D. and soil-moisture in soybean (Glycine max). Weed Logsdon. 1998. Oat and rye overseeded into soybean Technol. 6:838–846. as fall cover crops in the upper Midwest. J. Soil Water Lundquist, E.J., L.E. Jackson, K.M. Scow, and C. Hsu. 1999. Conserv. 53:276–279. Changes in microbial biomass and community com- Jones, R.J. 1942. Nitrogen losses from Alabama soils in position, and soil carbon and nitrogen pools aft er lysimeters as infl uenced by various systems of green incorporation of rye into three California agricultural manure crop management. Agron. J. 34:574–585. soils. Soil Biol. Biochem. 31:221–236. Kanwar, R.S., R.M. Cruse, M. Ghaff arzadeh, A. Bakhsh, Mamo, M., and G.D. Bubenzer. 2001. Detachment rate, D.L. Karlen, and T.B. Bailey. 2005. Corn-soybean and soil erodibility, and soil strength as infl uenced by living plant roots. Part II: Field study. Trans. ASAE alternative cropping systems eff ects on NO3–N leach- ing losses in subsurface drainage water. Appl. Eng. 44:1175–1181. Agric. 21:181–188. Martinez, J., and G. Guiraud. 1990. A lysimeter study of Karlen, D.L., and C.A. Cambardella. 1996. Conservation the eff ects of a ryegrass catch crop, during a winter- strategies for improving soil quality and organic wheat maize rotation, on nitrate leaching and on the matt er storage. p. 395–420. In M.R. Carter and B.A. following crop. J. Soil Sci. 41:5–16. Stewart (ed.) Structure and organic matt er storage McBride, R.G., R.L. Mikkelsen, and K.R. Barker. 2000. in agricultural soils. Advances in Soil Science. CRC The role of low molecular weight organic acids from Press Inc., New York. decomposing rye in inhibiting root-knot nematode Kaspar, T.C., J.K. Radke, and J.M. Lafl en. 2001. Small grain populations in soil. Appl. Soil Ecol. 15:243–251. cover crops and wheel traffi c eff ects on infi ltration, McCracken, D.V., S.J. Corak, M.S. Smith, W.W. Frye, and runoff , and erosion. J. Soil Water Conserv. 56:160–164. R.L. Blevins. 1989. Residual eff ects of nitrogen fer- Kaspar, T.C., T.B. Parkin, D.B. Jaynes, C.A. Cambardella, tilization and winter cover cropping on nitrogen D.W. Meek, and Y.S. Jung. 2006. Examining changes availability. Soil Sci. Soc. Am. J. 53:1459–1464. in soil organic carbon with oat and rye cover McCracken, D.V., M.S. Smith, J.H. Grove, R.L. Blevins, and crops using terrain covariates. Soil Sci. Soc. Am. J. C.T. MacKown. 1994. Nitrate leaching as infl uenced 70:1168–1177. by cover cropping and nitrogen source. Soil Sci. Soc. Kaspar, T.C., D.B. Jaynes, T.B. Parkin, and T.B. Moorman. Am. J. 58:1476–1483. 2007. Rye cover crop and gamagrass strip eff ects on Meisinger, J.J., W.L. Hargrove, R.L. Mikkelsen, J.R. Wil- liams, and V.W. Benson. 1991. Eff ects of cover crops NO3 concentration and load in tile drainage. J. Envi- ron. Qual. 36:1503–1511. on groundwater quality. p. 57–68. In W.L. Hargrove Kett erings, Q.M., J.M. Blair, and J.C.Y. Marinissen. 1997. (ed.) Cover crops for clean water. Proc. Int. Conf., Jack- Eff ects of earthworms on soil aggregate stability and son, TN. 9–11 April 1991. Soil and Water Conserv. Soc., carbon and nitrogen storage in a legume cover crop Ankeny, IA. agroecosystem. Soil Biol. Biochem. 29:401–408. Mele, P.M., and M.R. Carter. 1999. Impact of crop man- Kirkegaard, J.A., and M. Sarwar. 1998. Biofumigation agement factors in conservation tillage farming on potential of brassicas. I. Variation in glucosinolate earthworm density, age structure and species abun- profi les of diverse fi eld-grown brassicas. Plant Soil dance in south-eastern Australia. Soil Tillage Res. 201:71–89. 