194 SCIENCE PROGRESS. 



results into connection with other previously ascertained 

 facts. 



The main object of a chemical analysis is clearly to 

 show what is the quantity of plant food existing in the soil. 

 Physiologists are aware that the plant food in a soil occurs 

 in two distinct forms. A plant can, in the first place, feed 

 upon substances which are in solution. The water in a soil 

 contains a more or less considerable amount of carbonic 

 acid, and in this weak solution of carbonic acid certain of 

 the ingredients of the soil are soluble. Soil water generally 

 contains a good deal of calcium and magnesium carbonate ; 

 it contains nitrates, chlorides, and sulphates, with soluble 

 silicic acid. It generally contains no phosphates, and only 

 traces of potassium salts ; sodium salts may, however, be 

 present. If, therefore, the plant were entirely dependent 

 on the soil solution for its nourishment, it would be starved, 

 as two essential constituents of plant food, phosphates and 

 potash, are not supplied by this medium. 



The second mode in which a plant feeds is by the 

 solvent action of its roots. This extremely important 

 function of the roots has been far too little investigated. 

 Sachs was the first to show that the root hairs of certain 

 plants had the power of eroding polished plates of marble, 

 dolomite, and osteolite, by virtue of the acid sap which they 

 contained. Zoller, more than thirty years ago, ascertained 

 at Liebig's suggestion that calcium phosphate, ammonium- 

 magnesium phosphate, and the potash of a freshly-manured 

 soil, were dissolved when placed on a membrane the other 

 side of which was in contact with a weak solution of hydro- 

 chloric, acetic, or citric acid. It is generally, and probably 

 correctly held, that this solvent action of the roots is especi- 

 ally effective towards the phosphoric acid, potash and other 

 substances which have been previously absorbed by the 

 soil from solution, and which are thus held on the surface of 

 the soil particles. As to the nature or amount of the free 

 acid present in root sap little is definitely known. A. Mayer 

 lays most stress on the presence of oxalic acid, which he 

 found in several instances. 



The importance of this solvent action of the roots can 



