AVAILABLE PLANT FOOD. 101 



Soil A. Soil B. 



Insoluble matter ... ... ... 67'38 80-28 



Ferric oxide and alumina (FeA and A1 2 3 ) 15'61 8- 16 



Lime (CaO) ... ... ... -29 -13 



Magnesia (MgO) ... ... ... -31 -21 



Potash (K 2 0) ... ... ... -86 -48 



Phosphoric acid (P a O 5 ) '15 -12 



Not determined (soda, chlorine, &c.) ... T42 1-13 



100-00 100-00 



From these figures it would appear that soil A is better 

 provided with lime, potassium, and phosphoric acid than 

 soil B, and inasmuch as there is also more nitrogen present 

 one would conclude that soil B would receive much more help 

 from phosphatic and lime manures than soil A. 



Actual practice shows exactly the opposite, for it is found 

 that basic slag produces a large increase and great improve- 

 ment in the crop on the field from which soil A was taken, 

 while the field from which soil B was derived does not respond 

 to applications of basic slag. 



From such disagreements between the results of chemical 

 analysis and actual farming experience, which are often met 

 with, it is evident that to know what a soil contains is not 

 sufficient to enable one to form a judgment as to its fertility ; 

 one must know, in addition, something about the state of exis- 

 tence of the important items of plant food must know, in 

 fact, the amounts of these ivliich are directly available to the 

 plant. 



Many methods of extracting soil so as to obtain some 

 measure of the phosphoric acid and potash particularly, which 

 are in an assimilable form, have been suggested. One of the 

 most successful is that of Dyer." This method is based upon 

 the extraction of the soil with a one per cent, solution of citric 

 acid, which Dyer found to be about the average acidity of the 

 juices of the roots and root hairs of a large number of plants 

 examined by him. 



The method is thus carried out : 200 grammes of the air- 

 dried soil which has passed the 3mm. sieve are, without 



* Jour. Chem. Soc. 1894, Trans. 141. 



