Castrillón Quintana, OliviaBedoya Mejía, OswaldoMontoya Martínez, Diana Victoria2012-07-112012-07-112012-07-111909-0455http://hdl.handle.net/10567/512Introduction. The increase in the organic products demand has generated a higher consuming of composted materials, in order to increase the chemical and physical quality of the soils and guarantee better crops. Besides, laws are being created and implemented to rule the parameters for the organic manure production, processing and application in Colombia and worldwide. There are many reasons to control the process in a production plant: The smell, the standardization of properties to sanitize the final product and the compost quality. Objective. To evaluate the effect of the pH on the micro organisms growth in compostation systems under passive ventilation and no ventilation. Materials and Methods. The compostation process was made on the field with two piles which were taken to their maturation stage. The samples were collected and incubated in enriched BHI broth and with a pH adjusted to 5, 6, 7, 8 and 9. 24 hours later they were seeded to count the bacteria, moulds and leavenings. Another sample was collected at a field pH and the microorganisms were counted. In the statistic evaluation, the SAS program was used. Results. The pH effect is significant in the growth of bacteria, moulds and leavenings. For the bacteria, the most influent pH was 5 for both piles. For the moulds and leavenings it was 9 for the ventilated piles. Conclusion. The results show the significant effect of pH on the growth of colony forming units of mesophilic and thermophilic bacteria, moulds and leavenings. This fact cannot be evaluated keeping away the others that intervene in the compostation, such as ventilation, temperature, humidity and kind of substrate used as raw material.esCompostCorporación Universitaria LasallistaAbono orgánicoCompostUnidades formadoras de colonias (UFC)Compost - ProducciónCompost - PHMicroorganismosArticle