The effects of two management systems ( coventional tillage and non-tillage) on several soil properties were analyzed in a savanna site located in the high central plains of Venezuela. The site was characterized by Ultisol type of soil containing 70% sand, pH 5. l, 1.4 % carbon and 0.12% nitrogen. A subarea of 250 m2 was subject to conventional tillage for a period of 13 consecutive years. A second subarea of similar size was subject to conventional tillage for eight years followed by five consecutive years of non tillage. Implementation of conventional tillage caused, during the rainy season, a 68%, and 60% reduction on the soil microbial biomass and light organic matter fraction, respectively. Concomitantly, a decrease of 86% in the stabilíty of aggregates and 30% on the carbon associated to aggregates, were also detected. In contrast, non-tillage system significantly improved the structure as well as the organic matter fractions in the five cm of the uppermost soil horizon. Therefore, we might conclude that management of agricultura! soils by means of non-tillage constitutes the most beneficia! system for soil conservation and exploitation. ; El efecto de dos sistemas de labranza (siembra directa y convencional) sobre algunas propiedades del suelo fue estudiado en una sabana de los Llanos Altos Centrales Venezolanos. El suelo bajo estudio es un Ultisol con un 70% de arena, pH 5,l; 1,4% de carbono y 0,12% de nitrógeno. En este sitio, una subárea de 250 m2 estuvo sujeta a labranza convencional por 13 años continuos y otra área de igual tamaño se manejó con labranza convencional por 8 años, seguidos de 5 años continuos de siembra directa. La aplicación de labrado convencional desmejoró significativamente la calidad del suelo en la época de lluvia, observándose un 68% y 60% dereducción en la biomasa microbiana y en la fracción ligera del suelo, respectivamente. Se observó también una disminución del 86% en la estabilidad de los agregados y 30% del carbono asociado a los agregados. Por el contrario, la siembra ...