10 THE CHEMISTRY OF THE FARM 



phosphoric acid and potash to the plant, as these sub- 

 stances, especially the former of them, exist in the soil 

 in difficultly soluble forms, and are present in very 

 small quantity in the water contained in soils. 



An apparently selective power is exerted by plants, 

 some soluble ash constituents being taken up in much 

 larger quantity than others, which may actually be more 

 abundant in the soil. A striking example of this is the 

 assimilation of potassium in preference to sodium salts. 

 This selective action of the roots is quite explainable 

 by the known laws of diffusion. When ingredients of 

 the sap are removed out of solution by becoming part 

 of the tissues of the plant, the diffusion of such sub- 

 stances from the soil will continue ; while salts not 

 appropriated by the tissues can continue to enter by 

 diffusion only so long as the solution in the soil is 

 stronger than that in the plant. 



2. Assimilation of Nitrogen. — Besides furnishing the 

 plant with its ash constituents, the root has the impor- 

 tant function of supplying nitrogen ; this is nearly 

 always taken up in the form of nitrates. A plant is 

 capable of making use of nitrogen in the form of nitric 

 acid or ammonia ; it is also, according to several 

 experimenters, able to assimilate nitrogen when in the 

 form of urea, uric and hippuric acids, and several 

 other amide bodies. The facility, however, with which 

 ammonia and amide bodies are converted into nitric 

 acid in the soil is so great that nitrates become by far 

 the most important source of nitrogen at a plant's 

 disposal. In the case of soils very rich in organic 

 matter, as peat bogs and some forest soils, nitrates 

 may be entirely absent ; in these cases ammonia, or 



