LONG-TERM COMPARISON OF ROCK PHOSPHATE WITH SUPERPHOSPHATE ON CROP YIELD IN 2 CEREAL-LEGUME ROTATIONS

Monocropping and long-term use of highly water soluble commercial fert ilizers may reduce crop yields and contaminate surface and ground wate r. A long-term experiment compared the effectiveness of rock phosphate (RP) with superphosphate (SP) on crop yields, soil tests, and leaf nu trient concentrations in two rotations with or without addition of N o r lime in a strip-block design with phosphate as vertical factor and N or lime as horizontal factor applied over P treatments. Crops in rota tion A (1952-1967) included oats (Avena sativa L.), alfalfa (Medicago sativa L.), corn (Zea mays L.) 1st yr and corn 2nd yr while crops in r otation B (1968-1983) included corn and soybean (Glycine mar L.). Phos phate application as single source, RP and SP in combination, and RP w ith elemental sulphur ranged from 0 to 40 kg P ha(-1) yr(-1). In rotat ion A, RP was applied once every 8 yr at eight times the annual rate; in rotation B it was applied at half the annual rate of rotation A. Th e SP was applied annually in both rotations. In both rotations, P trea tments significantly increased crop yields. In rotation A, crop yields on RP treatments were similar to those on corresponding SP treatments except for oats. Similarly, yields of crops grown with RP alone were similar to those obtained with a mixture of RP with SP or S. But corn grown after alfalfa did not respond to N application nor did crop yiel ds respond to lime application. In rotation B, at low lime, corn and s oybean yields were similar on RP treatments to those on corresponding SP treatments, but at high lime, yields were significantly lower with RP than corresponding SP treatments. Further, crop yields on mixture o f RP and SP were similar to yields with RP alone, and the RP and S com bination produced lower soybean yields and similar corn yields than RP alone. In this long-term cropping sequence study, on acidic soil RP w as equally as effective as SP in supplying P to crops.