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Fluid Journal : Fluid Journal 1993-1995
1 Fluid Journal Winter 1994 by Dr. Stanley Barber " " " " "There must be phosphorus uptak There must be phosphorus uptak There must be phosphorus uptak There must be phosphorus uptak There must be phosphorus uptake by eby eby eby eby half of the root system." half ofthe root system." half of the root system." half of the root system." half of the root system." Dr. Stanley Barber, Purdue University distinguished professor e m eritus, stresses nutrient placement as key to maximizing crop yields, during interview with Fluid Journal. FJ: What factors affect phosphorus movement to the roots? The main factor affecting phosphorus movement to the roots is a process called diffusion, which is simply the motion of the molecules causing phosphorus movement along a concentrated gradient to the lower level at the root surface. This is gradual. In a day, you wouldn't expect phosphorus to move more than 500ths of an inch. So it's only the phosphorus nearer the plant root that gets there. This is significant when you realize corn or soybean roots occupy a mere one percent of soil volume. The rate at which P moves is going to depend upon the soil. In high-P soils it will move faster---and farther. Since phosphorus moves in soil water, it will diffuse faster in soils that hold more water and have what we call field capacity, which holds 30 to 50 percent water. By contrast, it will move slower in sandy soils with only about 10 percent water. FJ: What about deep placement of P in reduced tillage fields? A lot of people are thinking about it. You're talking about subsurface placement. FJ: Yes. Subsurface placement will put phosphorus where the soil is moist more of the time. If it is placed below the plow layer, the soil may also be lower in phosphorus. The difficulty with this type of placement is that it may place the phosphorus in only one percent or less of the soil volume and not enough roots will be absorbing phosphorus to be very effective for increasing yield. If five to twenty percent of the soil volume were fertilized, it could be effective; however, it may not show much benefit where surface application is also used. If it were done several times, it might fertilize enough soil to be effective. FJ: What about nitrogen applied with phosphorus? Will it enhance P uptake, for example? Placing nitrogen with phosphorus in the band will stimulate root growth in the fertilizer soil, since nitrogen stimulates root growth just as phosphorus does. The increased root growth increases phosphorus uptake. As the amount of root stimulation increases, the greater the difference in P or N levels between fertilized and unfertilized soil. Of course, remember, nitrogen also has its own properties. It can convert to nitrate, which can move from where it was placed. Thus, you don't have a concentrated band all the time like you do with phosphorus. FJ: We know that nitrification inhibitors may keep nitrogen in the ammonium form longer. Could this affect root growth? Added ammonium usually stimulates root growth in the band. Keeping nitrogen as ammonium keeps nitrogen in the band longer and should produce a greater effect on root growth and hence phosphate uptake. FJ: What about the longevity of phosphate bands? I've never researched this, but I would assume that the band would remain for several years. Phosphorus gradually gets fixed by the soil. The higher P concentration, such as occurs in the band, extends the time the band will be present. The longevity will also vary with the soil. FJ: You've studied band placement for years. What do you think of surface banding versus broadcast in reduced till? The surface banding studies of phosphorus I've done rely on enough tillage to mix it with up to 15 percent of the topsoil. This fertilizes enough soil to
Fluid Journal 1996-1998