Usually, Conservation Agriculture leads to soil organic matter improvement and the extension of the soil live with positive return in terms of fertility, water management and productivity with lower costs of production. Nevertheless, the reduction of the mineralization brought by tillage associated to the N immobilization in the biomass of cover-crops and the growth of the soil life can slow the returns. To avoid these risks of drawback it is necessary to understand those different factors and the relations in between them in order to adapt fertilization programs in consequences!