Rock Phosphate Availability as Influenced by Soil pH

Abstract The influence of the pH of the culture medium on the availability of rock phosphate to oats was investigated by means of two sets of pot cultures. In one set, quartz sand containing 1% of montmorillonite clay served as the culture medium. Samples of the clay, previously saturated with H + , were treated with varying amounts of Ca(OH) 2 to provide a series of samples having pH values of 4.9, 5.5, 6.2, 6.7, and 7.4, respectively. Addition of these clays to quartz cultures imparted their respective pH's to the cultures. In one series, rock phosphate served as a source of phosphorus, and in a second, superphosphate served similarly. All other nutrients were supplied in soluble forms. The yield with rock phosphate was highest at pH 5.5. With superphosphate yields were good throughout and approximately double those with rock phosphate. A soil pH of 6.0 or lower appears to be necessary for satisfactory utilization of rock phosphate. In a second set of pot cultures, Spencer silt loam subsoil of pH 5.1 and very low in organic matter and available phosphorus served as the culture medium. Other nutrients than phosphorus were supplied in available forms. When rock phosphate was applied 1 month prior to liming to pH 7, the oats grew much better and absorbed much more phosphorus than when the order of applications was reversed, showing that strongly acid soils act on rock phosphate rather rapidly, and that for rock phosphate to be effective acidity is needed. Thus, it is advisable to apply rock phosphate a year or more in advance of liming.