PHOSPHATE FERTILIZERS FOR THE TEXAS BLACKLANDS

Abstract

Field and greenhouse expts. showed that double superphosphate and monammonium phosphate were approximately equivalent in their availability to beans, vetch, oats and ryegrass grown on highly calcareous Houston black clay. Utilization of fertilizer phosphate by beans and vetch increased as the plants approached maturity, while the reverse was true for oats and ryegrass. Field expts. with isotopically labelled superphosphate applied to wheat and vetch clearly demonstrated that surface application was unsatisfactory. Maximum utilization was obtained by mixing the fertilizer intimately with the upper 8 in. of soil. Bandad fertilizer was most efficiently used by wheat when placed at an 8 in. depth, while 2-4 in. placement gave best results with vetch. Radiophosphorus of high specific activity was used to trace the utilization of monosodium phosphate for long periods. With uncropped soil, availability decreased 70% in 7 mos. Under continuous cropping with ryegrass, the magnitude of this decrease was considerably reduced.