The experiment was carried out in pots using three clayish soils from the municipality of Piracicaba, SP, Brazil. The objective of this work was to obtain information on the decay of organic residue (corn culture trash applied at the proportion of 50 ton/ha) and the influence of nitrogen sulphate at the levels of 0, 50, 100 and 150 kg N/ha on the added organic material rate of de cay. The experiment was installed at room conditions and protected from rain. The evaluation of the decay was made by determinations of the carbon content and of the C/N ratio of the soils during the incubation period (70 days). The samples were collected and analyzed weekly. The moisture of the soils was mantained at the level of 70% of the water retained at 1/3 atm. The main conclusions are as follows: 1) The addition of mineral nitrogen in the soils accelerated the losses of carbon from the soils, showing a greater effect during the initial incubation period. 2) The addition of nitrogen affected the content of residual carbon in the final incubation period. Increasing levels of added nitrogen lowered the remaining carbon content in the soil at the end of the incubation period. 3) The addition of nitrogen affected the reaction of the C/N ratios in all the used soil. The treatments that received mineral nitrogen showed closer C/N ratios during the incubation period. The amount of mineral nitrogen applied was related to the narrowing speed of the values in the initial incubation period. 4) There is a strong indication that little increments of the total nitrogen (with peaks at the 28th day) are related to the addition of mineral nitrogen in all the soils. 5) The evolution of the carbon content and of the C/N ratios can be taken as parameters in the evaluation of the organic material rates of decay for the used soils and within an incubation period of 70 days.