Yield Trends and Soil Chemical Changes Resulting from N and Manure Application to Continuous Corn1

Abstract Many Vermont dairy farms are located in a region where clay soils are common and represent much of the land topographically suited to row crops. Since these soils frequently have restricted drainage, soils are used for corn (Zea mays L.) that would be considered unsuitable in other regions. To evaluate long‐term yield trends and the effects of various treatments on crop and soil, corn silage was grown for 11 years on a somewhat poorly‐drained Panton clay soil (Typic Ochraqualf). Four rates of manure (0, 22, 44, and 66 metric tons wet weight/ha) and five rates of ammonium nitrate‐N (0, 56, 112, 168, and 224 kg N/ha) were combined factorially and applied annually to plots from 1965 through 1975. The highest yields were produced by combining 44 metric tons of manure and 224 kg N (44M to 224N). However, the 22M to 112N treatment yields were not significantly different from the highest treatment yields. The check treatment (OM‐ON) was initially 82% of the highest treatment yield, but by the end of the experiment was about 54% of the maximum yields. Annual yield fluctuations were correlated with variations in rainfall. The effect of inorganic‐N fertilizer on lowering soil pH was counteracted by manure. N application caused decreased extractable P, K, Ca, and Mg while manure application caused increased organic matter, P, Ca, and K levels. Manure did not increase soil Mg. Low yields should be expected when corn silage is grown on the Panton clay. Both manure and N applications were necessary to obtain maximum yields. Manure application at the rate of 44 metric tons/ha/year was necessary to maintain soil organic matter levels during continuous corn.