Historical; The microbiological population of the soil as a whole; Occurrence of specific microorganisms in the soil; Decomposition of plant and animal residues in soils and in composts; Humus: nature and formation; Decomposition of soil organic matter and evolution of carbon dioxide; Transformation of nitrogen in soil; nitrate formation and nitrate reduction; Nitrogen fixation - nonsymbiotic and symbiotic; Transformation of mineral substances in soil by microorganisms; Higher plants and soil microorganisms; Associative and antagonistic effects of soil microorganisms; Disease-producing microorganisms in the soil and their control; Stable manures, composts, and green manures; Microorganisms and soil fertility; Recent developments in soil microbiology.

Experiments were conducted in quartz sand and in a silt loam to determine effect of differing levels of P³² on the growth and metabolic activity of soil microorganisms. Determinations were made of carbon dioxide evolved and of numbers of microorganisms, and in some experiments, of amounts of soluble phosphate, nitrate, and ammonia. In quartz sand treated with phosphorus, dextrose, urea, and mineral salts, and with differing amounts of P³², activity levels of 0.005 and 0.05 mc gave no appreciable differences. At 0.5 mc P³² per mg of P³¹, the rate of CO₂ production was reduced during the first week, and differences in the soil flora were encountered. Recovery in CO₂ production then followed, but the soil flora differences persisted throughout 3 weeks of incubation. In Seymour silt loam treated with 0.5 mc of P³² per mg of P³¹, there was found no influence on microbial numbers or on carbon dioxide evolution, but the amounts of soluble phosphate, ammonia, and nitrate at the end of 3 weeks were lower than for 0.05-, 0.05-, or 0-mc levels of treatment. When the phosphorus treatment was supplemented with dextrose and urea, no differences in the P or N status of the aliquots could be demonstrated. With dextrose and urea added and a 5.0-mc level of activity, there was only slight reduction in CO₂ evolution and only slight differences in microbial numbers.

A decreased availability of plant food throughout the growing season has been observed for some stubble mulch practices. To determine if the microflora were involved as a possible factor in this plant food availability difference, comparative data were obtained from double-cut and turnplow soil samples. Under the conditions of this investigation the following microbiological factors may be related to the plant food tie-up problems: (a) The environmental conditions of better oxygen supply and more organic matter in the 0–6 inch horizon seem to favor the stimulation of the soil fungi in the mulch plots. This group of soil microorganisms might cause a temporary loss of nitrate-nitrogen as a result of protein synthesis by the mold cells. (b) Biochemical nitrification as determined with the perfusion apparatus of Lees and Quastel showed slightly lower amounts of nitrate-nitrogen formed from the mulch than from the turnplow samples. In addition, the soil pH should be considered as an environmental factor which may have considerable influence on the activity of the soil population. It is believed that these factors might be temporarily involved in the plant food tieup problems found to be associated with stubble mulch tillage practices at Virginia Polytechnic Institute.

A study was made of the effects of several soil fumigants in moderate and high dosages and of steam sterilization on the aggregation of the <50 µ particles of five soils. The fumigants were injected into dry soil in large screw cap bottles, water sufficient to bring the moisture content of the soil to 50% of capacity was immediately added, and the lids tightly secured. After 3 days the soil was air-dried, placed in 16-ounce jars in 420-gram portions, adjusted to 50% moisture capacity, and incubated. Microbial counts and aggregate analyses were made after 0-, 10-, 20-, 50-, 100-, and 250-day incubation periods. The fumigation treatments markedly affected the microbial population of the soils but had little or no effect on aggregation. In high dosages, D-D, chloropicrin, and ethylene dibromide slightly increased the aggregation of one soil, namely, Yolo loam at the 0-day incubation period only. Steam sterilization increased the aggregation of Yolo loam, an unnamed mountain soil, and Hanford sandy loam. In the latter two soils, the effect was not significant after the 0-day incubation period. Increased aggregation was associated with decreased wettability of the soil. The study indicates that aggregation is influenced more by products of microbial activity during the decomposition of organic waste material than by numbers of microorganisms. Fumigation apparently does not provide sufficient energy material in the form of dead microbial cells and adsorbed fumigant to change the aggregation status of a normal soil significantly, or destruction of organic cementing materials by increased microbial activity counteracts any aggregating effect.