FOOD REQUIREMENTS OF CROPS 217 



is nearly helpless in obtaining it, and the 35 pounds 

 would not necessarily come in contact with the roots 

 so that it could all be assimilated. While the amount 

 of plant food removed in crops cannot serve as the 

 basis for their manuring, valuable results are ob- 

 tained from a study of the different elements of fer- 

 tility removed in crops. In making use of the pre- 

 ceding table, other factors, as the influence of the 

 crop upon the soil and the power of the crop to ob- 

 tain its food, must also be considered. 



294. Plants Exert a Solvent Power in Obtaining 

 Food. It was supposed at one time that plants ob- 

 tained all of their mineral food from the mineral matter 

 dissolved in the soil water. See Section 87. Experiments 

 by L,iebig demonstrated that plants have the power of 

 rendering a large portion of their own food soluble, 

 provided it does not exist in forms too inert to under- 

 go 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 soluble mineral matter 

 present, also weighed and analyzed the crops. His 

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

 crop was obtained from forms insoluble in water. 73 

 Other experiments have shown that the leachings 

 from a fertile soil do not contain sufficient plant food 

 to grow a normal crop. 87 



