FOOD REQUIREMENTS OF CROPS 257 



For example, an acre of wheat contains 35 pounds of 

 nitrogen, while an acre of clover contains 70 pounds ; 

 if 70 pounds of nitrogen were applied to an acre of 

 clover and 35 pounds to an acre of wheat, poor results 

 would follow, because clover can obtain its own nitrogen 

 while wheat is less able to do so, and the 35 pounds 

 would not necessarily come in contact with the roots 

 so that all could be assimilated. While the amount of 

 plant food removed in crops cannot serve as the basis 

 for their manuring, valuable results are obtained from a 

 study of the different elements of fertility which they 

 contain. In making use of the preceding table, other 

 factors, as the influence of the crop upon the soil and 

 the power of the crop to obtain its food, must also be 

 considered. 



308. Plants exert a Solvent Power in Obtaining 

 Food. - - It is believed that crops procure some of their 

 food from minerals insoluble in water. Experiments 

 by Liebig demonstrate that plants have the power of 

 rendering a portion of their food soluble, provided it 

 does not exist in forms too inert to undergo chemical 

 change. Liebig grew barley in boxes so constructed 

 that all of the water-soluble 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 grown. One each of the 

 manured and unmanured boxes was left barren. He 

 collected all of the drain waters and determined the 



