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Fluid Journal : Fluid Journal 2005-2007
14 Fluid Journal FALL 2006 DR. JOHN KOVAR SUMMARY In previous studies, we used exchange resin membranes to evaluate the distribution of bioavailable P in the soil profile after a surface dribble application (P2O5 at 30 lbs/A) of ammonium polyphosphate (APP) starter fertilizer. Forty-three days after application in 2002, the highest concentration of bioavailable P was found more that four inches below the surface for a 15+30+10 starter treatment. At 68 days after application, higher P concentrations were measured at a depth of less than two inches below the surface for the 15+30+10 treatment, and more than three inches below the surface for a 60+30+10 treatment. The research was expanded in 2003 to include an orthophosphate (0+30+0) Fall Surface-Applied Fluid P Movement Into Soil Limits Potential Loss to Erosion Two years of measurements show that fall-applied fluid P moves into the soil, limiting losses and maintaining relatively higher levels of bioavailable P in top six inches of soil. Soil analyses in the spring of 2004, 20 and 23 weeks after band application of fluid P sources the preceding fall, showed that relatively higher levels of bioavailable phosphorus (P) were present up to five inches below the soil surface. In the spring of 2005, however, we could detect fewer differences in relative amounts of bioavailable P from measurements made 15 to 19 weeks after application. Low levels of soil water at the time of the measurements probably limited P diffusion. Nevertheless, the results of these two years of measurements suggest that banded fluid P applied to the soil surface after crop harvest will move into the soil profile where it will be less subject to loss in runoff or by erosion during the winter months, and yet be available to plants the following growing season. These studies are being repeated in 2006. treatment and three sampling dates. Higher concentrations of bioavailable P were measured near the soil surface for all three of the starter treatments 23 days after application. By 42 days after application, the highest concentration of P was found one to two inches below the surface for both the 15+30+10 and 60+30+10 starter treatments. At 63 days after application, the highest concentration of bioavailable P was measured at a depth of more than three inches below the surface for the 15+30+10 starter treatment. In contrast, no measurable differences in bioavailable P distribution were found in plots in which 0+30+0 or 60+30+10 was dribbled on the soil surface. Although the results of these previous studies were not consistent among treatments, they do present convincing evidence that surface dribble applications of P fertilizer increase P bioavailability below the soil surface for several weeks after application. Given that P diffusion in the soil is a relatively slow process, P source, volume of material (30 gal/A), and porosity of the soil probably played a role in P movement. In any case, increased levels of bioavailable P in the root zone would potentially benefit the plant throughout the growing season. This leads to the question of what happens to P bioavailability when a
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