Nitrate production as affected as sweetclover residues left on the surface of the soil

The development of nitrates in soil with sweetclover residues on the surface and cultivated by subsurface tillage was compared with nitrate production under conventional tillage where the sweet-clover residues were plowed under. The investigation was conducted on field plots at Lincoln, Neb., in first- and second-year corn after first- and second-year sweetclover over a 6-year period, and in wheat. Sampling for nitrates was done to a depth of 6 feet. The available nitrogen in the subtilled plots with first-year corn after first-year sweetclover was lower than on the plowed plots in 4 years out of 6. To a depth of 6 feet the mean available nitrogen for the 6-year period of the subtilled plots was about 7% lower than with the plowed plots. Also, on plots with second-year corn after first-year sweetclover, the available nitrogen to a depth of 6 feet was about 7% lower on the subtilled plots. The mean available nitrogen for a 5-year period in first-year corn after 2 years of sweetclover to a depth of 6 feet was 10% lower on subtilled plots than on plowed plots. In second-year corn after 2 years of sweetclover the mean available nitrogen to a depth of 6 feet for 3 years was about 8% lower on the subtillage than on the plowed plots. Nitrate determinations in 3 different years in corn land made about the first of November, after the corn was mature, showed a higher available nitrogen content on the subtillage than on the plowed land. Some of this seems to have carried over into the next season. This was shown by the fact that the increase in nitrate on the plowed land over subtilled was less in the second-year corn than in the first-year. In the seedbed for winter wheat in 3 different years the mean available nitrogen to a depth of 6 feet was slightly lower on the subtilled than on the plowed areas. The amounts of available nitrogen produced were large in every test and potentially sufficient to produce large yields of corn or other crops. That the nitrification process following the growing of sweetclover was slower under the subsurface tillage system than under the plowing system is to be regarded as a desirable circumstance in a region of limited rainfall. On the other hand, the high nitrate levels that were obtained under subsurface tillage through the growing of sweetclover are also desirable in areas of depleted soil where a residue cover is important for erosion control.