228 SOILS AND FERTILIZERS 



too inert to undergo chemical change. Liebig grew 

 barley in boxes so constructed that all of the water- 

 solnble plant food could be secured. Two of the boxes 

 were manured and two left unmanured. In one box 

 which received manure and one which received none, 

 barley was growm. One each of the manured and 

 unmanured boxes was left barren. He collected all of 

 the drain waters and determined the soluble mineral 

 matter present, also weighed and analyzed the crop. 

 His results showed that 92 per cent, of the potash in 

 the crop was obtained from forms insoluble in water. 72 



In the roots of all plants there are present various 

 organic acids. Between the rootlet and the soil there 

 is a layer of water. The plant sap and the soil water 

 are separated by plant tissue which acts as a membrane. 

 All of the conditions are favorable for osmosis. The 

 acid sap from the roots finds its way into the soil in 

 exchange for some of the soil water. This acid, excre- 

 ted by the roots, acts upon the mineral matter, render- 

 ing it soluble, when it is taken up by the plant. 

 Different plants contain different kinds and amounts 

 of organic acids as well as present different areas of 

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

 agricultural crops have different powers of assimila- 

 ting food. 



Plants not only possess the power of rendering their 

 food soluble but they are also able to select their own 

 food and to reject that which is unnecessary. For ex- 



