Soil properties and weather conditions are known to affect soil N availability and plant N uptake; however, studies examining N response as affected by soil and weather sometimes give conflicting results. Meta‐analysis is a statistical method for estimating treatment effects in a series of experiments to explain the sources of heterogeneity. In this study, the technique was used to examine the influence of soil and weather parameters on N response of corn (Zea mays L.) across 51 studies involving the same N rate treatments that were performed in a diversity of North American locations between 2006 and 2009. Results showed that corn response to added N was significantly greater in fine‐textured soils than in medium‐textured soils. Abundant and well‐distributed rainfall and, to a lesser extent, accumulated corn heat units enhanced N response. Corn yields increased by a factor of 1.6 (over the unfertilized control) in medium‐textured soils and 2.7 in fine‐textured soils at high N rates. Subgroup analyses were performed on the fine‐textured soil class based on weather parameters. Rainfall patterns had an important effect on N response in this soil texture class, with yields being increased 4.5‐fold by in‐season N fertilization under conditions of “abundant and well‐distributed rainfall.” These findings could be useful for developing N fertilization algorithms that would prescribe N application at optimal rates taking into account rainfall pattern and soil texture, which would lead to improved crop profitability and reduced environmental impacts.

The rate of corn (Zea mays L.) root growth in the field and root distribution in the soil as related to stage of plant growth has not been studied in detail. To obtain more information we measured the length, fresh weight, and distribution of corn roots at time intervals between planting and harvest in 1970 and 1971. The study was made on corn growing on Chalmers silt loam soil at Lafayette, Indiana. Grain yields were 6,160 kg/ ha in 1970 and 11,700 kg/ha in 1971. Root length and fresh weight increased rapidly for 80 days following planting, remained relatively constant for 14 days, and then decreased rapidly when the plants were in the reproductive stage. A maximum root density of 4.1 cm/cm3 occurred in the 0 to 15 cm zone at 79 days. The lower soil zones reached maximum root density 1 to 2 weeks later than in the 0 to 15 cm zone. Root density in the 0 to 15 cm zone was greater in cores taken midway between plants in the row than at other locations. Maximum root length was 153 cm/cm2 of surface area at 86 days.

High rates of N loss have been observed from N fertilizers applied directly on the surface in no‐till corn (Zea mays L.) production systems. Field experiments were conducted at four locations over a three‐year period to determine what effects N source and N placement had on N losses in both no‐till and conventional till corn production systems. Soils used were: Stoy loam, an Aquic Hapludalf; Clermont silt loam, a Typic Ochraqualf; Avonberg silt loam, an Aeric Fragiaqualf; Chalmers silty clay loam, a Typic Argiaquoll; and Lyles fine sandy loam, a Typic Haplaquoll. Nitrogen sources used were anhydrous ammonia (NH3), urea‐ammonium nitrate solutions (UAN), solid urea and solid ammonium nitrate (NH4NO3). Placement variables used were injection of NH3 and UAN 20 cm below the soil surface and broadcasting UAN, urea and NH4NO3 on the soil surface with no incorporation. Nitrogen rates used were 0 and 165 kg N/ha. Injecting NH3, or UAN below the surface resulted in consistently higher corn grain yields than applying UAN, NH4NO3 or urea directly on the soil‐residue surface. Percent N in leaf and grain also reflected an increase in N use efficiency with subsurface N placement. Percent N in leaf was significantly higher where NH3 or UAN were injected as compared to UAN or urea surface applied.

Information on changes in the rate of nutrient uptake per unit of plant root length (nutrient flux) during the growth of the plant is important for evaluating the capacity of the soil to supply sufficient nutrient to the root surface. Since little information was available, nutrient flux into the root was determined in the field by sampling corn (Zea mays L.) plants at 9 and 14 dates in 1970 and 1971, respectively. Root length, root fresh weight, shoot dry weight, and content of 10 nutrients in the shoot were determined on replicated samples at each harvest. Nutrient uptake rates per plant per day were calculated and divided by mean root lengths to get nutrient flux into the root as related to plant age. Nutrient flux was greatest at the first sampling, decreased rapidly with increased plant age until plants were 70 days old, and then remained relatively constant. Phosphorus flux into the root was 11.3 μmoles per meter of root per day at 20 days and 0.08 μmoles m−1d−1 at 80 days. Mean flux into the root was similar in both years despite large grain yield differences between years. As the flux may be biased downward with older plants because of lower uptake rates by more mature tissue, efforts were made to overcome the bias by assuming that only roots less than 5 days old actively absorbed nutrients. Flux calculations on this basis still decreased as the plant developed, but the differences were smaller.

No abstract