Fluid Fertilizers Improve Phosphorus Diffusion but not Lability in Andisols and Oxisols

Fluid phosphorus (P) fertilizers increase availability of P in calcareous soils compared with granular fertilizers, because of reduced P precipitation. It is unclear whether fluid P forms may also enhance fertilizer P availability in acidic soils. A 35‐d incubation experiment assessed the diffusion, lability, and solubility of P from granular (calcium phosphate and ammonium phosphate) and fluid (ammonium phosphate and polyphosphate) fertilizers in two Andisols and two Oxisols. A calcareous Inceptisol and an Alfisol were included for comparison. Phosphorus diffusion and lability was assessed using a novel visualization technique and isotopic exchange (24‐h equilibration), and residual fertilizer granules were analyzed at the end of the incubation. Most P (90%) diffused out of the granules, except for calcium phosphate fertilizers in the calcareous soil. In the strongly P‐sorbing soils (acidic and calcareous), P applied in fluid form diffused further from the application point compared with granular P. However, in the Andisols and Oxisols, the greater diffusion did not result in greater P lability and solubility; more P remained labile with the granular (34% on average) than with the fluid fertilizer (24%). In contrast, in the calcareous soil, the application of fluid P resulted in enhanced P lability and solubility. These results suggest that in Andisols and Oxisols, where strong adsorption rather than precipitation reactions limit fertilizer P availability, spreading the fertilizer over a larger soil volume by using fluid formulations may reduce P lability. No agronomic benefit over granular products is expected from the use of fluid P fertilizers in these soil types.