Laboratory, Greenhouse, and Field Studies With Mixed Fertilizers Varying in Water-Soluble Phosphorus Content and Particle Size

Greenhouse and field studies were undertaken to evaluate the effect of the water-soluble phosphorus content and particle size of mixed fertilizers on the availability of fertilizer phosphorus to plants. Supplementary laboratory experiments were conducted to study the behavior of phosphorus in granulated mixed fertilizers under various soil conditions. The rate at which the water-soluble fraction of phosphorus moved out of fertilizer granules was rapid, essentially being complete in 48 hours. No important difference in dissolution rate was noted for fertilizers with varying water-soluble phosphorus contents over the range of 3.3 to 15% moisture. The extent of movement of fertilizer phosphorus away from large granules and the concentration in soil around the phosphate was directly related to the percentage of water-soluble phosphorus in the fertilizer. Varying the water-solubility of phosphate fertilizer having a similar N-P₂O₅-K₂O ratio had no significant effect on dry weight yield of wheat plants grown in the greenhouse. The percentage of plant phosphorus derived from fertilizer was markedly influenced by the water-soluble phosphorus content and the particle size of the mixed fertilizer and soil reaction. Absorption of fertilizer phosphorus by wheat plants from 4–6 mesh granular material was directly proportional to its water soluble phosphorus content. For pulverant fertilizers the phosphorus water solubility had very little effect on the percentage of plant phosphorus derived from fertilizer. At a soil pH of 5.5, wheat absorbed a larger quantity of fertilizer phosphorus than when grown at pH 6.5 or 7.5. The phosphorus content of field grown sugar beet plants was significantly higher 1 month after planting on plots where fertilizers high in water-soluble phosphorus were used as compared to those from plots treated with fertilizers low in water-soluble phosphorus. Two months after planting, values for total phosphorus were similar for plant samples from plots receiving fertilizers either high or low in water-soluble phosphorus. Although the yield response to phosphorus was highly significant, there were no significant differences in sugar beet yields between fertilizer treatments.