No abstract

Crop responses and N uptake of corn (Zea maysL.) grown with moldboard plow tillage (PT) and minimal tillage (MT) were measured in a 4‐yr field experiment on a Mattapex silt loam soil (finesilty, mixed, mesic, Aquic Hapludult) near Salisbury, MD (Atlantic Coastal Plain region). Labeled ammonium sulfate or ammonium nitrate (15N depleted) was applied annually to the entire area (74 m2) of each plot at five N rates ranging from 0 to 180 or 270 kg N ha−1. Corn dry matter yield, N content, and isotope composition were determined in the entire plant at the 11 to 12 leaf stage and at silage harvest, as well as in the corn grain at maturity. Below 135 kg N ha−1, dry matter yields, total N uptake, and fertilizer N (FN) uptake were greater with PT than with MT, but with 180 kg N ha1−1there was either no difference between the tillage systems or MT was greater than PT. Fertilizer N additions had no consistent or practically significant effect on soil N (SN) uptake. At 135 kg N ha−1, average crop FN recoveries were 62% for PT and 53% for MT, but this difference decreased with increasing N rate until both tillage systems averaged about 53% recovery at 180 kg N ha−1. Average SN uptake was greater with PT culture than with MT culture, averaging 76 kg N ha−1and 59 kg N ha−1, respectively. The crop N requirement of corn was about 17 kg N ha−1greater for MT corn than PT corn due to a 10% greater yield of total dry matter with MT. Corn grown with MT required about 68 kg more FN ha−1than PT, due to a lower uptake of SN and a greater crop N requirement. A lower uptake of N on MT plots during the second half of the growing season suggests (i) a greater net mineralization of N on PT plots and/or (ii) a greater loss of N via leaching and/or denitrification on MT plots. A nitrogen concentration of about 11 to 12 mg N g−1in the silage total dry matter, or about 15 mg N g−1in the grain, can be used as an approximate value for N sufficiency. However, these values should be considered broad guidelines since significant deviations were observed in individual years.

Little is known about the relative effectiveness of granular (UG) or prilled urea (UP), urea‐ammonium nitrate solutions (UAN), and ammonium nitrate (AN) when top dressed for no‐till corn. In most of the field studies reported, the N‐fertilizer used with no‐tillage has been ammonium nitrate, ammonium sulfate, or ammonium‐based mixed fertilizers. We conducted field experiments for 3 years on three moderately well‐drained Mattapex, Bertie, and Delanco silt loam soils (all Aquic Hapludults), comparing topdressed UG, UP, UAN solution (30% N), and AN at four N rates (45, 90, 135, and 180 kg/ha) on yields of continuous corn (Zea mays L.). A zero N plot was included in each of the four replicates. Yields were significantly higher from AN than from other N sources in 1978 at Forage Farm and in 1976 and 1978 at Wye Institute. Average yield from UAN exceeded that from UP and UG at Wye Institute. In 1978, when rainfall was above normal, even at the highest rate of N (180 kg/ha) total N uptake was barely sufficient to obtain maximum yields at Forage Farm and Poplar Hill and was clearly inadequate at Wye Institute. With 180 kg/ha of N applied, N%TDM when grain had reached the dough stage was about 0.9 at Wye Institute and 1.1 at the other sites. Values associated with estimated maximum yields generally exceed 1.2%. In 1978, estimated recoveries of applied N were higher from AN than from the other N sources at Wye Institute and Forage Farm.

Many studies have indicated that organic matter levels in soils tend to rise when no-till (NT) management replaces plowing. It was hypothesized that organic P may thus play a more important role in the plant availability of P in NT soils. A laboratory study was conducted to investigate the organic and inorganic forms of P and their cycling in NT and conventionally tilled (CT) silt Joams. Soils from three long-term field experiments were sampled in the fall of 1984, obtaining core segments from 0 to 2, 2 to 4, 4 to 8, 8 to 12, 16 to 20, and 24 to 28 em. Soil analyses indicated significant differences in the distribution with depth of macro-nutrients between tillage systems. Total P, dilute acid-extractable P (DA-P), total N, and organic matter were more concentrated in the 0-to 2-cm layer under NT compared to CT management. Organic f did not accumulate to the same extent as did organic N and C, resulting in a lower organic P/organic C ratio under NT than under CT management. With increased rates of P fertilization on one soil, total P and DA-P increased dramatically in the upper soil layers, but a similar increase and stratification of organic P was not observed. Samples taken in the spring of 1985 indicated that, despite significantly higher microbial respiration in NT cores, there was no consistent change in the solubility of P over a 75-d incubation period. Thus, it was concluded that the incubation procedure inadequately measured P mineralization in these soils. However, water-soluble P was significantly greater in the upper 5 em under NT, even where no P fertilizer had been applied. This may have been a result of interference with P-adsorption reactions by accumulated organic matter. This study provided no evidence that organic P is of more importance to plant P availability under NT.

This study comparing corn yields from different rates of applied N under no‐tillage and conventional culture was prompted by numerous field observations in Maryland that no‐till corn, with low to moderate rates of N, often showed more pronounced N deficiency symptoms than did conventionally tilled corn. Little is known regarding the relative N requirements of untilled corn grown in cover‐crop residues as compared to conventional tillage. The purpose of this study was therefore, to determine whether tillage method affected optimal crop requirements for fertilizer N. First‐year effects of tillage method and fertilizer N applications (0, 45, 90, 135, and 180 kg/ha) on corn (Zea mays L.) yields, N uptake by the crop, mineral N accumulation in the soil, and total and potentially mineralizable soil N are reported. With suboptimal N rates, N deficiency symptoms were more severe throughout the season on untilled than on cultivated plots. However, the optimal level of applied N for grain and dry matter yields did not differ with tillage method. After grain harvest, the amount and distribution of residual mineral N in the root zone for a given rate of applied N were similar in tilled and untilled plots at two locations (Poplar Hill and Plant Research Farm). At the third site (Wye Institute), however, residual mineral soil N was higher with conventional than with no tillage. These differences were attributed to variation among sites in amounts of precipitation between planting and harvest, as it affected relative water infiltration in tilled and untilled plots. Total residual mineral N in the root zone tended to increase with N application rate. Between harvest (1973) and planting (1974), however, most of the residual mineral fertilizer N was removed by the cover crop or lost. Total soil organic N to a depth of 30 cm was unaffected by N rate or tillage method during one cropping season. At one rate, however, potentially mineralizable N was higher in untilled than in tilled plots. Studies are continuing on two of the sites, with l5N‐lepleted fertilizer, to determine relative changes with time in soil N status as influenced by tillage method.