104 AGRICULTURE 



that we supply in the form of artificial 

 manures is capable of serving at once, or 

 almost at once, as plant food. Bearing in 

 mind the fact, therefore, that there are 

 superabundant stores of nutritive materials 

 in most land, it is evident that we cannot 

 say that manuring is undertaken to furnish 

 plant food that is non-existent ; but, rather, 

 that manures are applied in order to provide 

 crops with food in a form in which it is not 

 naturally present in sufficient quantity in 

 the land. 



At one time it was believed that the 

 requirements of crops as regards manures 

 could jbe ascertained by chemically investigat- 

 ing the soil ; but the figures just quoted show 

 that the ordinary method of chemical analysis, 

 which merely shows the total percentages of 

 soil ingredients, is practically valueless for 

 this purpose. Of late years much attention 

 has been given to determining the availability 

 of the nutritive materials present in the soil, 

 and especially as regards phosphoric acid and 

 potash. In place of extracting these sub- 

 stances by means of strong hydrochloric acid, 

 analysis has proceeded on the lines of pre- 

 paring a solution containing a small percentage 

 of a weak solvent (1 per cent, of citric acid) ; 

 the percentage of phosphoric acid and potash 



