196 



PRINCIPLES OF AGRICULTURAL CHEMISTRY 



phosphoric acid averaged 12 bushels corn possibility, and actual 

 production averaged 14. Considering the fact that the, actual 

 yields given were estimates, and that the corn possibility is prob- 

 ably a little low, the agreement is good. This matter requires con- 

 siderably further study. 



Water-Soluble Constituents. The water-soluble constituents of 

 the soil are of significance from the fact that material can enter 

 the plant only in solution. The root is composed of cells, 

 through which there are no openings for the entrance of solids. 



\Yhen a liquid containing a substance in solution is 



Fig. 46. Enlarged plant cell, normal below, and with the protoplasm 

 contracted by nitrate of soda above. 



brought in contact with another portion of the same 

 liquid which does not contain that substance, the dissolved sub- 

 stance passes into that portion until all parts of the liquid have a 

 uniform composition. This is called diffusion. The same occurs 

 when the liquid is separated by a membrane, which the dissolved 

 substance is able to penetrate. Substances which cannot generally 

 pass through membranes are termed colloids, examples being 

 albumen, glue, etc. Salt, sugar, calcium sulphate, which can pass 

 through, are called crystalloids. If, then, a plant cell is brought 



