The Dissolution and Migration of Phosphorus from Granular Superphosphate in Some Michigan Soils

Abstract

AbstractThe purpose of this study was to determine the rate of dissolution and migration of phosphorus from granular superphosphate under different soil conditions. The effect of variation in soil type, soil moisture, compaction, and soluble phosphorus level in soils was considered.Laboratory studies were conducted in which fertilizer granules and tablets were placed in soils for varying time periods, after which the remaining phosphorus in the recovered granule and in the soil was measured by chemical or radioactive isotope techniques.Rapid dissolution of phosphorus from fertilizer granules in contact with moist soil was experimentally verified. At field capacity moisture contents, 50 to 80% of water soluble phosphorus moved out of the granules in 24 hours. Even in soils as low as 2 to 4% moisture, approximately 20 to 50% of the phosphorus moved from the granule into the soil in a one day period. It was found that both commercial and experimental superphosphate 4–8 mesh granules had a strong tendency to absorb moisture when placed in a saturated atmosphere. In relatively dry soils, moisture is drawn from the soil mass toward the granule, thereby forming a moist shell.The degree of soil compaction around the granule affects the rate of fertilizer dissolution. However, differences in compaction were found to have less effect than variations in soil moisture.Placing radioactive fertilizer granules in soils with extremely high available phosphorus contents or in contact with non‐radioactive superphosphate did not markedly reduce the rate of dissolution of granular phosphorus.Migration studies showed that phosphorus moved very little under the conditions studied. Maximum movement of about one inch occurred at soil moistures approximating field capacity on three soils. Most of this movement took place in the first week of a 4‐week period. Compacting soil tended to increase both dissolution and migration of fertilizer phosphorus. The larger the superphosphate granule, the more extensive was phosphorus migration.