In many parts of the country, because not enough live stock is raised, there is often too little manure to apply to the wheat land. Where this is the case commercial fertilizers must be used. Since soils differ greatly, it is impossible to suggest a fertilizer adapted to all soils. The elements usually lacking in wheat soils are nitrogen, phosphoric acid, and potash. The land may be lacking in one of these plant foods or in all; in either case a maximum crop cannot possibly be raised. The section on manuring the soil will be helpful to the wheat-grower. [Illustration: FIG. 197. A BOUNTIFUL CROP OF WHEAT] It should be remembered always in buying fertilizers for wheat that whenever wheat follows cowpeas or clover or other legumes there is seldom need of using nitrogen in the fertilizer; the tubercles on the pea or clover roots will furnish that. Hence, as a rule, only potash and phosphoric acid will have to be purchased as plant food. The farmer is assisted always by a study of his crop and by a knowledge of how it grows. If he find the straw inferior and short, it means that the soil is deficient in nitrogen; but on the other hand, if the straw be luxuriant and the heads small and poorly filled, he may be sure that his soil contains too little phosphoric acid and potash. =EXERCISE= Let the pupils secure several heads of wheat and thresh each separately by hand. The grains should then be counted and their plumpness and size observed. The practical importance of this is obvious, for the larger the heads and the greater the number of grains, the larger the yield per acre. Let them plant some of the large and some of the small grains. A single test of this kind will show the importance of careful seed-selection. [Illustration: FIG. 198 A WIDELY GROWN CROP] SECTION XXXVIII. CORN When the white man came to this country he found the Indians using corn; for this reason, in addition to its name _maize_, it is called _Indian corn_. Before that time the civilized world did not know that there was such a crop. The increase in the yield and the extension of the acres planted in this strictly American crop have kept pace with the rapid and wonderful growth of our country. Corn is king of the cereals and the most important crop of American agriculture. It grows in almost every section of America. There is hardly any limit to the uses to which its grain and its stalks are now put. Animals of many kinds are fed on rations into which it enters. Its grains in some form furnish food to more people than does any other crop except possibly rice. Its stalk and its cob are manufactured into many different and useful articles. A soil rich in either decaying animal or vegetable matter, loose, warm, and moist but not wet, will produce a better crop of corn than any other. Corn soil should always be well tilled and cultivated.