258 SOILS AND FERTILIZERS 



soluble mineral matter present, also weighed and analyzed 

 the plants. His results showed that 92 per cent of the 

 potash was obtained from forms insoluble in water. 73 



The soluble plant food from a fertile soil is not gen- 

 erally sufficient for plant growth. 85 When oats, wheat, 

 and barley were seeded in prepared sand and watered 

 with the teachings from a pot of fertile soil, they made 

 only a limited growth. Oats grown in prepared sand 

 and watered with soil leachings assimilated only 25 per 

 cent as much phosphoric acid as plants grown in fertile 

 soil. See Section 224. The character and concentra- 

 tion of the soil solution are, however, important factors 

 in crop production and some soils may contain sufficient 

 amounts of water-soluble elements to produce crops. The 

 relative amounts of food which plants take from the 

 soil solution and that which they render soluble have 

 not been extensively investigated. 



In the roots of plants there are various organic acids 

 and salts. Between the root and the soil is a layer of 

 water. The plant sap and the soil water are separated 

 by plant tissue, which serves as a membrane. All of the 

 conditions are favorable for osmosis. The sap from the 

 roots finds its way into the soil in exchange for some of 

 the soil water. The acid and other compounds, excreted 

 by the roots, act upon the mineral matter, rendering 

 portions of it soluble, and then it is taken up by the 

 plant. Different plants contain different kinds and 

 amounts of solvents, as well as present different areas of 

 root surface to act upon the soil, and the result is that 



