V. THE ANALYSIS AND COMPOSITION OF SOILS 93 



of the same materials extractable by 1 per cent citric acid solution. 1 

 Plants sown in such soil grew very slowly at first, but later on, probably 

 because of the gradual production of " available " plant food, they grew 

 much better. 



In warmer climates, the rate at which potash and phosphoric acid 

 become available is probably greater than in England, and this is prob- 

 ably why soils of such apparent poverty (on analysis) are able to 

 produce fair crops. This is very noticeable ia South Africa and also 

 in India. 



Many other methods of determining the available plant food in soils 

 have been proposed, in which weak solutions of hydrochloric acid, 

 carbonic acid and other substances are used as solvent, but careful 

 investigations have shown that all these methods, like that of Dyer, 

 yield empiric results. 



At present, therefore, it cannot be claimed that any method has 

 been designed which will determine directly the amount of plant food 

 which a given soil is capable of yielding to plants. Nevertheless, 

 Dyer's method and perhaps some of those in which aqueous solutions 

 of carbon dioxide are used as solvent, afford useful indications of the re- 

 lative powers of soils to supply the mineral requirements of crops under 

 similar conditions of temperature and humidity. The results obtained 

 by the application of such methods are, in most cases, much more in 

 accordance with the indications of actual field trials, than the figures, 

 obtained by a " complete " analysis of the same soil. 



Determinations occasionally made. A few other constituents 

 may require determination for special purposes : 



Determination of sulphuric acid. Sulphur is found in a soil as sul- 

 phates (most frequently calcium sulphate), in organic compounds, and 

 occasionally also as sulphides (e.g., iron pyrites). The sulphuric acid, 

 existing as such, may be determined by digestion of the soil with dilute 

 hydrochloric acid, filtration, and precipitation of the solution with barium 

 chloride, in the usual way employed for sulphuric acid determinations. 

 The total sulphur may be sufficiently well determined by digestion 

 with concentrated nitric acid for six hours, diluting with water, filter- 

 ing, and precipitating the filtrate with barium chloride. 



Determination of nitrates. The amount of nitrate present in a 

 soil at any given time is usually very small. 



If a determination be required, it is important that the soil be air- 

 dried as soon after its collection as possible, otherwise the process of 

 nitrification may continue and increase the amount of nitrates. It is 

 advisable to accelerate the process of drying by heating the soil to 30 

 or 40 C. and drawing a current of warm air over it. An arrangement 

 which the author finds very convenient for drying soils before analysis 

 is described on p. 83. With this apparatus the operation can be 

 rapidly completed without the possibility of contamination by dust 

 or products of combustion, and with little opportunity for nitrification 

 to appreciably increase the quantities of nitrates in the soil. 



1 Jour. Chem. Soc., 1908, Trans., 45. 



