The degree of degradation of soils can be measured by reference to a variety of chemical, physical, biochemical and biological properties, the last two being particularly sensitive to the changes that take place. Among these parameters, those related with the soil carbon content, such as total organic carbon and microbial biomass carbon, are of the greatest interest although others are also important indicators of microbial activity, including ATP and phosphatase activity. This study looks at the microbial activity of various soils in SE Spain which present different degrees of plant cover. The results demonstrate that vegetation is of fundamental importance for the development of microbial populations and their activity since soils with a substantial vegetal cover showed higher values for the above parameters than those with a reduced cover, which are more degraded. A seasonal study of the soil properties showed that the time of year exercises an important influence on the parameters that are indicative of microbial activity.