LONG-TERM NO-TILLAGE EFFECTS ON SOIL PROPERTIES AND CONTINUOUS CORN YIELDS

Lack of soil disturbance in no-tillage changes some of the most import ant basic soil properties. Short-term changes have been well documente d by previous research, but little is known about long-term changes. T his study was to determine the effects of 20 yr of continuous com (Zea mays L.) under no-tillage (NT) and conventional moldboard plow tillag e (CT) with 0, 84, 168, and 336 kg N ha-1 on soil properties and grain yields and, to the extent possible, compare 20-yr results with previo usly published 5- and 10-yr results. Soil organic C and N; extractable P; exchangeable Ca, Mg, and K; and pH were significantly higher with NT than CT in the 0- to 5-cm depth. Below the 5-cm depth, Mehlich III P; pH; and exchangeable Ca, Mg, and K were higher with CT than NT. Org anic C and N increased with increasing N rates. Conversely, pH and exc hangeable Ca and Mg declined with high N rates. Bulk density was not s ignificantly different among NT, CT, and bluegrass sod (Poa pratensis L.), but increased with depth. Comparisons of 1989 results with those obtained in 1975 and 1980 revealed that the soil's organic C was resto red to the level of the bluegrass sod following a decline of 19% with CT and 9% with NT between 1970 and 1975. Grain yields, which declined along with organic C, have not recovered. Changes in organic matter co ntent, with their many ramifications, are probably the most important long-term effects of tillage differences on basic soil properties.