124 FERTILIZERS 



a long time as though crop after crop could be removed 

 without injury. Often, in such neighborhoods, one farmer 

 produces twice as much to the acre as another close by. 

 This difference was not noticeable a century ago. One 

 soil has been abused ; the other has been cared for. It is 

 just as reasonable in the long run to work a horse day after 

 day feeding it no grain, but only hay or straw, as to work 

 the soil season after season without supplying it with plant 

 food. 



81. Kinds of Fertilizers. A fertilizer is any substance 

 added to the soil to maintain or to increase its power to 

 produce crops. Fertilizers are either direct or indirect. 

 A direct fertilizer is one that supplies available plant 

 food directly to the soil. Nitrate of soda is a common 

 example. 



An indirect fertilizer, such as lime, is one that benefits 

 the growth of plants through its effects on the soil by im- 

 proving texture, preventing sourness, promoting the growt li 

 of helpful bacteria, or by converting unavailable plant food 

 into available food, but not by furnishing substances to 

 plants directly. 



A complete fertilizer is one that supplies nitrogen, phos- 

 phoric acid, and potash, all three. One that furnishes but 

 one or two of these plant foods is termed an incomplete 

 fertilizer. 



Commercial fertilizers. The question of fertilizers has 

 become so important that large industries have been de- 

 veloped in the United States to manufacture certain kinds 

 of them. Nearly six hundred manufacturers are selling 

 more than $150,000,000 worth of fertilizers to American 

 farmers yearly. Such manufactured products are called 

 commercial, or artificial, fertilizers to distinguish them 

 from those like manures, which are produced by nature. 

 The manufacturers gather the raw materials from widely 



