25O SOILS AND FERTILIZERS 



left unfertilized. If both the corn and the wheat are 

 benefited by the nitrogen, the soil is in need of this ele- 

 ment. If, however, the wheat responds and the corn 

 does not, the soil is not in great need of nitrogen, but 

 does not contain an abundance in available forms. 



298. Deficiency of Phosphoric Acid. In experiment- 

 ing with phosphoric acid, turnips are grown on two 

 plots and barley on two plots. To one plot of each, 16 

 pounds of acid phosphate are applied. If both crops 

 show additional yields, the soil is in need of available 

 phosphoric acid. If only the turnips respond while the 

 barley is indifferent, the soil contains a fair amount. 

 Barley and turnips are used because there is such a 

 marked difference in their power to assimilate phosphoric 

 acid. 



299. Deficiency of Potash. In order to determine the 

 condition of the soil as to potash, potatoes and oats 

 may be used as the trial crops, and 8 pounds of sulphate 

 of potash should be applied to one plot of each. Addi- 

 tional yields indicate a poverty of available potash ; an 

 increased potato crop and an indifferent oat crop indicate 

 potash not in the most available form. If no additional 

 yields are obtained with either crop, the soil is not in 

 need of potash. 



300. Deficiency of Two Elements. If the preliminary 

 trial indicates a deficiency of two elements, as nitrogen 

 and phosphoric acid, in verifying these results, both 



