The Composition- and Use of Fertilizers. 259 



must decide which specific forms we shall apply. To illustrate, 

 in applying some form of nitrogen, shall we use nitrate 

 of soda, or sulphate of ammonia, or dried blood, or fish-scrap? 

 Shall we use as our source* of phosphoric acid bone-meal, dis- 

 solved bone, acid phosphate, dissolved bone-black or Thomas slag? 

 Shall we apply muriate of potash, or sulphate of potash, or car- 

 bonate of potash What principles are there to guide us in mak- 

 ing the best choice? 



In deciding what form of material to use for plant food, we 

 should be guided by: 



(1) The availability of the plant food; 



(2) The preference, if any, shown by different plants; and, 



(3) The comparative economy possible in purchasing different 

 materials. 



Availability of Different Forms of Plant Food. — As a 

 rule, commercial fertilizers are used for the purpose of increasing 

 the single season's crop to which they are applied. Most farmers 

 plan to give to each succeeding crop by itself the plant food it 

 needs, and to avoid supplying any one crop with more material 

 than it can use to best advantage. This is especially true of 

 those who do not own the farms which they work and who plan 

 each year's work by itself, without reference to the future. Under 

 such circumstances a farmer desires to use those forms of fertiliz- 

 ing materials which will be taken up most quickly and completely 

 by the crops. On the other hand, the farmer who owns his land, 

 frequently desires to use materials, the plant food of which will 

 be utilized gradually by crops and which will last through several 

 seasons. It will thus be seen that if one desires a fertilizer which 

 will act at once and be largely used up by the present crop, then 

 he will need to purchase his plant food in forms different from 

 those purchased by the man who desires more lasting benefits, 

 extending through several seasons. 



"We will now consider the relative rapidity with which different 

 forms of nitrogen, phosphoric acid and potash are available for " 

 the use of plants. 



(a) Availability of Different Forms of Nitrogen. 



(1) Nitrate of Soda is easily soluble in water and is utilized as 

 such directly by plants. Therefore, as soon as nitrate of soda 

 dissolves and reaches the root system of plants, it is immediately 



