Physical Analysis.--Chemical analysis is costly and unsatisfactory as a guide to fertilization. Physical analysis by a competent man may have distinct value, and especially to one lacking experience with his soil. The mapping of soils by national and state authorities has given pretty accurate knowledge of hundreds of soil types, their location and characteristics, and when a soil expert obtains a sample of soil and the history of its past treatment, he can assign it to its type and give to its owner dependable advice regarding its crop-adaptation and probable fertilizer requirements. The Use of Nitrogen.--There is no fully satisfactory way of determining the kind and amount of fertilizer that should be used at any particular time for any one crop. Perfection in this respect is no easier in attainment than in other matters. There are, however, means of arriving at conclusions that are a valuable guide. In a general way, nitrogen is in scant supply in all worn soils. Wherever the cropping has been hard, and manure has not gone back to the land, the growth in stalk and leaves of the plant is deficient. The color is light. Inability of a soil to produce a strong growth of corn, a large amount of straw, or a heavy hay crop, is indicative of lack of nitrogen in nearly every instance. The legumes, such as clover, and the stable manures are rich in nitrogen, and when the scheme of farming involves their use on all the land of the farm, no need of purchased nitrogen may arise in the production of staple crops. In the black corn soils the nitrogen content originally was high. Lands that naturally are not very fertile rarely have enough available nitrogen. Where timothy is a leading crop, the demand for nitrogen is heavy. A cold spring or summer, checking nature's processes in the soil, may cause a temporary deficiency in available nitrogen in land that usually has a sufficient supply. Associating a rank growth of stalk and leaf with an abundance of nitrogen, the experienced man can form a pretty safe opinion regarding the probable profitableness of an investment in this element. It costs nearly four times as much per pound as either of the two other constituents of a fertilizer, and so far as is feasible it should be obtained through the legumes and stable manure.