Sorbed and Solution Phosphorus and Their Relationships to Crop Response
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1971
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University of Hawaii at Manoa
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Adsorption of phosphorus from aqueous solutions by several Hawaiian and Indian soils was studied in relation to P concentration, equilibration time, ionic environment and temperature. The effect of a prior P application on the sorption of P subsequently applied was studied using samples of a calcareous Superstition sand, previously used in a pot experiment in which P was a variable. The phosphate sorption curves were used for estimating the P requirement of soils for maximum crop growth of pearl millet (Pennisetum typhoides) and Desmodium (Desmodium intortum). To approximate equilibrium it was necessary to allow six and eight days reaction time for acid soils and calcareous montmorillonitic soils respectively. Substitution of K for Ca in the equilibrating solution always resulted in increased P adsorption. Also an increase in the salt concentrations (KCl and CaC12) increased P retention by soils. Phosphate sorption isotherms suggested that for non-calcareous soils at low solution P concentrations, P sorption was limited mostly to monolayer adsorption by P reactive sites. At higher concentrations an abrupt increase in P retention occurred. The shape of the isotherms suggested either presence of groups of P reactive sites which are energetically different and/or multilayer adsorption. In calcareous soils at low solution P concentrations the mechanism of P retention seems to be adsorption, with precipitate formation, probably CaHPO4, becoming important as P concentration in solution increased. Provided the equilibration time is long, phosphate added to a calcareous soil, even in low amounts, may form nuclei for additional P precipitation. Almost all of the P adsorption and desorption isotherms followed the Langmuir equation. This enabled the calculation of P adsorption maxima, which were found to be a good means of expressing the P buffering capacity of soils. Thermodynamic studies showed the differential heats of P adsorption to vary after 0.8 fractional saturation of P reactive sites. Phosphate sorption curves were used as a basis for fertilizing soils on which millet was grown. The equilibrium solution P concentrations at which 95% maximum yields occurred varied with soils. Still it might be possible to use a general solution concentration value (0.6 ppm) for most of the agricultural soils as their P adsorption maxima are usually < 2500 ppm. The percentage saturation of the P adsorption maximum for 95% yields of millet was inversely related to the adsorption maximum. Yields of Desmodium in comparison with millet showed an Interaction existing between crops and adjusted solution P concentrations required for maximum yields in different soils.
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