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

Abstract

Applications of ammoniacal forms of N along with P in the same injection 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 ammonium polyphosphate or APP + UAN, at a constant P application rate, was injected into soil-filled boxes using an apparatus that allowed a needle to be pulled through the soil below the surface, simulating subsurface band application. Soils boxes were sampled by sequentially removing concentric cylindric cores of increasing radii centered around the line of fertilizer injection. Sampling times were 1, 4, 12, 24, and 36 weeks. Available P was determined by either the Bray-1 or the Olsen procedure. Available Zn, Fe, Cu, and Mn were also determined with the DTPA procedure. For both soils, soil pH generally decreased in the first 12 weeks and then increased during the remaining incubation period. Treatment comparisons within rings over time showed that APP + UAN generally gave significantly higher extractable P when compared with the APP treatment. Total available P, calculated as the sum of the products of soil dry mass and extractable P for each ring, was significantly higher 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), where 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 faster for this treatment than for the APP + UAN treatment. Treatment effects on DTPA-extractable Fe and Mn generally corresponded to changes in soil pH with concentrations increasing with decreasing pH. For DTPA-extractable Zn and Cu, however, concentrations were often lower for the treatment with the lowest pH