An attempt has been made to determine the intensity of the biological processes in the soil during the most important periods of the Egyptian farm rotation. The fluctuations of the nitrate content in the surface soil have been taken as the most important index of this activity.In all cases the moisture content of the soil limited these processes more than any other factor.There was observed throughout the season in a cotton field a relatively large amount of nitrate, more than sufficient for the immediate needs of the cotton plant. The lack of response on the part of the Egyptian cotton crop to nitrogenous fertilisers may be accounted for in part, if not entirely, by the fact that nitrification in the soil is well ahead of the needs of the crop.Nitrification under wheat and maize shows in general the same characteristics in Egypt as in other parts of the world; there is no accumulation of nitrate in the soil.The winter fallow, depending for its water on the rainfall, may be a period of steady nitrification when the amount of the rainfall is sufficiently high.