'19 CLAY MINERALS AND PHOSPHATE AVAILABILITY: I. ADSORPTION OF PHOSPHATE IONS BY CLAY MINERALS·

Recent investigations.6,12.17 ' Indicate that clay minerals are not only important in cation exchange reactions in soils but they are perhaps a. major factor in governing the availability of phosphate in many 8011s. From these data. it appears that the kind of clay mineral predominating in the soil is of considerable importance. Soils In which montmorillonite predominates have a high cation exchange capacity, while kaolinite clays have a high anion exchange. Kaolinitic soils have a high capacity to remove phosphate from solution and to fix applied soluble phosphate fertilizers. The recovery of superphosphate by crops is frequently not over 20 per cent, and a recovery of less than 3 per cent has been re-ported 12 where data showed the predominating clay mineral present in the soil was kaolinitic In character. The silica-sesquioxide ratio of the colloid is not an entirely reliable index as to the type of clay present but is highly indicative of it. In kao-linitic clays 14 the silica-sesquioxide ratio is approximately 2. Gile 6 re-ports data showing low efficiency of superphosphate when applied to solls in which the colloid present has a low silica-sesquioxide ratio and a much greater efficiency when the ratio was higher. The types of clay minerals prE-sent were not further identified. Gans 4, about twenty-five years ago, suggested a relationship between colloid composition and phosphate avail-ability. Roszmann 16 suggested the possibility of phosphate retention by clays in 1927. Highly weathered soils are very often kaolinitic in character. In many soil series 1.11 more than one type of clay mineral is present. This is to be expected. trom the reports 6. 11. 14. 16 on clay mineral formation. Alexander, et all report the presence of kaoUn in appreciable amounts i