Micronutrient Concentrations in Soil Solution After Ammonium Phosphate Applications

Two fluid ammonium polyphosphates, 11-17-0 (11-37-0 oxide basis), a commercial 10-15-0 (10-34-0), and an equimolar mixture of mono- and diammonium phosphate (MAP-DAP), each with a solution pH of about 6.2, were well mixed with 1-kg lots of several soils to supply 2,000 ppm of P, which approximates the P concentration in soil near a fertilizer band at a P rate of 50 kg/ha. Each soil was then moistened to 0.2 or 0.3 atm and stored in plastic bags. After periods up to 28 days, soil samples were placed in a lucite cell and a portion of the soil solution was expressed by N₂ gas. These solutions were analyzed for Cu, Fe, Mn, Zn, organic C, and total and orthophosphate P. Micronutrient concentrations in solutions of acid soils were temporarily increased by both ortho- and polyphosphates. Increases were related in part to the amount of soil organic matter solubilized by these fertilizers. Organic C in solutions of treated soils increased in the order: MAP-DAP, 10-15-0, and 11-17-0, which was related to the original percentage of the total P in polyphosphate form (0, 50, and 78, respectively). Micronutrient concentrations in solutions of acid soils were highest during the first week and decreased to those in untreated soil by 28 days. Soluble micronutrient concentrations in neutral and calcareous soils were not affected by phosphate applications even after 1 day. Thus, solubilization of soil micronutrients by polyphosphates does not appear to play an important role in micronutrient nutrition of crops, especially in calcareous soils. Phosphorus concentrations in soil solution generally were higher in soils treated with ortho- than with polyphosphates. Decreasing polyphosphate concentrations in solutions of polyphosphate-treated soils with time indicated hydrolysis and precipitation of water-soluble polyphosphates; less than 25% of the soluble P was in polyphosphate form after 2 weeks, with higher polyphosphate concentrations found in 11-17-0 than in 10-15-0 treated soils during the first 2 weeks after application.