Soil microbiological activity was measured for eight seasons, 1927 to 1934, in order to study some underlying causes of the comparative effects of alfalfa, brome, timothy, and western rye grass on the yield and nitrogen content of succeeding wheat crops.When previously fallowed soil was seeded to alfalfa and grasses, the moisture and nitrate content of the soil were reduced, and generally remained at a relatively low level until the sods were plowed up. In the drier seasons the nitrates were reduced to a very low level or disappeared entirely in the grass and alfalfa plots. The nitrate content of the alfalfa plot soils was generally greater than that of the grass plots, and the brome grass plots were generally lower in nitrates than the timothy and western rye grass plots. The wheat plot soils generally contained more nitrate than the grass and alfalfa plots, especially in the drier seasons. When the sods were plowed up, nitrates accumulated in the alfalfa plots to a greater extent than in the grass plots and to a lesser extent generally in the brome plots than in the timothy and western rye plots. The greater nitrate content of the soil under wheat following alfalfa was observed for a period of three or four years in separate sets of plots plowed up two years apart. The nitrate level of the soil under wheat had a tendency to drop in mid-summer, often reaching its lowest point in July. The fallow plot soils were always higher in moisture than any of the cropped plots at the end of each season, and higher in nitrates in the latter half of each season.The concentration of water-soluble phosphorus was greatest in the surface soil and seemed to be slightly higher under alfalfa and grasses than under wheat, but the total concentration was small and there was no very definite seasonal trend.The numbers of fungi and bacteria, as determined by the plate count method for five seasons, 1929 to 1933, did not fluctuate very much in certain plots and seasons, but fluctuated greatly in others. The greatest fluctuations in fungal counts were observed under the first crop of wheat following brome grass, and in bacterial counts also under the first crop of wheat following sods, in the relatively moist season of 1931. Plate counts of actinomycetes did not fluctuate very greatly during the one season in which they were determined. The numbers of fungi were generally higher in the alfalfa plots than in the grass plots, but the differences between the grasses were apparently insignificant. Under the first crop of wheat following sods, large Mucor colonies predominated in the alfalfa plot soil plates and the counts were relatively low. Brome grass plot soils gave by far the highest counts of fungi, which consisted mainly of small Penicillium colonies, under the first crop of wheat following sods in 1931. The differences between numbers of bacteria in the alfalfa and grass plots were not very significant. The moisture content of the surface soil fluctuated greatly during most of the seasons. There was evidence of correlation between fluctuations in bacterial numbers and moisture, especially in certain seasons, in all the cropped soils. There was less evidence of such correlation in the case of fungi, except under the first crop of wheat following brome grass in 1931. Fallow soil, though normally higher in moisture content in the latter part of each season, did not differ significantly from the grass-cropped soils in counts of fungi and bacteria. Although surface samples usually gave the highest counts, the deeper soil samples (to a depth of three feet) gave fairly high counts of both fungi and bacteria. During the season of 1930, amoebae were determined by the dilution plate count method; more than 1,000 and less than 10,000 per gram were nearly always found in both cropped and fallow soils.The total nitrogen content of the plot soils showed considerable variation (owing to random sampling) from year to year, but no definite trend downwards or upwards during this period of seven years. The surface soil in every case contained most nitrogen and the subsoil least.
Effect of thinning on the amount of mineral nitrogen
