Effects of long-term cropping on chemical aspects of soil quality

The objective of this study was to determine the effects of long-term (1950s to 1992) cropping by farmers, using their own cultural and fertilization practices, on soil quality as measured by soil OC, total N, NaHCO3-extractable P, and pH. We sampled farmers fields at 27 Oklahoma sites (representing six soil associations) in the 1950s and again in 1992. We also sampled soil at 2 sites with native sod to determine changes in soil properties since initial cultivation. Most of the sites were in areas commonly cropped to continuous wheat (Triticum aestivum L.). One of the 27 sites was planted to tall fescue (Festuca arundinacea Schreb), in the late 1970s. On the 26 cropped sites, OC and total N levels did not change significantly during the time period. Sodium bicarbonate extractable P increased significantly on 2 soil associations but remained unchanged on the other associations. Soil pH decreased on 2 associations and remained unchanged on the others. These results indicate that, except for reducing pH on some soils, farmers are growing continuous wheat without degrading these measures of soil quality. In the study of changes since initial cultivation, virgin Grant silt loam lost 45% of its surface (0-13 cm) OC in 25 yr and 59% in 92 yr of cultivation. Respective losses in total N were 23 and 46%. Kirkland silt loam surface (0-15 cm) soil had lost 42 and 47% of its initial OC after 58 and 92 yr in cultivation. Respective losses of total N were 29 and 36%. After cropping, both OC and total N levels in surface soils were similar to those in the subsoils. This was also true on the 26 cropped soils. At the site converted to grassland, OC and total N increased sharply, extractable P was unchanged, and pH was increased, probably as a result of lime application. Results obtained on this site may be indicative of changes occurring on land in the Conservation Reserve Program (CRP).