CHAPTER X. 



ACTIVE PLANT FOOD AND WATER-SOLUBLE CONSTITUENTS 

 OF THE SOIL. 



The complete analysis of the soil, or its partial analysis by 

 strong acids, does not show clearly the immediate needs of the soil 

 for plant food. Various weak solvents have been used for this 

 purpose, with some measure of success. 



Active plant food is a term used to designate the potash and 

 phosphoric acid soluble in fifth-normal nitric acid. We are not 

 yet able to ascribe different values to the organic nitrogen com- 

 pounds of the soil. 



Dilute Citric Acid. Numerous attempts have been made to 

 ascertain soil deficiencies by means of weak solvents, such as 

 water, water containing carbon dioxide, weak solutions of citric, 

 hydrochloric, nitric, or oxalic acids, etc. 



Dyer 1 found that the root acidity of 100 plants expressed as 

 citric acid, varies from 0.34 per cent, with Solanaceae to 3.4 per 

 cent, with Rosaceae, and averages 0.91 per cent. He based on 

 this work a method of estimating the available plant food by 

 extracting the soil with a i per cent, solution of citric acid. The 

 method was applied to Rothamsted soils. 2 The following are 

 some of the results : 



BROADBAI.K WHEAT PLOTS. 



The amount of phosphoric acid and potash dissolved by i per 



1 Jour. Chem. Soc., 1894, 115. 



* Bulletin 106, Office Exp. Sta., U. S. Dept. Agr. 



