The measurement of NH4 + -N in soil, and plant digests is one of the greatest needs in laboratories conducting agricultural and environmental research. Many laboratories do not have access to automated equipment for colorimetric analysis of soil and plant digests. The objective of this research was to modify an automated colorimetric analysis procedure for determining NH 4 +-N in soil and plant digests for manual use, and compare the proposed technique with the standard distillation-titration technique. The modified 961 Copyright© 1989 by Marcel Dekker, Inc. Downloaded by [University of Montana] at 18:13 13 December 2014 962 BAETHGEN AND ALLEY procedure is based on the color reaction between NH 4 + -and a weakly alkaline mixture of Na salicylate and a chlorine source in the presence of Na nitroprusside. Wavelength scans indicated a very well defined peak for determinations at 650 nm. Time scans showed that color development in the manual procedure was rapid, 12 to 40 minutes depending on temperature, and that the color development remained stable for at least 120 minutes. Regression analysis of the results from 18 soil and 20 plant tissue sample determinations by distillation-titration and the proposed method indicated NH 4 + -N recoveries of 99% or higher. The results obtained using the colorimetric procedure were very similar to the values obtained by distil ling and titrating the digests for both soil and plant samples as indicated by the large coefficients of determination (R 2 = 0.99).
Information to determine optimum row spacing and seeding rates for soft red winter wheat (Triticum aestivum L. em Thell.) in high‐yielding environments in the mid‐Atlantic Coastal Plain region is not available. Three field experiments were conducted in the Coastal Plain of Virginia (on Typic Hapludult and Aquic Quartzipsamments soils) to evaluate row spacing and seeding rate influences on grain yield of winter wheat grown with intensive management. Seeding rates ranging from 186 to 558 seeds m−2 were studied in row spacings of 10 and 20 cm with additional treatments of 744 and 1116 seeds m−2 in 10‐cm row spacings. Intensive management included adequate supplies of P, K, and micronutrients as preplant fertilizer, insecticide, and fungicide use, and split‐spring N applications. Measurements included wheat grain yield and yield components. Ten‐centimeter row spacings produced 0.6 to 0.8 Mg ha−1 higher grain yields than 20‐cm row spacings at similar seeding rates. Yield levels ranged from 5.0 to 8.1 Mg ha−1. Heads m−2 and kernels head−1 varied with seeding rate to a greater extent than weight kernel−1. Yields did not vary over a wide range of plant populations, but were reduced with low (469 heads m−2) or high (897 heads m−2) harvest populations. Seeding rates of 372 to 744 seeds−2 in 10‐cm rows were sufficient to produce high yields with the ‘Tyler’ cv. grown under intensive management.
A recently developed system to predict the optimum N fertilizer rate for winter wheat (Triticum aestivum L. emend. Theil) at Zadoks growth stage (GS) 30 is based on the relationship between measured economic optimum N rate at GS 30 and wheat tissue N content measured at GS 30. However, winter wheat often needs an earlier application of spring N to achieve optimum yield. We therefore developed a test to determine which fields need this earlier (GS 25) application, and to predict the optimum N rate with split‐application management using the tissue test or as a single spring application. The optimum N rate at GS 25 was measured over 5 yr, both with and without GS 30 N applications. These measured optimum N rates were regressed against a variety of possible predictor variables measured in the same fields. Tiller density at GS 25 was a good predictor of optimum N rate at GS 25 in a split spring application program. Using this relationship along with the GS 30 tissue test to make N recommendations for winter wheat increased estimated profit relative to using the tissue test alone. Soil NO3 measured to 0.9 m depth was the best predictor of optimum N rate at GS 25 when that is to be the only spring N application, and improved estimated profit relative to applying 90 kg N ha−1 at all sites; however, the economic performance of split spring N applications was substantially better than for any single spring applications. The recommendation system developed by integrating these component relationships is powerful and flexible, and provides field‐specific N rate recommendations for all spring N applications to winter wheat, regardless of management decisions about splitting spring N applications.
Variable rate technology enables management of individual soil types within fields. However, correct classification of soil types for mid‐Atlantic coastal plain soils are currently impractically expensive using an Order I Soil Survey, yet variable rate fertilizer application based on soil type can be highly effective. The objectives of this study were to determine if apparent electromagnetic conductivity (ECa) alone or combined with previous year crop yields using global positioning system technology can provide a useful alternative to detailed soil mapping. The site contained alluvial soils ranging from Bojac 1 and 2 (coarse‐loamy, mixed, thermic, Hapludults) to Wickham 3 and 4 (fine‐loamy, mixed, thermic, Ultic Hapludalfs). The two fields totaled approximately 24 ha. A statistical nonparametric classification method, called recursive binary classification trees, was used to determine how well soil types could be classified. Electromagnetic conductivity readings and crop yields were positively correlated. Broad patterns in the relationship between soil types and ECa readings and crop yields existed for all crop combinations considered. Lower ECa readings and crop yields corresponded to the Bojac soils, while higher ECa readings and crop yields were categorized as Wickham soils. Electromagnetic induction alone correctly classified the soils into broad categories of Bojac or Wickham with over 85% accuracy. When ECa was combined with crop yield data, correct classification rose to over 90%. More precise classification into Bojac 1, Bojac 2, and Wickham soils yielded slightly lower correct classifications ranging from 62.6 to 81.2% for ECa alone, and 80.3 to 91.5% when combined with various crop yields.
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