Rotations may often be planned with reference to the main or cash crop. Thus in the Aroostook (Maine) potato district the rotation is potatoes, oats and clover. The chief purpose of the oats and clover is to keep down the blight in potatoes and add through the clover nitrogen and organic matter to the soil. A system of cropping that is best when the owner operates the farm may not be desirable when the farmer is a tenant. When a farm is rented, the lease should provide that clover or other legumes occur with sufficient frequency to keep up the supply of nitrogen without the purchase of a considerable quantity in chemical fertilizers. The lease should be so drawn as to make it necessary for the tenant to keep live stock in order to realize the largest profit. The landlord should provide an equitable proportion of the mineral fertilizers when such are required. The provisions of the lease and the character of the rotation will necessarily vary with circumstances, but the following system of tenant farming which has been employed for many years in Maryland will illustrate the principles just stated: The lease provides for a five-course rotation consisting of maize, wheat, clover, wheat, clover. The landlord and the tenant share the maize and wheat equally, but the clover for hay or pasture goes entirely to the tenant, unless hay is sold, when it is divided equally. They each provide one-half the commercial fertilizer and one-half the seed, except clover seed, which the tenant is required to furnish. This lease provides for two clover crops out of every five crops raised, thus supplying nitrogen abundantly, and the terms of the lease are such that it is necessary for the tenant to keep live stock to consume these clover crops in order to secure the most profitable returns. The feeding of the clover makes it necessary to feed some or all the maize and may lead to buying additional concentrates. Stable manure is thereby supplied for the field which is to raise maize, while mineral fertilizers may be applied to the fields sown to wheat. On the limestone soils of the eastern states 50 pounds each of phosphoric acid and potash per acre applied to the wheat, and 10 loads of stable manure per acre to the maize will probably be found sufficient to maintain the crop producing power of the soil. In laying out a farm for a rotation it is desirable to plan the number of fields or tracts that will go in a rotation and try to get these as nearly equal size as possible. Having decided upon the number of years the rotation is to run and having adjusted the fields or tracts accordingly, it is quite possible to modify the proportion of crops by adding one crop and dropping another at the same time. Thus, if there are six 20-acre fields, any one of the following rotations might be used and the change from one to another easily made: