SOILS AND FERTILIZERS 389 



Most of the States have efficient systems of fertilizer inspection 

 by which the farmer may determine whether he is paying more for 

 a commercial fertilizer than it would cost him if he purchased simple 

 fertilizer materials and mixed them himself. 



Beneficial Effects. The reason for the beneficial effect of fertil- 

 izers has long been held to be due to the supplying of mineral plant 

 food to the soil in a more soluble or available form than is found in 

 soils of low productive capacity. There aye three great facts which 

 have never been satisfactorily explained under this hypothesis: (a) 

 Stable manure as a rule has a much higher fertilizer value than would 

 be indicated by the amount of mineral plant food elements it con- 

 tains. (6) Plants contain usually as much, or more, of silica, iron, 

 aluminum, magnesium, and sodium as of nitrogen, potash, phos- 

 phoric acid, or lime, and the former group are as necessary to plant 

 life and are quite as insoluble in their soil compounds as the latter; 

 they are rarely used successfully as fertilizers, and they are not con- 

 sidered commercially as being in the class of plant foods, (c) Cer- 

 tain nonplant food substances, such as copper sulphate, ferrous sul- 

 phate, common salt, and some three or four hundred substances 

 which have been tried from time to time do give occasionally very 

 beneficial results. However, it is not always possible to explain any 

 fact by hypothesis, however broad we make it, and as we know that 

 nitrogen, po.tash, and phosphoric acid are essential elements of plants, 

 and as we often get bigger yields after applying substances contain- 

 ing them to the soil, we have come to look upon the question as 

 proved that they act solely or principally through feeding the plant. 



Another fact which we have known for a long time and which, 

 we have not been able satisfactorily to explain is that dwarfed plants 

 or plants on infertile soils often contain a larger percentage of ash 

 than better plants grown on more productive soils. This would in- 

 dicate that on such poor soils they are able to take up more potash, 

 phosphoric acid, and lime than they are able to elaborate into good 

 plant tissue. The most advanced scientific work now shows that fer- 

 tilizers have another and an important function in keeping the soil 

 sweet and sanitary for the plants. 



The natural supply of mineral plant food comes directly from 

 the solution of the mineral particles of the soil. The solubility of 

 these minerals gives a very dilute solution, but adequate in amount 

 and rapidity of solution for the need of field crops. Furthermore, 

 the soil has a great absorbant power, and soluble fertilizer minerals 

 applied to the soil do not long remain in the free or capillary water 

 in which plants are supposed to feed. That is, when a soluble phos- 

 phate salt is added to a soil in the usual amount, or even in much 

 greater amount than is ordinarily applied, it can not again be ob- 

 tained by simply washing the soil, as it is withdrawn from the solu- 

 tion and firmly held by the surface of the solid grains, and the soil 

 moisture is not sensibly enriched by the application. 



It is this variation which makes fertilizer practice so uncertain. 

 It is inexplicable from the plant-food idea. 



