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Fluid Journal : Fluid Journal 2005-2007
than one inch below the surface for the 0+50+0 treatment, and at approximately one-half inch for the 17+50+17 treatment (Figure 1). In the spring of 2005, little P was extracted by the exchange membranes placed in the soil 15 weeks after application (data not shown). When the measurements were made, soil water content ranged from less than 8 percent to about 10 percent, which is well below the estimated field capacity (measured at --33kPa) water content of 27.4 percent. With such low soil water content, P diffusion to the exchange membrane would be limited. The final 2005 measurements were made 19 weeks after application, when soil water content was somewhat higher. Higher concentrations of bioavailable P were measured, mainly in plots that received the dribble band application of 0+50+0 (Figure 1, bottom row). Even with three replications, and a relatively uniform fertilizer band, additional measurements may be necessary to identify areas with more bioavailable P, especially when measurements are made several months after the fluid fertilizer is applied. Based on the data collected thus far, the movement of P into the soil profile should be similar for both the ortho- and polyphosphate materials. The precipitation reactions of ortho-P, which occur more slowly when APP fertilizer sources are used, had little effect on the mobility of the P applied to this central Iowa soil. Summary Both ortho- and polyphosphate fluid sources increased soil concentrations of bioavailable P with depth. The highest levels were found three weeks after fall application, but could be detected the following spring. Soil water content plays a significant role in the ability of exchange resin membranes to capture P and reveal soil P distribution. These results emphasize the agronomic acceptability of both surface starter P applications close to the row and fall applications of P after harvest. Dr. Kovar is soil scientist at the USDA/ ARS National Soil Tilth Lab in Ames, IA
Fluid Journal 2002-2004
Fluid Journal 2008-2009