PLANT FOOD IN SOILS. i 97 



made immediately the sample has been taken, or the soil 

 must be at once dried with suitable precautions (Trans. 

 Chan. Soc, 1882, 351). Even, however, when this is done, 

 the results obtained will fail to indicate the intrinsic quality 

 of the soil, as the amount of nitrate present at any moment 

 is largely influenced by the character of the preceding 

 weather, and by the cropped or uncropped condition of the 

 land. Much better results would be obtained if the analyst 

 were to ascertain what was the rate of nitrification in the 

 soil under certain fixed conditions. These rates would at 

 once indicate grades of fertility, and the accumulation of 

 results would soon enable the analyst to associate them with 

 a definite fertility of the soil. 



We have spoken of the plant food taken up from ready 

 formed solutions in the soil : we turn next to the undissolved 

 plant food of the soil brought into solution by contact with 

 the acid surfaces of the root hairs. If the analyst is to form 

 any idea of the quantity of effective phosphoric acid or 

 potash which a soil contains, he must employ some solvent 

 which shall imitate the solvent action of the root hairs. In 

 approaching such a problem we are at once confronted with 

 many difficulties. We do not fully know the composition 

 of the root sap of any plant, and there can be no doubt but 

 that the sap of different plants differs very materially in 

 composition, and that to this very cause is due some of the 

 remarkable differences in the capacity of various plants to 

 supply themselves with food from the soil. 



Dr. Bernard Dyer has recently published ( Traits. Chem. 

 So-c., 1894, 1 15) determinations of the acidity of root sap in 

 one hundred and three plants, representing twenty natural 

 orders. 



A summary of his results is given in the following 

 table : — 



