Results of an 11 year fixed experiment indicated that the spontaneous nutrient supplying capacity of the typical black soil farmland tested varied in the following way, its N supplying capacity declined from 92% to 50%~60% in the plot without nitrogenous fertilizer, its phosphorus supplying capacity decreased from 98% to 80%~90% in the plot of no phosphorous fertilizer, and its potassium supplying capacity dropped slowly from 100% to about 90% in the plot of no potassium fert ilizer. Nutrient equilibrium of the soil and its changes controlled nutrient gain or loss in the farmland The correlation coefficient of the model between gain and loss of available phosphorus and of total phosphorus is Y= 2.895+0.134X, the correlation coefficient of the model between gain and loss of readily available potassium and of total potassium is Y= 18.81+0.163X.