Spatial distributions of soil properties at the field and watershed scale may affect yield potential, hydrologic responses, and transport of herbicides and NO−3 to surface or groundwater. Our research describes field‐scale distributions and spatial trends for 28 different soil parameters at two sites within a watershed in central Iowa. Two of 27 parameters measured at one site and 10 of 14 parameters measured at the second site were normally distributed. Spatial variability was investigated using semivariograms and the ratio of nugget to total semivariance, expressed as a percentage, was used to classify spatial dependence. A ratio of <25% indicated strong spatial dependence, between 25 and 75% indicated moderate spatial dependence, and >75% indicated weak spatial dependence. Twelve parameters at Site one, including organic C, total N, pH, and macroaggregation, and four parameters at Site two, including organic C and total N, were strongly spatially dependent. Six parameters at Site one, including biomass C and N, bulk density, and denitrification, and 9 parameters at Site two, including biomass C and N and bulk density, were moderately spatially dependent. Three parameters at Site one, including NO−3 N and ergosterol, and one parameter at Site two, mineral‐associated N, were weakly spatially dependent. Distributions of exchangeable Ca and Mg at Site one were not spatially dependent. Spatial distributions for some soil properties were similar for both field sites. We will be able to exploit these similarities to improve our ability to extrapolate information taken from one field to other fields within similar landscapes.

Lack of water because of erratic rainfall frequently limits corn (Zea mays L.) production on Typic Paleudults in the Atlantic Coastal Plain. Traditionally, wide (96 cm) row spacing and low plant population have been used to prevent water stress, but recently landowners have begun to invest in irrigation systems. Changes in row spacing, plant population, or fertilization practices may be required to achieve maximum water‐and nutrient‐use efficiency with those systems. We evaluated plant population treatments averaging 7.0 and 10.1 plants m−2 in single and twin rows on a Norfolk (fineloamy, siliceous, thermic Typic Paleudult) loamy sand during 1980, 1981, and 1982. Three water management [nonirrigated, irrigated using tensiometers (TENS) to measure soil‐water potential for scheduling, and irrigated using a computer‐based water balance (CBWB) for scheduling], and two fertilization programs were also evaluated in a four‐factor split‐plot design. Water management and plant population interacted significantly. Planting in twin rows increased grain yield an average of 0.64 Mg ha−1 (10 bu/A), but planting more than 7.1 plants m−2 significantly increased grain yield only in 1980. Irrigation increased grain yield 150, 161, and 8% in 1980, 1981, and 1982, respectively, as a result of increased kernel weight and number of kernels per ear. Increasing total N, P, and K application beyond 200, 30, and 167 kg ha−1, respectively, did not significantly influence grain yield or yield components. Yield advantages of narrow rows can be obtained on Coastal Plain soils which require subsoiling by using a twin‐row planting configuration. Irrigation can be scheduled using either tensiometers (soil‐water potential) or a computerized water balance without significantly changing corn grain yield, nutrient accumulation, or yield components.

Allocation of photosynthate among leaves, stems, and roots is critical in seedling establishment. Corn (Zea mays L.) seedlings were grown in different spacing patterns in a field and with different reflected far‐red (FR) to red (R) light ratios to test the effects of a modified FR/R ratio on photoassimilate allocation. Green leaves absorbed most of the R and reflected much of the FR. Therefore, close‐spaced plants received more reflected FR and higher FR/R ratios. Seedlings that received the higher FRJR ratios developed longer and narrower leaves, longer stems, and less massive roots. Stem elongation was an early response to increased FR/R ratio even though fight did not impinge directly on the stems, which were initially at or below the soil surface and covered by several layers of leaves. Row orientation did not significantly alter FR/R ratio or seedling morphology because corn leaves are not heliotropic and did not function as directional FR reflectors, as was observed for soybean [Glycine max (L.) Merr.] in a previous study. An increase in the FR/R ratio reflected up to seedlings from the soil surface also resulted in increased shoot size and shoot/root biomass ratio. Early morphological responses of corn seedlings to FR/R ratio in reflected light are relevant to seedling establishment and are not dependent on the cause of the altered ratio.

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The maximum amount and rate of nutrient accumulation by irrigated corn (Zea mays L.) must be known so that farmers do not waste money or pollute water resources by applying excessive amounts offertilizer. Aerial whole plant samples were therefore collected from irrigated field experiments conducted on Norfolk (Typic Paleudults) loamy sand in t980, t98t, and t982, to determine seasonal dry matter, N, P, and K accumulations for corn yielding tO Mg ha-1 or more in the southeastern Coastal Plain. Rates of accumulation were derived by differentiating compound cubic polynomial equations that described seasonal accumulation patterns. Total dry matter accumulation averaged 23.t and 24.9 Mg ha-1 for two population treatments that averaged 7 X t0 4 or tO X t0 4 plants ha-1 • Aerial N, P, and K accumulation respectively averaged 228, 58, and 258 kg ha-1 in t980; 264, 37, and 372 kg ha-1 in t98t; and 225, 37, and 335 kg ha-1 in t982. Grain yields averaged t3.4, 11.7, and tO.~ Mg ha-1 in t980, t98t, and t982, respectively. Lower P accumulations in t98t and t982 were the result of lower grain yields that were apparently caused by excessive K accumulation. Calculated peak dry matter, N, P, and K accumulation rates were 650, tO, 1.6, and 28 kg ha-1 day-1 in this study, compared to rates of 247, 4.5, 0.6, and 3.2 kg ha-1 , respectively, in previous midwestern studies. Peak accumulation rates during both vegetative and reproductive growth stages emphasize that cultural, nutrient, and water management practices must be coordinated to provide a minimum stress production environment for high corn yield.