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Title: Interactions between phosphate adsorption and cation adsorption by soils and implications for plant nutrition 
Author: Stoop, Willem Adriaan
Date: 1974
Abstract: Interactions between phosphate adsorption and cation adsorption were studied in four Hawaiian soils. These belonged to the following soil groups: Hydrandept, Gibbsihumox, Eutrustox and Haplustoll. Relevant ion adsorption mechanisms were investigated by electrochemical techniques and direct adsorption measurements by means of adsorption isotherms. Implications for the availability of P, Ca, Mg and K for plants were investigated in pot experiments. Interactions between phosphate adsorption and cation adsorption were important in highly weathered soils containing mainly variable charge clay minerals such as Fe and Al hydrous oxides. These soils could be characterized by their zero point of charge (ZPC). The ZPC will increase with increasing contents of oxidic colloids. Consequently the adsorption of anions, particularly phosphate, will increase. For the soils in this study ZPC and P adsorption decreased in the following order: Hydrandept > Gibbsihumox > Eutrustox > Haplustoll Calcium was adsorbed specifically by the hydrous oxides present in the highly weathered soils, especially the Hydrandept and Gibbsihumox. The ZPC value of oxidic soils therefore should be determined in CaCl2 solutions to prevent the interference from adsorbed Ca in soil samples. This interference did affect ZPC measurements with NaCl or Na2SO4 as supporting electrolytes since the non-specifically adsorbed Na ion does not compete with Ca adsorbed in the Stern layer. Consequently no ZPC was determined but an isoelectric point (IEP) which refers to the charge o at a distance of several A units away from the colloid surface. A similar problem applied to the extraction of exchangeable Ca by NH40Ac. Non-specifically adsorbed NH4+ ions did not compete effectively with specifically adsorbed Ca, causing incomplete Ca extraction from these soils. With decreasing ZPC values cation adsorption became relatively more important than anion adsorption. Consequently, adsorbed phosphate, which lowered the ZPC, was associated also with increased cation adsorption. The adsorption of 700 ppm P by the Gibbsihumox lowered the ZPC of that soil from 4.10 to 3.85 and increased the cation adsorption by 1.35 me/100g soil at pH 5.5. This amounts to 0.6 me/mmol P adsorbed/ 100g soil. The increase in cation adsorption was more pronounced for specifically adsorbed divalent cations than for non-specifically adsorbed monovalent cations. Because of specific Ca adsorption, applications of NH 4 phosphates had greater effects on the adsorption of other cations (Na, K, NH4 , Mg, Ca) than had Ca phosphates. Hence if all P in these highly weathered soils was applied as Na(or NH4) phosphates, deficiencies of Mg, Ca and certain trace elements could be induced. In addition dispersion of inorganic and organic fractions occurred in the Gibbsihumox. Phosphate solubility in the oxidic soils depended mostly on salt concentration and cation valency. It was decreased by the accompanying cation according to a lyotropic series: K < Mg ≤Ca. This cation effect on P solubility decreased with increasing contents of layer silicate clay minerals and/or increasing base saturation of the soil. Consequently it was almost absent in the Haplustoll, which contains montmorillonite and kaolinite. The cation effect on P solubility has direct practical implications for oxidic soils because it affects P availability to plants at low P application rates, as was demonstrated by pot experiments with the Eutrustox. Increasing Ca concentrations were significantly related to lower P solubility and as a result lower P uptake and lower yields of Sudangrass. Consequently the Ca : P molar ratio in applied fertilizers has implications for P and also cation availability. It was demonstrated that by manipulating the Ca : P ratio one could increase nutrient (P, K and Mg) uptake and yields of Sudangrass and decrease the leaching losses of cations (Na, K, NH4 , Mg, Ca). As a result the efficiency of applied fertilizers can be improved considerably.
Description: Typescript. Thesis (Ph. D.)--University of Hawaii at Manoa, 1974. Bibliography: leaves 191-204. xvii, 204 leaves ill
URI: http://hdl.handle.net/10125/12038
Rights: All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Keywords: Phosphatic fertilizers, Cations, Soil absorption and adsorption, Plants -- Nutrition

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