50:1–10. Kladivko, E.J., J.R. Frankenberger, D.B. Jaynes, D.W. Meek, Mendes, I.C., A.K. Bandick, R.P. Dick, and P.J. Bott omley. B.J. Jenkinson, and N.R. Fausey. 2004. Nitrate leach- 1999. Microbial biomass and activities in soil aggre- ing to subsurface drains as aff ected by drain spacing gates aff ected by winter cover crops. Soil Sci. Soc. Am. and changes in crop production system. J. Environ. J. 63:873–881. Qual. 33:1803–1813. Miguez, F.E., and G.A. Bollero. 2006. Winter cover crops Kuo, S., U.M. Sainju, and E.J. Jellum. 1997. Winter cover in Illinois: Evaluation of ecophysiological characteris- crop eff ects on soil organic carbon and carbohydrate tics of corn. Crop Sci. 46:1536–1545. in soil. Soil Sci. Soc. Am. J. 61:145–152. Moschler, W.W., G.M. Shear, D.L. Hallok, R.D. Sears, and Kuo, S., and E.J. Jellum. 2000. Long-term winter cover G.D. Jones. 1967. Winter cover crops for sod-planted cropping eff ects on corn (Zea mays L.) production and corn: Their selection and management. Agron. J. soil nitrogen availability. Biol. Fertil. Soils 31:470–477. 59:547–551. Lafl en, J.M., G.R. Foster, and C.A. Onstad. 1985. Simula- Mutchler, C.K., and L.L. McDowell. 1990. Soil loss tion of individual-storm soil loss for modeling the from cott on with winter cover crops. Trans. ASAE impact of soil erosion on crop productivity. p. 285–295. 33:432–436. In S.A. El-Swaify et al. (ed.) Soil erosion and conserva- Nelson, D.W., and L.E. Sommers. 1996. Total carbon, tion. Soil and Water Conserv. Soc., Ankeny, IA. organic carbon, and organic matt er. p. 961–1010. In Lal, R., J.M. Kimble, R.F. Follett , and C.V. Cole. 1999. The D.L. Sparks et al. (ed.) Methods of soil analysis. Part potential of U.S. cropland to sequester carbon and 3. Chemical methods. SSSA Book Ser. 5. SSSA, Mad- mitigate the greenhouse eff ect. Ann Arbor Press, ison, WI. Chelsea, MI. Nyakatawa, E.Z., K.C. Reddy, and K.R. Sistani. 2001. Till- age, cover cropping, and poultry litt er eff ects on

335 selected soil chemical properties. Soil Tillage Res. Ruff o, M.L., and G.A. Bollero. 2003. Modeling rye and 58:69–79. hairy vetch residue decomposition as a function Parkin, T.B., T.C. Kaspar, and J.W. Singer. 2006. Cover crop of degree-days and decomposition-days. Agron. J. eff ects on the fate of N following soil application of 95:900–907. swine manure. Plant Soil 289:141–152. Sainju, U.M., B.P. Singh, and W.F. Whitehead. 2002. Long- Parr, J.F., and R.I. Papendick. 1978. Factors aff ecting the term eff ects of tillage, cover crops, and nitrogen decomposition of crop residues by microorganisms. fertilization on organic carbon and nitrogen concen- p. 109–129. In W.R. Oschwald (ed.) Crop residue man- trations in sandy loam soils in Georgia, USA. Soil agement systems. ASA, Madison, WI. Tillage Res. 63:167–179. Patrick, W.H., Jr., C.B. Haddon, and J.A. Hendrix. 1957. Sainju, U.M., and B.P. Singh. 2008. Nitrogen storage with The eff ect of longtime use of winter cover crops on cover crops and nitrogen fertilization in tilled and certain physical properties of Commerce loam. Soil nontilled soils. Agron. J. 100:619–627. Sci. Soc. Am. J. 21:366–368. Sarwar, M., J.A. Kirkegaard, P.T.W. Wong, and J.M. Des- Paustian, K., H.P. Collins, and E.A. Paul. 1997. Manage- marchelier. 1998. Biofumigation potential of brassicas. ment controls on soil carbon. p. 15–49. In E.A. Paul et III. In vitro toxicity of isothiocyanates to soil-borne al. (ed.) Soil organic matt er in temperate ecosystems: fungal pathogens. Plant Soil 201:103–112. Long-term experiments in North America. CRC Press, Sharpley, A.N., and S.J. Smith. 