The kinetics of precipitation of hydroxyapatite (Ca5(PO4)3OH, HAP) may be important in explaining the oversaturation of many natural waters with respect to pure HAP. Consequently, the objectives of this study were to determine the rate of HAP precipitation in the absence and presence of organic acids commonly found in soil solutions, which might act as potential inhibitors of solid phase precipitation. In well‐stirred solutions, the forward rate of HAP precipitation (R) between pH 7.4 to 8.4 was described with the experimentally derived rate equation: R = skfγ2γ3[Ca2+][PO3‐4], where s = HAP surface area (m2L−1), kf = forward rate constant (L2mol−1m−2s−1), γ2 and γ3 = di‐ and trivalent ion activity coefficients calculated from the Davies equation, and brackets represent concentrations of free hydrated species (mol L−1). The average kf in the absence of organic acids was 173 ± 11 L2mol−1m−2s−1. In the presence of 0.6 mM, 0.68 mM and 0.98 mM CTS (total soluble organic C) added as fulvic, humic, or tannic acid, the kf decreased to 2, 19, and 16 L2mol−1m−2s−1, respectively. The mechanism of precipitation inhibition in the presence of these acids appears to be adsorption of the organic ligand on the HAP seed crystals, thereby blocking sites for crystal growth. Smaller molecular weight organic acids such as citric, gallic, syringic, adipic, and azealic acid were not as effective at inhibiting HAP precipitation per unit CTS present, which may indicate the importance of larger molecular weight acids on actual physical coverage at the HAP surface. The inhibition of HAP precipitation in the presence of humic, fulvic, and tannic acids may explain why many soil solutions are supersaturated with respect to pure HAP, and why increases in P concentrations are observed in many agricultural studies in the presence of sludge or manure.

No abstract

Uptake and partitioning of N, P, and K by Upland cotton (Gossypium hirsutum L.) have been studied, but no such work has included American Pima cotton (G. barbadense L.). Our objective was to describe the N, P, and K uptake and partitioning into various plant parts for two representative Upland and Pima cotton cultivars. Upland ‘Deltapine 90’ (DPL 90) and Pima ‘S‐6’ were grown at two south‐central Arizona locations for 3 yr. Beginning 14 to 20 d after emergence, the aboveground portions of cotton plants were harvested and separated into stems, leaves (including petioles), burs (carpel walls), lint, and seeds. The bur fraction also included squares, flowers, immature bolls, and burs from mature bolls. Total N, P, and K analyses were conducted on each fraction (except lint). Nutrient concentration, uptake, and partitioning by cotton was modeled on a heat unit accumulation basis (HUAP). Up to about 1500 HUAP, leaves were major N sinks, leaves and the bur fraction were major P sinks, and leaves and stems were major K sinks for both cultivars. After 1500 HUAP, the bur fraction and seeds were major N and P sinks and the bur fraction was the major K sink. For DPL 90, the total N, P, and K uptake was 201, 31, and 254 kg ha−1 and for Pima S‐6, it was 201, 32, and 226 kg ha−1, respectively. Nutrient requirements for producing 100 kg lint ha−1 were 15‐2.3‐19 kg ha−1 N‐P‐K for Upland cotton and 21‐3.3‐23 kg ha−1 for Pima cotton.

American Pima cotton (Gossypium barbadense L.) is an extra‐long staple cotton produced in the southwestern USA and in other regions around the world. Pima cotton generally yields less than Upland (G. hirsutum L.), but there have not been any studies conducted to document the basis for these differences. Field trials were conducted at two south‐central Arizona locations from 1990 through 1992 for the purpose of comparing growth and yield between representative cultivars of Upland and Pima cotton. The aboveground portion of Upland ‘Deltapine 90’ (DPL 90) and Pima ‘S‐6’ were harvested and separated into stems, leaves (including petioles), burs (carpel walls), lint, and seeds. The bur fraction also included immature fruiting forms. Dry matter accumulation was modeled as a function of heat units (HU) accumulated after planting (HUAP). Both cultivars exhibited a linear increase of total dry matter over the duration of sampling. By the end of sampling, DPL 90 produced more total, stem, seed, lint, and reproductive (bur + seed + lint) dry matter than Pima S‐6; the dry matter that accumulated in leaves, bur fraction, and vegetative structures (leaf + stem) did not differ. The reproductive/vegetative ratio (RVR) was found to be similar for both cultivars, increasing rapidly with HU accumulation. Comparisons of lint dry matter accumulation as a function of RVR and harvest index (HI) revealed that DPL 90 yields more lint than Pima S‐6 due to a greater total biomass production and more efficient partitioning of dry matter into reproductive organs.

A field experiment was conducted for the purpose of determining the worst‐case potential for solute leaching under furrow‐irrigated conditions, and to assess the spatial variability associated with solute movement. The experiment was carried out on a Mohall sandy loam soil (fine‐loamy, mixed, hyperthermic Typic Haplargid) within a field of upland cotton (Gossypium hirsutum L.), with uniformity of all management factors including fertilizer N and irrigation water. Six mainplot areas were identified throughout the study area to provide characterization of spatial variability. Within each main plot, five subplots (1 m2 each) were further identified. To each subplot area, 20 g KBr was applied in 500 mL of solution using the Br‐ as a biologically conserved tracer. Soil from each subplot was sampled to a depth of 180 cm and separated into 30‐cm depth increments. Soil samples were then subjected to a 1:1 (soil/water) extraction and analyzed for Br‐ and NO‐3‐N. Appreciable amounts of solute movement were measured, with a very high degree of spatial variability. The highest degree of leaching potentials was measured early in the season, when soil water depletions were the lowest and crop root development would not be extended past very shallow regions of the soil profile. The results reinforce the need to split fertilizer‐N applications throughout the course of a growing season, and to base fertilization rates and frequency on properly calibrated plant‐tissue tests.