Forty two soil isolates (31 bacteria and 11 fungi) were studied for their ability to solubilize rock phosphate and calcium phosphate in culture medium. Eight bacteria and 8 fungi possessed solubilizing ability. Pseudomonas cepacia and Penicillium purpurogenum showed the highest activity. There was a correlation between final pH value and titratable acidity (r=-0.29 to -0.87) and between titratable acidity and soluble phosphate (r=0.22 to 0.99). Correlation values were functions of insoluble phosphate and of the group of microorganisms considered. A high correlation was observed between final pH and soluble phosphate only for the rock phosphates inoculated with the highest concentration of solubilizing bacteria (r=-0.73 to -0.98).

The influence of different media and incubation temperatures on the quantification of microbial populations in sorghum, eucalyptus and forest soils was evaluated. Microbial growth was compared by using complex (tryptone soybean agar, TSA, casein-starch, CS, and Martin) and saline (Thorton, M3, Czapeck) media and incubation temperatures of 25 and 30 degrees C. Higher numbers of total bacterial and fungal colony-forming units (CFU) were observed in sorghum soils, and of spore-forming and Gram-negative bacteria in forest soils than other soils. Actinomycetes counts were highest in forest soil when using CS medium at 30 degrees C and in sorghum soil at 25 degrees C in M3 medium. Microorganism counts were dependent on the media and incubation temperatures. The counts at temperatures of 30 degrees C were significantly higher than at 25 degrees C. Microbial quantification was best when using TSA medium for total and spore-forming bacteria, Thorton for Gram-negative bacteria, M3 for actinomycetes, and Martin for fungi.

Growth conditions (carbon source and pH of the medium) affected the level and distribution of derepressed acid and alkaline phosphatases in Neurospora crassa. Regardless of the pH, the production of both acid and alkaline phosphatases was stimulated by sucrose. Irrespective of the carbon source used, at pH values higher than 7.4 the secretion of alkaline phosphatase was stimulated, while the production and secretion of acid phosphatase was restricted. The converse was true at pH values lower than 5.7. The secretion of one of these enzymes did not exclude the simultaneous secretion of the other at pH values of around 7-0. These results suggest that the pH of the medium may play an important role in the survival of the organism in phosphorus-limited environments.

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227Some factors influencing the growth and production of extracellular lipase by Rhizopus oligosporus were studied. Highest yields of enzyme were obtained when Tweens were the carbon source. Soybean meal extract supported good growth and enzyme production. Carbohydrates, vegetable oils, proteins or amino acids did not stimulate lipase production. The fungus grew well with carbohydrate-or protein-supplemented media but not with oils, unless emulsified with a non-metabolizable gum. The production of biomass in static cultures was maximum at 35-40 "C after 4 d at pH 5.5. The yield of lipase was maximum at 25 "C after 3 d at pH 6.5. Shaking cultures enhanced growth but decreased lipase production.

The solubilization of inorganic phosphates by microorganisms supplies phosphates for plant nutrition and increases their growth. The solubilization of CaHPO4 (Ca-P) and AlPO4 (Al-P) by Aspergillus niger using several carbon and nitrogen sources was studied. Solubilization of Ca-P was enhanced when the carbon sources were mannitol, maltose, galactose and glucose (in that order). Galactose, sucrose and maltose were the carbon sources that enhanced the solubilization of Al-P. More extensive growth, acid production, and decrease in pH were obtained in the Al-P medium than in the Ca-P medium, however, the quantity of solubilized phosphate was 12% less. Phosphate solubilization was related to acid production, pH drop and fungal growth in the culture medium. The results of a study carried out under abiotic conditions showed that organic acids solubilize more Ca-P than Al-P. Evaluating the effect of the nitrogen source, the solubilization of Ca-P or Al-P decreased in the following order: glycine > NH4Cl > NaNO3 and NH4NO3 > urea > (NH4)2SO4, respectively. Ammoniacal nitrogen (NH4 + -N) sources were the most effective in the production of acids and in lowering of the pH.

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SUMMARYCompaction is one of the most destructive factors of soil quality, however the effects on the microbial community and enzyme activity have not been investigated in detail so far. The objective of this study was to evaluate the effects of soil compaction caused by the traffic of agricultural machines on the soil microbial community and its enzyme activity. Six compaction levels were induced by tractors with different weights driving over a Eutrustox soil and the final density was measured. Soil samples were collected after corn from the layers 0-0.