1991. Eff ects of cover crops Boca Raton, FL. on surface water quality. p. 41–49. In W.L. Hargrove Pesant, A.R., J.L. Dionne, and J. Genest. 1987. Soil and (ed.) Cover crops for clean water. Proc. Int. Conf., Jack- nutrient losses in surface runoff from conventional son, TN. 9–11 April 1991. Soil and Water Conserv. Soc., and no-till corn systems. Can. J. Soil Sci. 67:835–843. Ankeny, IA. Pott er, M.J., K. Davies, and A.J. Rathjen. 1998. Suppressive Shipley, P.R., J.J. Messinger, and A.M. Decker. 1992. Con- impact of glucosinolates in brassica vegetative tis- serving residual corn fertilizer nitrogen with winter sues on root lesion nematode Pratylenchus neglectus. J. cover crops. Agron. J. 84:869–876. Chem. Ecol. 24:67–80. Singer, J.W., S.M. Nusser, and C.J. Alf. 2007. Are cover Power, J.F., J.W. Doran, and P.T. Koerner. 1991. Hairy vetch crops being used in the US corn belt? J. Soil Water as a winter cover crop for dryland corn production. J. Conserv. 62:353–358. Prod. Agric. 4:62–67. Smith, M.S., and J.M. Tiedje. 1979. The eff ect of roots on Power, J.F., and J.A. Zachariassen. 1993. Relative nitrogen soil denitrifi cation. Soil Sci. Soc. Am. J. 43:951–955. utilization by legume cover crop species at three soil Staver, K.W., and R.B. Brinsfi eld. 1990. Patt erns of soil temperatures. Agron. J. 85:134–140. nitrate availability in corn production systems: impli- Qi, Z., and M.J. Helmers. 2010. Soil water dynamics under cations for reducing groundwater contamination. J. winter rye cover crop in central Iowa. Vadose Zone Soil Water Conserv. 45:318–323. J. 9:53–60. Staver, K.W., and R.B. Brinsfi eld. 1998. Using cereal grain Quemada, M., and M.L. Cabrera. 1997. Temperature and winter cover crops to reduce groundwater nitrate moisture eff ects on C and N mineralization from sur- contamination in the mid-Atlantic coastal plain. J. face applied clover residue. Plant Soil 189:127–137. Soil Water Conserv. 53:230–240. Raimbault, B.A., T.J. Vyn, and M. Tollenaar. 1990. Corn Strock, J.S., P.M. Porter, and M.P. Russelle. 2004. Cover response to rye cover crop management and spring cropping to reduce nitrate loss through subsurface tillage systems. Agron. J. 82:1088–1093. drainage in the northern U.S. Corn Belt. J. Environ. Raimbault, B.A., T.J. Vyn, and M. Tollenaar. 1991. Corn Qual. 33:1010–1016. response to rye cover crop, tillage methods, and Thorup-Kristensen, K., J. Magid, and L.S. Jensen. 2003. planter options. Agron. J. 83:287–290. Catch crops and green manures as biological tools Ram, D.N., M.T. Vitt um, and P.J. Zwerman. 1960. An eval- in nitrogen management in temperate zones. Adv. uation of certain winter cover crops for the control of Agron. 79:227–302. splash erosion. Agron. J. 52:479–482. Timmons, D.R., R.F. Holt, and J.J. Latt erel. 1970. Leaching Ranells, N.N., and M.G. Wagger. 1997a. Nitrogen-15 of crop residues as a source of nutrients in surface recovery and release by rye and crimson clover cover runoff water. Water Resour. Res. 6:1367–1375. crops. Soil Sci. Soc. Am. J. 61:943–948. Tisdall, J.M., and J.M. Oades. 1979. Stabilization of soil Ranells, N.N., and M.G. Wagger. 1997b. Grass-legume aggregates by the root systems of ryegrass. Aust. J. bicultures as winter annual cover crops. Agron. J. Soil Res. 17:429–441. 89:659–665. Tonitt o, C., M.B. David, and L.E. Drinkwater. 2006. Rasse, D.P., J.T. Ritchie, W.R. Peterson, J. Wei, and A.J.M. Replacing bare fallows with cover crops in fertil- Smucker. 