Density changes around phosphorus granules and fluid bands in a calcareous soil

Abstract

We employed x-ray computed microtomography (X-ray CT) to observe differences in moisture around fertilizer P granules (monoammonium phosphate, MAP) versus injection zones of fluid P fertilizer (technical grade monoammonium phosphate, TG MAP) in a calcareous soil over time. X-ray CT allows nondestructive visualization of small columns containing soils and fertilizers. We were able to visualize the increase in density around the highly hygroscopic fertilizer granule over time. It appeared that both water flow toward the granule and precipitation of P could be responsible for the development of about 1 mm thick high density zone immediately adjacent to the granule. The mass flow of water toward the granule may have slowed or restricted the diffusion of fertilizer P from the granule, thus increasing the chances for P fixation through precipitation reactions. Also, the granule became less dense with time indicating the progress of granule dissolution. In contrast, injection of fluid fertilizer (TG-MAP) in soil did not result in moisture changes over time as evidenced by a lack of X-ray CT detectable density differences in the soil column. These data support previous findings that, when P is supplied in granular form, P diffusion and isotopic lability in calcareous soils are reduced compared with equivalent liquid fertilizer formulations, probably due to precipitation reactions induced by osmotically induced flow of soil moisture into the fertilizer granule.