XANES Speciation of Phosphorus in Reduced-till System: Placement and Source Effect.

Phosphorus (P) management in reduced tillage systems has been a great concern for farmers. Despite various advantages of reduced-till systems, it has been found that surface application of P, mostly in granular forms, leads to an accumulation of P in the surface 0 to 5 cm soil layer and a depletion of available P deeper in the profile. Inconsistent results have been seen regarding effects of fertilizer placement on crop growth and yield. Therefore, knowledge of the dominant solid P species present in soil following application of P fertilizers and linking that to potential P availability would help us to efficiently manage P in reduced tillage systems. X-ray absorption near edge structure (XANES) spectroscopy is an advanced, nondestructive chemical speciation technique that can be used for characterization of chemical species of P in soils. The objective of this research was to understand the influence of placement (broadcast- vs. deep band-P), fertilizer source (granular- versus liquid-P) and time on reaction products of P under field conditions. Changes in soil pH, resin extractable P, total P, and speciation of P were determined at different distances from the point of fertilizer application at 5 week and 6 month after P application (at rate 75 kg/ha) to a soil system that was under long-term reduced tillage. Resin extractable P was lower for broadcast treatments as compared to deep band treatments for both the time periods. Resin extractable P was greater for the liquid P treated soils when compared to the granular P treated soils. Speciation results showed that granular-P fertilizers tended to form vivianite like products whereas liquid forms found to remain in adsorbed-P like forms in soil after 5-wk of application. Over 6 month time period, reaction products of broadcast-granular and liquid fertilizers transformed to apatite-, monetite-, and variscite-like forms while deep band fertilizers continued to remain in adsorbed-P like forms.