FERTILIZERS . 127 



for over a century, it is, nevertheless, doubtful whether quite 

 the right path has been followed by investigations which en- 

 deavored to determine just how those benefits arose. Air and 

 soil are the media through which the growing plant receives its 

 nourishment, but this is more than a mere mechanical process. 

 In some cases at least there must be some sort of digestion or 

 decomposition of foods before there can be assimilation. Silica, 

 highly insoluble and apparently the least suited of all the min- 

 eral constituents of the earth to enter the vital organism of the 

 plant, however finds its way into the plant tissues. Phos- 

 phorus, one of the most important mineral foods of plants, exists 

 in the soil, or is applied in fertilizers, almost exclusively in the 

 form of mineral phosphates, but appears in the plant largely 

 in organic combination, while the mineral phosphates which do 

 appear are not those which pre-existed in the soil, such as 

 those of lime, iron and alumina, but chiefly those of potash. It 

 is also found that soils of different composition, texture and 

 structure supply different quantities of water to the plant, irre- 

 spective of the percentage of water actually present in the 

 soil. As water conveys the nutritive solutions to the plant, 

 when the supply of water is inadequate, there may also be a 

 deficiency of nutrient materials. It is probable, then, that ferti- 

 lizers, by temporarily increasing the concentration of the so- 

 lution, increase the food supply. Such fertilizers seldom per- 

 manently affect the nature of the solution, and the concentration 

 with respect to the mineral plant food constituents per unit of 

 solution is considered approximately constant. In the same 

 and in different soils, however, the water content varies widely, 

 and usually the greater the water content, the more diluted is 

 the solution. 



In 1902 such exceedingly delicate and sensitive methods for 

 analyzing soils in the field were devised that "the amounts of 

 nitrates, phosphates, sulphates and the like, which may be pres- 

 ent, as indicated by water solutions, can be determined to 

 within four or five pounds per acre one foot deep. ' ' Fertilizers 

 applied in the spring can be traced from the place of applica- 

 tion down through the different depths of the soil which they 

 invade. Much progress has been made toward determining by 

 analysis the fertilizers needed by a particular soil. 



