A simple method for determination of ammonium in semimicroKjeldahl analysis of soils and plant materials using a Tecator or Technicon 40-tube block digester is described. It involves use of an inexpensive steam distillation apparatus that permits direct distillation of ammonium from the tubes used for Kjeldahl digestion in 40-tube block digesters. The method is rapid and precise, and it gives results that agree closely with those obtained by the customary method of ammonium N analysis involving transfer of the Kjeldahl digest before distillation.

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Low silicon (Si) content in rice, sugarcane and other Si-accumulating crops can adversely affect crop performance because it creates reduced tolerance to both abiotic and biotic stresses. Assessing the Si status of a crop typically depends upon accurately measuring Si accumulation in plant tissue. Methods involving wet digestion of plant tissue followed by colorimetric determination of Si have proven attractive because they are both rapid and do not require costly, specialized instrumentation. Some popular wet-digestion methods are reported to provide highly variable and inconsistent results. A systematic study to identify and address the sources of variability associated with wet digestion for Si analysis found that modifications that reduce excessive foaming during wet digestion of plant tissue in strong alkali and peroxide significantly reduce variability. Unstable and variable color development associated with the molybdenum blue reaction is a concern in the colorimetric determination of Si in digests. Experiments showed that the inclusion of ammonium fluoride to facilitate the release of polysilicic acid prior to colorimetric determination results in reproducible readings that stabilize within 60 min and remain stable for at least 5 h. With this modification, the accuracy and precision of values obtained colorimetrically are comparable or superior to those obtained by inductively coupled plasma-optical emission spectroscopy (ICP-OES) analysis. A two-phase wet-digestion procedure is described for Si tissue analysis that is robust, accurate, and precise and requires equipment commonly found in most agricultural laboratories.

Increased understanding of the fate of N in cotton (Gossypium hirsutum L.) fields will improve N efficiency, optimize crop development and yield, and may help to avoid excessive N fertilization. This study quantified the effects of N fertilization rate (0, 84, and 168 kg ha−1) on seasonal uptake and partitioning of N and dry matter in field‐grown cotton during 1989 and 1990. The N rates studied were part of a larger experiment initiated in 1987 where plots received annual preplant applications of 0, 28, 56, 84, 112, 140, or 168 kg N ha−1 Experiments were conducted on Commerce silt loam (fine‐silty, mixed, thermic, nonacid, aeric Fluvaquent) on the Louisiana State University Agricultural Center Northeast Research Station near St. Joseph, LA. Plants were collected at five dates in each year at about 20‐d intervals. Shed plant debris was also collected. The N content and aerial biomass of plant components were determined. Maximum N uptake occurred between 49 and 71 d after planting and was 2.9 and 4.3 kg ha−1 d−1 for cotton receiving 84 and 168 kg N ha−1, respectively. At maturity, plants receiving 84 kg N ha−1 contained 160 kg N ha−1 in aerial biomass and an additional 50 kg N ha−1 in abscised plant debris. The total amount of N assimilated by plants receiving 84 kg fertilizer N ha−1 averaged 111 kg N ha−1 more than plants receiving no fertilizer N for an apparent fertilizer efficiency greater than 100%. By the end of effective bloom, plants receiving 168 kg N ha−1 assimilated 15 to 40% more N, primarily in leaves and lower bolls, than plants receiving 84 kg N ha−1 This excess assimilated N was recovered in surface litter and in N‐enriched plant components. Of the plant components studied, leaf‐blades most consistently reflected the amounts of fertilizer N applied.

Loess soils of the Midsouth USA are easily eroded, contain little organic matter, and are drought susceptible. Conservation tillage and cover crops may increase cotton (Gossypium hirsutum L.) yields on these soils. A field study was conducted from 1995 through 2001 on Gigger silt loam (fine‐silty, mixed, thermic Typic Fragiudalfs) to study effects of tillage, cover crops, and N rates on yield and fiber properties. Cotton was grown without tillage (no‐till) and with surface tillage (surface till) following annual winter cover crops of wheat (Triticum aestivum L.), hairy vetch (Vicia villosa Roth), or volunteer winter vegetation (native) and with fertilizer N rates of 0, 39, 78, 118, or 159 kg ha−1. Tillage, cover crop, and N rate affected cotton yield. The till × N rate, till × cover crop, and cover crop × N rate interactions were significant. Yields of cotton receiving zero fertilizer N were 28 to 49% higher under surface till than under no‐till. In contrast, cotton receiving optimum N produced 5 to 11% higher yields with no‐till than with surface till. Following native cover, 118 kg N ha−1 optimized cotton lint yields under no‐till (1249 kg ha−1) whereas only 78 kg N ha−1 was necessary to optimize yields under surface till (1117 kg ha−1). Following wheat, 118 kg N ha−1 optimized lint yields under both no‐till (1246 kg ha−1) and surface till (1185 kg ha−1). Following hairy vetch, no fertilizer N was needed to optimize yields under no‐till (1238 kg ha−1) and surface till (1160 kg ha−1). Tillage, cover crop, and N rate did not appreciably compromise or improve fiber quality. After 7 yr, lint yields did not decrease in systems employing no‐till and cover crops, and combined with optimal fertilizer N, these conservation practices increased cotton yields.