SynopsisUsing dalapon, 2,4-D, and amitrole instead of sweep tillage for weed control during fallow periods in a wheat-sorghum rotation did not have a measurable effect on soil pH, organic matter, wet sieve aggregation, or microorganisms. Chemical weed control reduced the number of nematodes and the nitrate content in the surface inch of soil. In laboratory studies, the three herbicides did not cause prolonged reductions in nitrification unless applied at excessively high amounts.

Immobilization of tracer nitrogen added as ammonium sulfate or as nitrate was followed by analysis of inorganic nitrogen in Moreno sandy loam and Sacramento clay during the course of incubation experiments. In Sacramento clay receiving ammonium sulfate plus straw the nitrifying bacteria were able to compete effectively with the immobilizing flora for nitrogen, whereas in the Moreno soil this was not the case. In both soils immobilization attained a maximum in 6 to 10 days. There was some increase in inorganic nitrogen thereafter, but none of this resulted from remineralization of the tracer nitrogen added initially. In greenhouse experiments competition for added tracer nitrogen between a growing crop and the immobilizing flora emphasized preferential utilization of ammonium nitrogen by soil microorganisms and of nitrate nitrogen by the crop. In Moreno sandy loam, overall recovery of the added tracer was higher under treatments favoring immobilization than in pots receiving only fertilizer.

In an investigation of the mechanism of action of chitin in the biological control of Fusarium solani f. phaseoli in soil, statistically significant control of a root rot caused by the fungus was noted when the aminopolysaccharide was added to soil up to 2 weeks prior to planting. No significant suppression by the microflora responding to the amendment was observed in longer periods of preincubation. Saprophytic soil fungi were also inhibited during the first 2 weeks after chitin incorporation. Actinomycetes, chitinase-producing microorganisms, and chitinase activity were markedly stimulated during that period of time, but the population of bacteria was not altered appreciably. It is suggested that mycolytic activity and toxin production may be implicated in the selective influence of chitin in suppressing certain soil fungi. Acetate effectively controlled disease, but it appears that the mechanisms of action of chitin and acetate are distinct.

Soil flooding is a common practice in crop production and in measures designed for the control of certain plant pathogens. Because of the alterations in the moisture, aeration, and nutrient status of the soil, as a consequence of such practices, marked microbiological changes take place. These changes in the microflora are both qualitative and quantitative. Among the apparently selective effects of flooding are the eradication of fungi such as Sclerotinia sclerotiorum (9) and nematodes like Heterodera marioni (2). Flood-fallowing, moreover, has been proposed as a means for the elimination from soil of Fusarium oxysporum f. cubense (11).The specific mechanisms by which the microbiological changes are brought about are unknown. That the reduction in population size or complete elimination of some specific microorganism is the result of microbial reactions is apparent from the significant suppression of the test species in non-sterile as contrasted with sterile soil. The effects of O2 depletion through microbial metabolism (10) or the influence of bacterial toxins (3) have been suggested as means by which indigenous microorganisms may affect the persistence of alien species. In the present investigation, a detailed study of the ecological changes in submerged soil was initiated to provide a better understanding of the principles involved in the control of specific fungi by soil submergence.