Availability of phosphate rocks in soils of varying degrees of acidity

The results show very clearly that there is not a close relationship between soil acidity and the increase in the availability of phosphorus in phosphate rocks. For example, with phosphate rock 451 in only one instance of higher acidity, pH 5.80, was there as much phosphorus absorbed as from the soil with a pH of 7.14. Similarly, sample B-14 was an ineffective source of phosphorus, whereas sample 943 supplied sufficient available phosphorus for the plants to make fair growth. Although there is no close relationship between increased soil acidity and availability of phosphorus in phosphate rocks, it appears that there is some tendency for the availability of phosphorus in phosphate rocks to increase with an increase in soil acidity, but even this will not be true in all cases. For example, the results from phosphate rock 451 show a decrease in availability at all degrees of acidity excepting pH 5.80. In addition, all but sample 985 show a decrease in relative availability at pH 4.76. It appears evident from the results of the growth studies and from the amount of phosphorus absorbed by the plants that it is only in extremely acid soils that the acidity of the soil may increase the relative availability of the phosphorus in phosphate rocks. Furthermore, from the results presented, rock phosphate cannot be recommended indiscriminately as a fertilizer on acid soils similar to those used in these experiments, since the soil acids have not generally increased the amounts of phosphorus absorbed by plants. The phosphate rocks used in this experiment were selected to determine if the soil acids would increase availability of phosphate rocks in spite of their fluorine content, which has been shown previously to be associated with the availability of phosphorus in phosphate rocks. The results reported also show that the availability of the phosphorus in phosphate rocks is very closely associated with their fluorine content. The results in Table 6 are arranged in the order of decreasing fluorine content of the phosphate rocks, sample 943 having the smallest percentage of fluorine and sample B-14 having the largest. If the results under each degree of acidity are examined, it will be noted that regardless of the degree of acidity, there is a general trend for the availability of the phosphorus in the phosphate rock to decrease as the percentage of fluorine in the rock increases. In appears evident, therefore, that only those phosphate rocks low in fluorine content should be used as direct phosphate fertilizers. The fact that availability of phosphate rocks seem to be closely associated with the fluorine content of the rock may explain some of the variations which have been found in the availability of rock phosphate by different investigators. An experiment conducted with a phosphate rock having a high percentage of flourine, such as sample B-14, would not show "rock phosphate" to be anywhere near as efficient as a source of phosphorus as phosphate rock 943 or 985.