PHOSPHORUS AND MICRONUTRIENT AVAILABILITY FROM DUAL APPLICATION OF NITROGEN AND PHOSPHORUS USING LIQUID FERTILIZERS

Applications of ammoniacal forms of N along with P in the same injecti on zone can enhance P availability relative to applications of P alone . The spatial and temporal variability of available P was assessed in acid soil and in alkaline soil, using ammonium polyphosphate (APP) as a P source and urea ammonium nitrate (UAN) as a N source. Either ammon ium polyphosphate or APP+UAN, at a constant P application rate, was in jected into soil-filled boxes using an apparatus that allowed a needle to be pulled through the soil below the surface, simulating subsurfac e band application. Soils boxes were sampled by sequentially removing concentric cylindric cores of increasing radii centered around the lin e of fertilizer injection. Sampling times were 1, 4, 12, 24, and 36 we eks. Available P was determined by either the Bray-1 or the Olsen proc edure. Available Zn, Fe, Cu, and Mn were also determined with the DTPA procedure. For both soils, soil pH generally decreased in the first 1 2 weeks and then increased during the remaining incubation period. Tre atment comparisons within rings over time showed that APP+UAN generall y gave significantly higher extractable P when compared with the APP t reatment. Total available P, calculated as the sum of the products of soil dry mass and extractable P for each ring, was significantly highe r for APP+UAN compared with APP at all sampling times. The decline in total available P with time was best described by y = a(e(-bt) + c), w here y is the total available P (mg) at time t (weeks) and a, b, and c are curve-fitting parameters. The value of b was greater for the APP treatment for both soils, indicating that total available P declined f aster for this treatment than for the APP+UAN treatment. Treatment eff ects on DTPA-extractable Fe and Mn generally corresponded to changes i n soil pH with concentrations increasing with decreasing pH. For DTPA- extractable Zn and Cu, however, concentrations were often lower for th e treatment with the lowest pH.