PLANT FOOD IN SOILS. 201 



must yield results much more nearly representing the sol- 

 vent action of plant roots than are obtained when the soil 

 is extracted with concentrated hydrochloric acid, may be 

 freely granted, but it is perhaps open to doubt whether the 

 solution is the best which could be employed. Actual ex- 

 periments can alone decide such a question. We may, 

 however, remark that the strength of citric acid adopted is 

 distinctly greater than the total acidity found in the root sap 

 of agricultural crops. We would also point out that plant 

 sap must contain combined as well as free organic acids, and 

 the fact that bases are dissolved out of the soil by the action 

 of this sap shows that a supply of combined acid is constantly 

 maintained. Now we have just seen that the action of a 

 soluble citrate (or, at all events, of ammonium citrate) upon 

 ferric and aluminic phosphate, and upon calcium phosphate, 

 is quite distinct from the action of free citric acid, the citric 

 acid especially attacking the calcium phosphate, while the 

 citrate had a preponderating action upon the ferric and 

 aluminic phosphates. It is thus open to doubt whether the 

 selection of free citric acid only, is perfectly judicious. In 

 this connection it may be mentioned that P. Wagner, in 

 1 886, proposed the use of a solution containing 0*2 per cent, of 

 free citric acid, with 3*0 per cent, of citric acid combined with 

 ammonia, for the purpose of ascertaining what proportion of 

 the phosphate of a manure was to be reckoned as immedi- 

 ately available for plants ; and he was led to adopt this pro- 

 portion of free and combined acid because it gave the same 

 results as to the comparative value of manures as were 

 shown by actual experiments with agricultural plants. 



We now turn to the results obtained by Dr. Dyer by 

 the use of a one per cent, solution of citric acid. These 

 results are of great interest. " He has worked on a special 

 series of soil samples taken from twenty-two plots in the 

 barley field at Rothamsted in the autumn of 1889, and re- 

 presenting the first nine inches in depth. In these samples 

 he has determined both the total phosphoric acid and potash, 

 and also the phosphoric acid and potash extracted by a one 

 per cent, solution of citric acid. 



As an example of the immense difference to the crop 



