200 FERTILIZERS 



148. Direct Fertilizers. — Compounds which supply plant 

 food to the soil and thus have a direct action on plant growth 

 are called direct fertilizers, and are usually compounds 

 containing nitrogen, phosphoric acid, or potash which are 

 the three elements most commonly lacking in soils, either 

 because of low total content like nitrogen and phosphoric 

 acid, or because of unavailability like potash. The addition 

 of the other essential elements is rarely necessary, with the 

 exception of calcium. 



149. Indirect Fertilizers or Amendments. — Compounds 

 which are not added primarily to supply plant food, but 

 which cause some other plant food to become available, and 

 which correct a harmful condition in the soil, or act as a 

 stimulant to plant growth by other causes than merely nutri- 

 tive, are called indirect fertilizers or amendments. Calcium 

 in various forms is usually called an indirect fertilizer, 

 although as noted in Chapter XII, calcium may frequently 

 serve as a plant food. Sodium chloride, manganese salts, and 

 sulphur, are all classed as indirect fertilizers or amendments. 



150. Commercial Fertilizers. — Under this head come all 

 those fertilizers which the farmer buys — compounds which 

 are of great commercial importance. Compounds which are 

 produced on the farm, such as barnyard manure or green- 

 crop manures, are not rated as commercial fertilizers. 



151. Complete Fertilizers. — A complete fertilizer is one 

 which contains nitrogen, phosphoric acid, and potash. 

 These three elements are the only ones which the farmer 

 needs to consider as being necessary to purchase for plant 

 food. Nitrogen and phosphoric acid exist in soils in very 

 small amounts and are hence very likely to be lacking in 

 sufficient quantity to nourish crops. Potassium on the other 

 hand is rarely lacking, but is very frequently present in 

 such unavailable form that plants cannot obtain enough for 

 normal growth. Of the other essential elements, none is 

 ever actually lacking in soil for the nutrition of the plant, 

 except calcium. But since calcium is usually referred to as 

 a soil amendment or indirect fertilizer, discussion of this 

 element as a plant food will be postponed. 



In preparing a complete fertilizer for the market the 



