COMPARATIVE RESPONSE OF CORN AND SOYBEAN TO SEED-PLACED PHOSPHORUS OVER A RANGE OF SOIL TEST PHOSPHORUS

The responses of corn and soybean to seed-placed fertilizer were compa red over NaHCO3-extractable soil phosphorus (P) levels ranging from 3 to 35 ppm in a two-year experiment. Early season corn and soybean shoo t-P concentrations were increased with increasing soil test P and were increased with seed-placed P regardless of soil test P, although the increases were greater for corn than soybean. Corn grain yield increas ed with increasing soil test P to a plateau level and increased with s eed-placed P regardless of soil test P. A side-band (5 cm x 5 cm) appl ication of 39 kg P ha(-1) at a low soil test P increased yield more (P <0.15) than application of 7 kg P ha(-1) with the seed. A side-band ap plication of 9 kg P ha(-1) at a medium soil P test did not increase yi eld, Soybean yield was increased with increasing soil test P one year out of two, but did not respond to seed-placed P in either year. The y ield response of corn was attributed to the increased P concentration prior to the 6-leaf stage.