SUMMARY 203 



mix the basic compounds named above with sodium nitrate 

 or with potash salts, for the latter absorb moisture and the 

 whole mass will harden to a solid mass which requires 

 crushing before use. 



155. Choice of Fertilizers. — In the following discussion of 

 various fertilizers there are considered for the most part 

 only their effects on the plant, the soil, and on each other. 

 Their relative value as forms of plant food is considered 

 onlv on these grounds. But it must be remembered bv the 

 farmer that the economical factor should enter into the choice 

 of a fertilizer. As a matter of scientific fact one form of 

 fertilizer may be better than another, but its cost may be 

 greater than the increased benefits derived from its use. 

 With a thorough knowledge of the several forms of each 

 element, of the effects of each crop to be grown, of the 

 soil, and of the cost, a farmer can decide for himself which 

 is the best kind to use under the circumstances. 



156. Summary. — Inasmuch as one of the vital factors in 

 crop growth is the supply of plant food in an available form, 

 it is necessary to maintain this supply by the addition 

 of such elements as are removed most largely and exist in 

 the soil in small or unavailable amounts. The principal 

 plant food elements added to the soil ordinarily are nitrogen, 

 phosphoric acid, potash, and calcium. Substances which 

 supply plant food are called direct fertilizers; those which 

 affect plant food reserves already in the soil, or improve 

 the condition of the soil in other wavs, are called indirect 

 fertilizers or amendments. 



Direct fertilizers may be obtained in the complete form 

 where all plant food elements are mixed together, or as single 

 fertilizers, containing only one plant food element. Fertilizers 

 can be used singly or mixed at home. There are certain 

 advantages and disadvantages either in buying ready mixed 

 goods or single materials and mixing them afterward. 



Lime and basic materials must not be mixed with nitrog- 

 enous compounds containing ammonia or with other forms 

 of nitrogen readily convertible into ammonia. Neither 

 should lime be mixed with acid phosphate, since an insoluble 

 fertilizer results. In choosing a fertilizer the farmer must 



