Assuming a given type of management, the question is, How much of the required nitrogen will be obtained from the legumes in the cropping system, how much from the manure, and how much must be purchased in commercial fertilizers? No satisfactory cropping system can be devised at the present prices of farm products and cost of fertilizers for the production of the ordinary cereals and hay that does not include the production of some legume. Assuming a legume in the cropping scheme, the fertility of the soil may be maintained by yard manure alone or by commercial fertilizers alone. Illustrations of both methods are to be found in actual practice. Generally speaking, however, the use of yard manure supplemented with commercial fertilizers will be found more scientific and in the end the most economical. A factor entering into this problem will be the amount of purchased feed. If considerable amounts of purchased feeds are used and the resulting manure carefully preserved and judiciously applied, the commercial fertilizers required will be reduced to the minimum. A concrete illustration may bring out the philosophy underlying farm schemes better than abstract problems. The following outline shows a five-course rotation with the method of fertilization which the results of the Pennsylvania Station indicated would be advisable, at least on limestone soils in eastern United States. 1. Maize yard manure, 8 tons per acre. 2. Oats nothing. 3. Wheat acid phosphate, 350 lbs. muriate of potash, 100 lbs. 4. Clover and timothy nothing. 5. Timothy nitrate of soda, 150 lbs. acid phosphate, 150 lbs. muriate of potash, 50 lbs. This rotation is suggested for the purpose of maintaining a farm that is already in a fairly fertile condition and one on which there is no considerable amount of purchased feed. Where concentrates are purchased liberally, yard manure should be available to use on the timothy and meadow in place of the commercial fertilizers. Where there is plenty of manure and it is desired to increase the amount of maize and hay and reduce the amount of oats and wheat, the following rotation and method of fertilization would be indicated: 1. Maize acid phosphate, 200 lbs. 2. Maize yard manure, 8 tons. 3. Oats nothing. 4. Wheat acid phosphate, 350 lbs. muriate of potash, 100 lbs. 5. Clover and timothy nothing. 6. Timothy nitrate of soda, 150 lbs. acid phosphate, 150 lbs. muriate of potash, 50 lbs. 7. Timothy yard manure, 8 tons.