PLANT FOOD IN SOIL 89 



Food can be taken up by the roots of plants only 

 when it is in solution, or in a condition capable of 

 being dissolved by contact with the acid sap of the 

 root hairs. Matter which is in neither of these con- 

 ditions is useless to the plant, though it may after- 

 wards become available by the chemical actions within 

 the soil. Most of the ingredients of soil are in an 

 insoluble condition : this fact is really of the utmost 

 advantage, as else soils would lose their fertility by 

 heavy rain. 



The chemical analysis of soils usually aims at deter- 

 mining the total amount of the various matters pre- 

 sent in a soil, or else the quantities soluble in strong 

 hydrochloric acid ; it does not therefore succeed in 

 furnishing a measure of the soil's fertility. Dyer has 

 lately employed with success a 1 per cent, solution of 

 citric acid for soil analysis ; this solution has an 

 acidity somewhat similar to that possessed by root 

 sap. By extracting the soils of the experimental 

 wheat and barley fields at Kothamsted with this 

 solution, he found that when the soil contained '03 

 per cent., of phosphoric acid, soluble in 1 per cent, 

 citric acid, manuring with phosphates was not 

 needed ; but that when only '01 per cent, was present, 

 phosphatic manure was urgently required. When the 

 potash soluble in 1 per cent, citric acid amounted to 

 •004 per cent, potash manures were apparently not 

 required for barley ; in the case of wheat, '005 or 

 •006 per cent, apparently sufficed. Wood, working 

 with calcareous Norfolk soils, found that '008 per cent, 

 of potash soluble in 1 per cent, citric acid was quite 

 insufficient for barley, but •OlS was sufficient. 



