Evaluation of some biological tests as parameters for microbial activities in soils. I. Laboratory experiments.

The experiments were confined to the effects of the addition of different sources of carbon (glucose, wheat straw, and sawdust) on the microbial activities in soils: loamy sand, loam and saline clay were used. The parameters used for this purpose were total bacterial counts, dehydrogenase test, CO2 evolution, and oxidation of organic carbon. Salinity of soils had deleterious effects on all the parameters used. Addition of organic matter resulted in marked increases in bacterial counts, formazan production, and CO2 evolution. Glucose showed the most rapid effect and highest levels. Wheat straw yielded the highest results and sawdust always showed the least effect.

Evaluation of some biological tests as parameters for microbial activities in soils. II. Field investigations.

Investigations were designed to study the effect of certain factors on the microbial activities in soil. The parameters, used as an index of the microbial activities, were total bacterial counts, dehydrogenase activity, oxidation of organic carbon, and CO2 evolved/7 days. Bahteem Farm clay soil was examined for determining the effects of depth, type of fertilization, and crop rotation on the microbial activities. It appears that the microbial activities, as indicated by the tested parameters, were more pronounced in the surface 15 cm-layer than in the subsurface layer (15-30 cm). Results of all the parameters tested showed markedly higher increases with farmyard manure than with nitrogenous fertilizer and in the control, without significant differences between the latter two. Moreover, the time of sampling had no effect on the results obtained for all parameters. Different types of rotations did not exert significant variation in total bacterial counts, though more than one crop per year increased the organic carbon content of soil and mostly the dehydrogenase activity, whereas the evolution of CO2 tended to decrease. At Gabal el-Asfar Farm, the effect of irrigation with sewage effluent, for long periods, on the microbial activities of sandy soil was investigated. Sewage water stimulated the total bacteria, raised the dehydrogenase activity, the organic carbon, and the production of CO2. In North El Tahreer and Mariut Sectors, the effect of both the type and age of cultivation on the microbial activities in the calcareous soils were examined. Cultivation raised the figures of all the tested parameters progressively with time of cultivation. It was also noticed that crops exerted more beneficial effects on microbial activities than orchards, and the dehydrogenase test was the most reliable parameter to reveal this fact.

Biological activities during ripening of composts.

Three investigations were carried out. The first determined the relationships between dehydrogenase acitivty throughout the course of composting wheat straw and the changes in both organic carbon and total nitrogen. The second investigation tested the validity of the first investigation results. The third dealt with the effect of the degree of ripening of a compost, expressed by age and C/N ratio, on its dehydrogenase activity, and production of CO2. The results revealed continual decreases in the organic carbon, coincident with increases in total N and dehydrogenase activity. Noticeable chemical and biological changes occurred in the first and third month. Levels of dehydrogenase were highest in rice straw and lowest in cotton stalk compost, while the wheat straw compost occupied an intermediate position. Aging of the compost resulted in increases in the dehydrogenase figures and decreases in the amounts of CO2 produced.