Reaction Products of Applied Phosphate in Limed Soils

Abstract

A study was made of the solubility of phosphate in limed soils to which liberal amounts of superphosphate had been added over a long period of time but to which no phosphate had been applied for at least 5 years prior to sampling. The control limed soils to which no phosphate had been applied were also included for comparison. A 20‐g sample of soil was extracted with 200 ml of 0.01M CaCl2 for 108 hours and the concentration of phosphate and pH were determined in the clear supernatant solution after centrifugation. Although the extracts of all soils were found to be undersaturated with respect to hydroxyapatite, most of the extracts were found to be supersaturated with respect to fluorapatite in the presence of solid phase CaF2. By assuming the formation of an ideal solid solution with fluorapatite and hydroxyapatite as end‐members, it was shown that fluorapatite should be the stable phase even in the absence of solid phase CaF2 provided that the F‐ concentration in the soil solution is > 10‐8 M. Consistent with this deducation, the solubility of phosphate in many of the limed soils examined in this study agreed well with that expected for fluorapatite in the presence of 10‐5M F‐ solution, which was the concentration of F‐ found in several of the extracts that were examined. It may be concluded, therefore, that fluorapatite was the ultimate reaction product of the applied superphosphate in these limed soils.

Phosphate fluxes in the soil-plant system: A computer simulation

“…This is especially true when phosphate is being removed by plants and periodically added in the form of fertilizers and decaying plant residues. Such compounds as variscite, strengite, hydroxyapatite, and fluorapatite should be the stable end-products of phosphate reactions in soils, and in certain conditions crystalline minerals do form in soils (Lindsay andMoreno, 1960, Murrman andPeech, 1968). However, these minerals form slowly at normal soil phosphate concentrations and temperatures (Low and Black, 1950, Aslyng, 1954, Lindsay and Moreno, 1960, Larsen, 1967, Wilson, 1968.…”

Suppression of superphosphate-phosphorus fixation by farmyard manure II. Some studies on the mechanisms

“…Octocalcium phosphate seemed to be a fairly stable form in this soil since it was the predominant form in the native soil and it did not convert to hydroxyapatite even after 96 days of soil incubation. Formation of apatite in calcareous soils ( 8,15) may, therefore, be an extremely slow process ( 10 ).…”

“…In calcareous soils, the phosphatic fertilizers are generally converted to less soluble forms such as dicalcium phosphate ( 7,11 ), octocalcium phosphates ( 22 ), and apatite ( 8,15). In the presence of higher organic matter content in soils, the conversion of labile phosphate to Jess available compounds was comparatively slower, The solubility of phosphorus from these compounds was also high, Thus LARSEN (8) reported that organic matter reduced the formation of apatite by 10 per cent and increased appreciably 395 the dissolution of phosphorus from newly formed crystals of apatite.…”

Evidence of Dicalcium Phosphate Precipitation in a Calcareous Soil

“…WATER-SOLUBLE phosphate added to soil quickly disappears from the liquid phase, but it is a matter of conjecture whether the mechanism of this reaction is predominantly adsorption on the surface of the soil particles or a true precipitation of sparingly soluble salts. In calcareous soils the salts most likely to be formed are dicalcium phosphate, octocalcium phosphate, hydroxyapatite, and fluorapatite (As yng, I 954; Clark and Peech, 1955;Lehr and Brown, 1958;Hagin and Hadas, 1962;Murrmann and Peech, 1968). When a dilute phosphate solution or finely divided, soluble phos hate is added to soil, no reaction product has yet been directly detecte!…”