12 Transactions of the South African Philosophical Society. 



Great Brak Eiver) were extracted by the hydrochloric acid raethod 

 already referred to. Thus the following percentages of plant food 

 (Grade II.) were obtained : — 





Lime. 



Potash. 



Phosphoric 

 Oxide. 



A 



-050 



•023 



•012 



B 



-094 



•061 



•015 



C 



-044 



•017 



•008 



On evaporating soils taken from the same localities with a mixture 

 of hydrochloric and nitric acids, and thereafter boiling the residues 

 with hydrochloric acid, very much higher results were arrived at. 





Lime. 



Potash. 



Phosphoric 

 Oxide. 



A 



-25 



•14 



•11 



B 



-19 



•25 



•029 



G 



-25 



'27 



•055 



A clear indication that these soils, which were granitic in origin, 

 contained a considerable quantity of plant-food constituents of the 

 third grade. 



In analysing a soil with the object of determining its ''reserve 

 stock" {i.e., Grade II.) of plant food, a clear description of the 

 method of extraction is always essential. The standard method 

 adopted by Professor Hilgard and other American investigators 

 indicates the maximum amount of plant food that is present in any 

 given soil in a condition capable of extraction by plants : this is one 

 reason for the adoption of the method.* A somewhat similar state- 

 ment may be made in respect of the "immediately available" 

 (Grade I.) plant food in the soil. The results obtained, for instance, 

 by Dr. Dyer's citric acid method, may be looked upon as indicating 

 not the reserve stock but the immediately available plant food, and 

 as showing the maximum amount immediately available, in much 

 the same way as hydrochloric acid-'of 1-115 specific gravity shows 

 the maximum that can be considered to be present as a reserve 

 stock. Extraction of a soil with water likewise shows immediately 

 available plant food therein, although not the full amount. How 

 much less may be extracted from a soil by oue method than by 

 another will be gathered from the following results obtained in 



* The method adopted as a standard in the Government Laboratories here 

 differs from Professor Hilgard's only in being carried out at the ordinary tem- 

 perature, whereas his is conducted at steam temperature. In this respect our 

 method is more along the line of that employed by the German experiment 



stations. 



