Because winter legume cover crops may provide si!Jnificant quantities of N while conserving soil and water resou¢es, the role of legumes in conservation tillage production systems warrants renewed attention. The objectives of this study were (i) to determine the N contribution of several legumes to a subsequent crop of grai~ sorghum (Sorghum bicolor (L.) Moench) under no-tillage management and (ii) to determine the influence of legume cover crops on soil fertility status. A field experiment was conducted in which four winter legumes, one nonlegume, and a no cover crop treatment were studied over a 3-yr period. In addition, four fertilizer N rates (0, 28, 56, and 112 kg N ha-1 ) were applied to no-till grain sorghum. Grain sorghum did not respond to fertilizer N when following a legume cover crop but responded to as much as 99 kg N ha-1 when following a nonlegume cover crop or no cover crop. A mean estimate of 72 kg N ha-1 was obtained for the fertilizer N replaced by the legume. The effects of the legume cover crops on soil fertility status included: (i) a lower pH, (ii) a redistribution of K+ to the soil surface from deeper in the soil profile, and (iii) a lower C/N ratio in soil organic matter. Since fertilizer N represents a sizeable portion of the fossil fuel energy required for nonleguminous row crop production, the estimated N contribution of legume cover crops represents a significant energy savings, enhancing the conservation value of a no-tillage production system.

Winter legume cover crops have been shown to provide significant amounts of N to subsequent nonleguminous crops, but benefits beyond those directly attributed to N are rarely cited. A 3 yr field study at two Georgia locations utilizing a randomized, complete‐block, split‐plot design with four replications was begun in 1985 to measure the equivalent fertilizer N supplied by winter annual legumes and to monitor changes in soil physical and chemical properties. Corn, Zea mays L. was grown on a Rome gravelly clay loam soil (fine‐loamy, mixed, thermic Typic Hapludult) at the Limestone Valley location, and grain sorghum [Sorghum bicolor (L) Moench] on a Greenville sandy clay loam soil (clayey, kaolinitic, thermic Rhodic Paleudult) in the Coastal Plain. Main plots were cover crops of hairy vetch (Vicia villosa Roth.), crimson clover (Trifolium incarnatum L), berseem clover (Trifolium alexandrinum L), winter pea [Pisum sativum subsp. arvense (L) poir], wheat (Triticum aestivum L), and fallow. Subplots were broadcast NH4NO3 fertilizer of varying rates. Hairy vetch and crimson clover replaced the greatest amount of fertilizer N averaging 123 and 99 kg N ha−1, respectively. More water‐stable aggregates were found following cover crops than fallow in the 0‐ to 0.025‐m depth at the Coastal Plain location. Higher infiltration rates were found following cover crops than fallow at both locations and infiltration rates were greater following hairy vetch than following wheat at the Coastal Plain site. An adapted winter legume cover crop can replace all of the fertilizer N necessary for optimum rain‐fed grain sorghum and up to two‐thirds of that required for corn production, and improve soil physical properties.

No abstract

The N contained in winter cover crops (particularly legumes) is a potentially important source of N for succeeding crops. The purpose of this study was to determine N release from crimson clover (Trifolium incarnatum L.) residue under no‐tillage and conventional tillage conditions. Residues contained in nylon mesh (53 µm) bags placed either on the surface of no‐tillage plots or buried at plowlayer depth in conventional tillage plots were removed for dry matter, C, and N determinations at 1, 2, 4, 8, and 16 weeks after placement in mid‐May. The rate of N disappearance was more rapid under conventional than no‐tillage conditions. The percentage of initial residue N remaining at 4 and 16 weeks under conventional tillage conditions was 40 and 31, respectively. The corresponding values for no‐tillage were 63 and 36% at 4 and 16 weeks. The C/N ratio of the residue remained relatively unchanged over the 16‐week period under no‐tillage conditions. However, under conventional tillage conditions, the C/N ratio declined from 15.9 to 12.7. Under humid, subtropical conditions, release of N from both surface and buried residue from winter legumes, is sufficiently rapid to be of significant benefit to the summer crop.

The efficiency of properly applied urea N fertilizers is generally equal to the efficiency of ammonium nitrate. Proper applications, however, most often include incorporation soon after application. In no‐tillage systems where cultivation is not a common practice, these fertilizers are often applied as surface sidedress applications and are sometimes used as carriers for contact herbicides. These field studies which were conducted for 3 years on a Cecil sandy loam soil (Typic hapludult) were designed to compare the efficiency of prilled urea, urea‐ammonium nitrate solution (UAN), and prilled ammonium nitrate (AN) as affected by methods of application in no‐tillage corn (Zea mays L.) production. Treatments included shallow incorporated and unincorporated band applications of all N sources plus a broadcast spray application of UAN. Nitrogen rates were 90, 180, and 270 kg/ha. The results for corn grain yield showed that, in comparing the three N sources, the order of efficiency is generally urea < UAN solution ≤ ammonium nitrate. However, in comparing the three methods of application (surface band, incorporated band, and surface broadcast spray) for the UAN solution, little difference was observed between the surface or incorporated band applications. The surface broadcast spray treatment, on the other hand, resulted in considerably less grain yield and N uptake as judged from the ear leaf N concentrations. These results indicate that, at least for the conditions of this study, urea is not an efficient N source for no‐tillage production systems and is especially inefficient when applied in solution as a broadcast spray.