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Fluid Journal : Fluid Journal 1999-2001
3 Fluid Journal Winter 2001 very sizable quantities of P can be absorbed by only a portion of the root system. It has been suggested that total Pneedsofaplantcanbemetviaa single root. It appears that when roots contact a P droplet, root proliferation can be expected, as well as an increase in root growth in that part of the soil. However, exhaustion of P in that soil area affected by the P droplet or dry particle could be a limiting factor. Therefore root-droplet contact would have to be made with several droplets to ensure adequate P supply to the plants. Calculations indicate that over 80 root-droplet contacts are needed for each corn plant at a yield level of 150 bu/A to ensure an adequate supply of P. This assumes an application rate of 30 lbs/A of P2O5 in a 15-inch band spacing (which provides a 0.5 inch distance between droplets with 17 milligrams P2O5 per droplet), a plant population of 25,000 plants/A, and a total P uptake of 80 lbs/A of P2O5 with 10 percent use of the applied P). In contrast, we suggest that plant roots may follow a continuous band with only one root contact. However, with discontinuous bands, where fertilizer is placed in droplets or as dry particles too far apart to interact with each other, a new root contact may be needed for each droplet or particle. If true, since it is generally known that roots contact only 1 to 2 percent of soil volume, it appears reasonable that individual non-interacting droplets or particles would have a much lower probability of being exploited by the plant root than continuous P bands. The benefit of adding UAN to starter fertilizers to produce about 1:1 N:P2O5 ratios (place at least 2 inches from seed for greater than 20 lbs/A of N on a sandy soil, and same distance for greater than 40 lbs/A on a non-sandy soil) may be due in part to the improved distribution of P reported in these studies from higher rates of P application. This does not diminish the importance of additional ammonium N in P uptake or the possible effects of high ammonium N concentra-tions on P fixation reactions, which may in turn keep more P in an available form. Better distribution, continued ammonium N presence, and possible delay of P fixation reactions are added factors in improved P response and the benefits of band placement of fluid P fertilizers. Dr Eghball is a research soil scientist, USDA/ARS and Dr Sander is professor emeritus of agronomy at the University of Nebraska. ! Wheat Grain Yield (bu/A) Applied P (lb P O /A) 25 Broadcast Knife Seed 22 34 46 58 70 51 44 37 30 Figure 6. Effect of method of P application on wheat grain yield, Sander, et al. Corn Grain (bu/A) Applied P (lb P O /A) 25 Broadcast Knife 202428323640 140 130 120 110 100 Figure 7. Effect of method of P application on corn grain yield, Raun, et al. Wheat Grain Yield (bu/A) Seed Knife Applied P (lb P O /A) 25 18243036 42 48 54 Figure 8. Effect of method of P application on wheat grain yield, Leikam, et al. 84 78 72 66 60 54 48 42
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