The objective of this research work was to evaluate the effects of doses and time of application of N on the C/N ratio of the straw cover and on the growth and productivity of maize plants growing in a no tillage system. The experiment was carried out at the Experimental Farm of the College of Agriculture of the Sao Paulo State University (UNESP) on its campus of Botucatu, state of Sao Paulo, Brazil. The treatments were distributed in the field according to a randomized complete block design in a split plot arrangement. The treatments consisted of four doses of N (0, 20, 40, and 60 kg ha -1) applied to oat crop and N doses (60, 80, 100, and 120 kg ha -1) sidedressed to corn. The development and productivity of the maize crop in a no-tillage system were found to be dependent of the C/N ratio and the straw cover. The response of the maize plants to the early application of N is dependent on doses and time of application.