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Fluid Journal : Fluid Journal 1993-1995
1 Fluid Journal Fall 1995 pplied phosphate in the soil typically moves only a short distance. This movement is primarily by a diffusion process. Rate of phosphate diffusion is influenced by 1) amount of applied P, 2) soil water content/bulk density, and 3) chemical reaction of P with the soil. Studies indicate that deep-placed phosphate is better distributed for crop uptake than surface-applied phosphate because the latter tends to accumulate in surface soils. Phosphate diffusion and movement also influence residual P effect, P longevity, and the probability that P- affected soil will be collected during soil sampling. The availability of residual-P bands is especially important with present-day emphasis on controlling soil erosion with surface residues and reduced tillage. While banding is well known to by Drs. B. Eghball, D.H. Sander and J. Skopp A Phosphorus R Phosphorus R Phosphorus R Phosphorus R Phosphorus Rate Affe cts Phosphorus ate Affects Phosphorus ate Affe cts Phosphorus ate Affe cts Phosphorus ate Affe cts Phosphorus Movement Movement Movement Movement Movement Nebraska researchers look at effects of different application rates and different soils on phosphorus diffusion as well as phosphorus longevity. Summary: Diffusion distances and apparent diffusion coefficients increased in a linear manner as P application rate increased. Longevity of banded P was shortest for the Sharpsburg soil with the highest P absorption capacity, and longest for the Coly soil with the lowest P-adsorption capacity. While most of the P movement occurs in the first few weeks after application, calculations indicate that bands may remain available for plant phosphate uptake from 2.6 to 6.5 years, depending on the soil and applied phosphate rate. increase P efficiency during the year of application, little research is available on the residual nature of P bands, especially when placed below tillage depth or in no-till systems. Studies have shown that P banded below tillage depth has had greater residual value than broadcast P or seed-applied P. Objectives of this experiment were to determine 1) diffusion coefficients, 2) P movement, and 3) longevity of banded P in different soils. Rate effect Yield. It appears that greater yield responses due to higher phosphate rates may not be because of phosphate fertilizer exhaustion at lower phosphate rates. Instead, the cause may be the larger volume of P-affected soil associated with higher phosphate application rates. Diffusion. In our studies, diffusion distance increased in a linear manner as phosphate application rate increased, with no significant differences observed between the three soils studied (Figure 1). Longevity of banded phosphate also increased in a linear manner as phosphate application rate increased (Figure 2). Soil effect Adsorption. Amount of P adsorbed was highest for Sharpsburg soil and lowest for Coly soil. The P-adsorption coefficient for Sharpsburg soil was about two and five times higher than for Nora and Coly soils, respectively. Diffusion. After 94 days, banded phosphate moved only a few inches from the center of applied P bands. Maximum movement was about 1.6 inches at 123 lbs/A of phosphate in the Nora soil (Figure1). Volume of soil being affected to a depth of 12 inches after 94 days ranged from 1.1 percent (Coly and Sharpsburg) to 5.2 percent (Nora). However, probability of contacting any part of the band at soil sampling ranged from 11.7 percent (Sharpsburg) to 25.9 percent (Nora). Research has shown that the surface area of P-affected soil influences plant uptake of added P since it affects the probability of root/P contact. Longevity. Band longevity averaged across the phosphate rates was lower for Sharpsburg and Nora soils than for Coly soil (Figure 2). The Sharpsburg soil had the shortest longevity, averaging only 3.3 years across P rates, compared with 5.6 and 4.1 for Coly and Nora, respectively. It seems that the greater P-adsorption capacity of the Sharpsburg soil reduced the longevity of added phosphate. As shown in Figure 2, P longevity ranged from 2.6 years (Sharpsburg) to 7.4 years (Coly). Longevity was highest in the soil with the lowest P-adsorption capacity (Coly) and lowest for the soil with the highest P-adsorption capacity (Sharpsburg). Sampling important Because of the residual nature of these phosphate bands, it is important to
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