2000. Rye cover crop and nitrogen fertiliza- izer-intensive cropping systems: A meta-analysis of tion eff ects on nitrate leaching in inbred maize fi elds. crop yield and N dynamics. Agric. Ecosyst. Environ. J. Environ. Qual. 29:298–304. 112:58–72. Reddy, K.N., R.M. Zablotowicz, M.A. Locke, and C.H. Unger, P.W., and M.F. Vigil. 1998. Cover crop eff ects on soil Koger. 2003. Cover crop, tillage, and herbicide eff ects water relationships. J. Soil Water Conserv. 53:200–207. on weeds, soil properties, microbial populations, and Utomo, M., W.W. Frye, and R.L. Blevins. 1990. Sustaining soybean yield. Weed Sci. 51:987–994. soil nitrogen for corn using hairy vetch cover crop. Reeleder, R.D., J.J. Miller, B.R.B. Coelho, and R.C. Roy. Agron. J. 82:979–983. 2006. Impacts of tillage, cover crop, and nitrogen on Varco, J.J., W.W. Frye, M.S. Smith, and C.T. MacKown. 1989. populations of earthworms, microarthropods, and Tillage eff ects on nitrogen recovery by corn from a soil fungi in a cultivated fragile soil. Appl. Soil Ecol. nitrogen-15 labeled legume cover crop. Soil Sci. Soc. 33:243–257. Am. J. 53:822–827. Ritt er, W.F., R.W. Scarborough, and A.E.M. Chirnside. Villamil, M.B., G.A. Bollero, R.G. Darmody, F.W. Simmons, 1998. Winter cover crops as a best management prac- and D.G. Bullock. 2006. No-till corn/soybean systems tice for reducing nitrogen leaching. J. Contam. Hydrol. including winter cover crops: Eff ects on soil proper- 34:1–15. ties. Soil Sci. Soc. Am. J. 70:1936–1944. Rosecrance, R.C., G.W. McCarty, D.R. Shelton, and J.R. Wagger, M.G., and D.B. Mengel. 1988. The role of nonle- Teasdale. 2000. Denitri fi cation and N mineralization guminous cover crops in the effi cient use of water and from hairy vetch (Vicia villosa Roth) and rye (Secale nitrogen. p. 115–127. In W.L. Hargrove (ed.) Cropping cereale L.) cover crop monocultures and bicultures. strategies for effi cient use of water and nitrogen. ASA, Plant Soil 227:283–290. CSSA, and SSSA, Madison, WI.

336 The Use of Cover Crops to Manage Soil | T.C. KasparChapter and J.W. | Authors Singer Soil Management Practices

Wagger, M.G. 1989a. Time of desiccation eff ects on plant Williams, S.M., and R.R. Weil. 2004. Crop cover root chan- composition and subsequent nitrogen release from nels may alleviate soil compaction eff ects on soybean several winter annual cover crops. Agron. J. 81:236–241. crop. Soil Sci. Soc. Am. J. 68:1403–1409. Wagger, M.G. 1989b. Cover crop management and nitro- Wyland, L.J., L.E. Jackson, W.E. Chaney, K. Klonsky, gen rate in relation to growth and yield of no-till corn. S.T. Koike, and B. Kimple. 1996. Winter cover crops Agron. J. 81:533–538. in a vegetable cropping system: Impacts on nitrate Wagger, M.G., M.L. Cabrera, and N.N. Ranells. 1998. leaching, soil water, crop yield, pests and manage- Nitrogen and carbon cycling in relation to cover crop ment costs. Agric. Ecosyst. Environ. 59:1–17. residue quality. J. Soil Water Conserv. 53:214–218. Yoo, K.H., J.T. Touchton, and R.H. Walker. 1988. Runoff , Wander, M.M., S.J. Traina, B.R. Stinner, and S.E. Peters. sediment and nutrient losses from various tillage 1994. Organic and conventional management eff ects systems of cott on. Soil Tillage Res. 12:13–24. on biologically active soil organic matt er pools. Soil Zhu, J.C., C.J. Gantzer, S.H. Anderson, E.E. Alberts, Sci. Soc. Am. J. 58:1130–1139. and P.R. Beuselinck. 1989. Runoff , soil, and dis- Wendt, R.C., and R.E. Burwell. 1985. Runoff and soil solved nutrient losses from no-till soybean losses for conventional, reduced, and no-till corn. J. with winter cover crops. Soil Sci. Soc. Am. J. Soil Water Conserv. 40:450–454. 53:1210–1214.

337 338 Chapter | Authors