Comparative availabilities of organic and inorganic phosphates as shown by the Neubauer method
1941
The availabilities of ten organic phosphates in comparison with common inorganic phosphate fertilizers were determined by the Neubauer method in quartz sand, Norfolk sandy loam, and Chester loam. A few phosphates were also tested in Cecil clay loam and Las Vegas loam. The organic phosphates studied were glycerophosphate, hexose diphosphate, nucleic acid and its sodium salt, monophenyl and diphenyl orthophosphates, diphenyl pyrophosphate, and monoethyl, diethyl, and triethyl phosphates. The inorganic phosphates used for comparative purposes were mono-, di-, and tricalcium phosphates, hydroxyapatite, calcined phosphate, and sodium metaphosphate. In any given soil or sand all of the organic phosphates, with the exception of triethyl phosphate, showed availabilities similar to those of the common inorganic phosphates. Triethyl phosphate which does not hydrolyze in aqueous solution showed a negligible availability as a plant nutrient. The phenyl phosphates, particularly the monophenyl phosphate, were toxic in the lighter soils and at the higher concentration. The results as a whole emphasize the prime importance of the soil itself in determining the ability of plants to utilize added organic and inorganic phosphates. Since, as previous results (7) have shown, most organic phosphates are fairly rapidly converted into inorganic phosphates, their behavior in plant nutrition studies is usually not markedly different from that of ordinary inorganic phosphates. In the case of both types of materials, a high content of colloid or clay, especially if iron oxides and aluminum oxides are abundant, leads to marked phosphate fixation.
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