IMPACT OF FERTILIZER PLACEMENT AND TILLAGE SYSTEM ON PHOSPHORUS DISTRIBUTION IN SOIL

Repeated use of band applications, both surface and subsurface, of immobile nutrients applicable to a variety of tillage systems raises concerns about the collection of soil samples used for making fertilizer recommendations. This study was conducted to measure the effect of tillage system and fertilizer placement on soil test P in a major part of the root zone. Soil samples were collected from control treatments and from treatments where phosphate fertilizer supplying 29 kg P ha(-1) yr(-1) had been broadcast, applied as a surface band, or applied in a subsurface band for three consecutive years in a corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation. These placements were used in both a chisel and a ridge-till planting system. Soil samples were collected from depths of 0 to 7.5, 7.5 to 15.0, 15.0 to 22.5, 22.5 to 30.0, and 30.0 to 45.0 cm at distances of 0, 7.5, 15.0, 22.5, 30.0, and 37.5 cm from the row. Soil test P, to a depth of 15 cm at each position from the row, was significantly affected by fertilizer placement. Placement also had a significant effect on soil test P to a depth of 30 to 45 cm at a distance of 30 cm from the row. Tillage system had a significant effect on measured soil test P at sampling positions near the row. These results indicate that both tillage system and fertilizer placement could have a major effect on soil test values. If band locations are known, they should be avoided, if possible, during sample collection.