Nitrogen losses from Alabama soils in lysimeters as influenced by various systems of green manure crop management

The amounts of nitrate nitrogen leached from Norfolk sandy loam, Hartsells fine sandy loam, and Decatur clay loam under different systems of soil and crop management were determined over a 4-year period by the use of lysimeters. Summer legumes, winter legumes, and sodium nitrate were used as sources of nitrogen. Sudan grass was grown during the following summer as a test crop in order to determine the influence of the various treatments on possible crop yields and nitrogen recovery. The loss of nitrogen by leaching was closely related to the texture of the soil. The amount of nitrogen lost decreased as the clay content of the soil increased. Only negligible amounts of added nitrogen leached from Decatur clay loam when the soil was cropped, but when this soil was left fallow without the addition of nitrogen, there was a progressive annual increase in the leaching of nitrates which amounted to 56 pounds of nitrogen in 1940. This clearly shows that when nitrates are present they can be readily leached from a clay soil as well as from a sandy soil. Differences in the rate of decomposition of organic material in the soils studied probably account for the higher nitrate losses in sandy soils than in the clay soil. When summer legumes were added to the sandy soils, nitrogen losses were affected by the kind of legume and time of turning under the legume. The amount of nitrogen leached increased as the percentage of nitrogen contained in the different legumes increased which was in the following order: Crotalaria, cowpeas, and soybeans. When sovbeans were turned under in the fall, as an average, 70% of the added nitrogen was leached from Norfolk sandy loam and 52% from Hartsells fine sandy loam. This loss was reduced to 38% on both soils when the soybeans were not turned under until spring. With the soil types studied, the growing of a winter cover crop following fall-turned legumes prevented all but small amounts of nitrogen from leaching. Oats were more effective than vetch in preventing this loss, but the yields of sudan grass were much greater following vetch. When soybeans were turned in the fall and followed with a crop of vetch, which was turned in the spring, the loss of nitrogen increased annually due to the large accumulation of nitrogen and formation of excessive amounts of nitrates. For the same fertilizer and cultural treatments, the yield of sudan grass on the different soils increased in the following order: Norfolk sandy loam, Hartsells one sandy loam, and Decatur clay loam. For any one soil type, the yield of sudan grass was directly related to the amount of added nitrogen that was not leached. Higher yields were obtained on sandy soils by applying 36 pounds of nitrogen as sodium nitrate than from 75 pounds of nitrogen in the form of summer legumes. Vetch, grown during the winter and turned under in the spring, produced higher yields of sudan grass than either summer legumes or sodium nitrate. The nitrogen content of the different soils was maintained at a constant level for 4 years when sodium nitrate was used as the source of nitrogen. The net gain in nitrogen was highest when vetch was grown as the source of nitrogen. There was a net gain in nitrogen by the soils when summer legumes were turned under and this varied inversely with the loss of nitrogen by leaching. From these 55% to 60% of the rainfall percolated through Norfolk sandy loam, and from 35% to 45% through the Hartsells and Decatur soils. It is evident from this that the small loss of nitrate from Decatur clay loam was not caused by a lack of percolation. From these results it is apparent that summer legumes should be turned in the spring on sandy soils, unless a winter cover crop is to be grown, that winter cover crops greatly reduce the loss of nitrogen by leaching, that the loss of nitrates is related to the texture of the soil; and that soils should not be left fallow during seasons of heavy rainfall.