PLANT FOOD IN SOILS. 195 



hardly be over-rated. Most of the phosphoric acid in soil 

 exists as a basic ferric phosphate, insoluble in water and in 

 carbonic or acetic acid, and but for the existence of this 

 solvent power in roots would remain useless to vegetation. 

 The potash, and we may add the ammonia, of soils is held 

 in almost equally insoluble combinations ; but analytical 

 chemists are aware that the whole of the ammonia, and 

 more or less of the potash, becomes soluble as soon as 

 the soil is placed in a weak solution of hydrochloric or nitric 

 acid. The acid sap of the roots is thus equally required 

 to bring about the solution of these important soil con- 

 stituents. 



It is clear from w T hat has been now said that it should 

 be the aim of a soil analysis to show what is the amount of 

 soluble plant food which a soil contains ; soluble, that is, 

 either in water, or in the acid root sap of the crops. Only 

 when this is done can the present fertility of a soil be plainly 

 indicated by its analysis. 



There is no difficulty in ascertaining the amount of plant 

 food soluble in water. By following Schlcesing's plan, and 

 displacing, by means of water, the solution actually present 

 in the soil, we can, if we please, examine the natural solution 

 in the soil. Or by extracting a powdered soil with water 

 on a vacuum filter, we may speedily remove the whole of 

 the freely soluble salts, including nitrates and chlorides. 

 This mode of proceeding will suffice to show what is the 

 amount of immediately available nitrogen which a soil con- 

 tains, as this will be represented by the nitric nitrogen 

 found on analysis. The following example, taken from the 

 series of analyses made of the soils of the Rothamsted 

 barley field in March, 1882, will show the enormous differ- 

 ences between the quantity of the total nitrogen of the soil 

 and the effective nitrogen existing as nitrates, and also the 

 dependence of fertility upon the latter. 



