Soil function and productivity depends on microbial communities for nutrient cycling, antagonistic effects and decomposition of organic matter among others. Mass and diversity of soil microbial communities depend on substrate characteristics determined by the resource input of the dominant plant species. Plant species effects and microbial can also be detected at much more localized spatial scales within habitats. The Coastal Plateau of the Guánica Dry Forest in Southwestern Puerto Rico is ideal for this type of study as dwarfed trees of different species are growing on cracks in the calcareous rocks isolated from each other. The aim of this preliminary study was to optimize the fumigation extraction method for dry land soils and to determine the soil microbial biomass associated to tree species representing different litter quality. Soil samples were obtained underneath the leaf litter of 4 isolated tress species. The substrate was shallow and predominantly humic. Microbial biomass was calculated following the fumigation extraction method described by Vance et al (1987). Our results confirm that the fumigation extraction method is appropriate for the determination of soil microbial biomass in our substrates and that microbial carbon is correlated to the percentage of water content.