CHANGES IN SOIL CHEMICAL PROPERTIES DUE TO 40 YEARS OF FERTILIZATION

Abstract

Continuous fertilization for 40 years with N and P resulted in significant changes in soil pH, lime requirement, organic matter, Bray and Kurtz no. 1 P, and DTPA-extractable micronutrients. A pasture soil receiving up to 224 kg N.ha-1 as ammonium nitrate and 45 kg.ha-1 as superphosphate since 1946 was analyzed for soil test values as a function of depth. Soil pH values were observed to decrease when N applications exceeded plant requirements. In plots receiving continuous additions of 224 kg.ha-1, the pH decreased from 6.5 to 4.1. The lime requirement of the soils reflected pH changes. Organic matter was found to be unaffected by P fertilization, but increased with increasing N rate. Bray and Kurtz no. 1 P concentrations were extremely high in the surface 10 cm of P-fertilized plots, and increased concentrations were observed to a depth of 60 cm. DTPA-extractable Fe was negatively correlated with pH, and increased DTPA-Fe was observed to a depth of 20 cm. DTPA-Mn also tended to increase in soils with lower pH, and both total and DTPA-extractable Mn appeared to have leached from the 0− to 10-cm to the 10− to 20-cm increment in soils receiving the highest rate of N application. A strong interaction of N, P, and depth was observed for DTPA-Zn, but DTPA-Cu was unaffected by fertilization. In 1965, the plots were split, with one-half of each plot continuing to receive fertilizer and the other half having the fertilization permanently discontinued. The soil pH, lime requirement, Bray and Kurtz no. 1 P, and DTPA-extractable Fe and Mn all showed a significant response to discontinuation of fertilizer additions. The pH tended to increase, whereas the lime requirement, Bray and Kurtz no. 1 P, and DTPA micronutrients decreased during the 20 yr without fertilizer.