This work involved studies of 12 phosphate rocks (PRs), four each of low, medium, and high reactivity, obtained from various deposits (Kodjari, Tahoua, North Carolina, Gafsa, Khouribga, Tilemsi Valley, Central Florida, Sechura, Minjingu, North Florida, Hahotoe, and Parc W). The reactivity of the rocks was estimated by four methods (neutral ammonium citrate first and second extractions, ammonium citrate pH 3, 2% formic acid, and 2% citric acid). The unit cell a dimensions and the empirical formulae of the rocks were calculated from x-ray diffraction data and statistical models of francolites; the results agreed closely with those published. The concentrations of nontrace metals (Al, Ba, Ca, Fe, K, Mg, Na, and Sr), and of many trace metals (Cd, Co, Cu, Cr, Li, Mn, Ni, Pb, Rb, and Zn) varied considerably in the PR samples. Laboratory studies on the release of P with 19 low-molecular-weight organic acids containing mono-, di-, and tri-carboxylic groups and three mineral acids (HCl, HNO[subscript]3, and H[subscript]2SO[subscript]4) showed that the effectiveness of the organic acids to release P is determined by the chemical structure, type and position of the functional group of the ligand, and by the relative stability of the metallo-organic complexes formed. The monocarboxylic acids were less effective than the dicarboxylic, which, in turn, were less effective than the tricarboxylic acid group, with the exception of oxalic acid which was equally effective as sulfuric acid. Analysis of the rocks before and after free carbonate removal showed that more P was released from the low reactive PRs than from the high reactives. Removal of free carbonate from the rocks significantly altered the relationship between P and Ca and Mg released. Laboratory studies on the release of P from PRs added to three Iowa surface soils (Gosport, Clinton, and Grundy soils) with or without organic acids and incubated at 25°C for 15, 30, and 45 days showed that the percentage of plant-available P (P[subscript] i test) released from the rocks varied considerably and was related to the reactivity of the rocks. Incubation time of 30 days corresponded in many cases to the maximum percentage of P released. A greenhouse study with corn (Zea mays L.) grown for 60 days on three soils treated with six PRs (Kodjari, North Carolina, Gafsa, Tilemsi Valley, Central Florida, or Hahotoe) mixed with or without organic acid (oxalic or citric) showed that application of organic acids to soils did not significantly increase the uptake of native P by plants. Corn response (dry matter and P yields) to addition of oxalic or citric acid varied with P rates and PR sources, suggesting that organic acids have potential as amendments for increasing plant-available P in PRs added to soils.
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