The farmer should make the best use of farm manures and through tillage to render plant food available for his crops before turning to commercial fertilizer for additional plant food.
If he grows leguminous crops for green manuring, for feeding stock or for cover crops, he can in many cases secure, chiefly through them, sufficient high priced nitrogen for the needs of his crops, and it is necessary only occasionally to purchase moderate amounts of phosphoric acid, potash and lime.
For special farming and special crops it may be necessary to use the commercial fertilizer more freely.
It is impossible to say here just what amounts or what kinds of fertilizer should be purchased, because no two farms are exactly alike as to soil, methods of cropping or methods of tillage.
There are certain factors, however, which will serve as a general guide and which should be considered in determining the kind and amount of fertilizer to buy.
- The crop.
- The soil.
- The system of farming.
THE CROP
Crop roots differ in their powers of feeding, or their powers of securing plant foods. Some roots can use very tough plant foods, while others require it in the most available form. Some roots secure nitrogen from the air. The cowpea roots, for example, can take nitrogen from the air and they can use such tough phosphoric acid and potash that it seldom pays to feed them directly with fertilizers.
A bale per acre crop of cotton requires for the building of roots, stems, leaves, bolls, lint and seed: