IIYDROGEX lOX CONCENTRATION IX S.A. SOILS. 203 



1. A yellow colouration may result. In this case the pH 

 value is less than Q'5 and must be determined by the successive 

 use of the three acid indicators named above. 



2. A blue colouration may be given. The pH value is then 

 greater than 7"5 and is determined by the successive use of the 

 two alkaline indicators. 



3. A green colour may be given. In this case the pH lies 

 between 60 and 7'o. Mixtures of the stock solutions giving 

 different pH values between 6"5 and 7'5 are then prepared, and 

 10 drops of indicator are added to 10 c.c. of each mixture, until 

 a mixture is obtained which gives an exact colour match with 

 the unknown solution. The pH of this mixture is knoM n and 

 hence the pH of the soil extract. 



A similar procedure is followed with each of the other indi- 

 cators and in this way the pH of any soil extract may be very 

 exactly determined. 



It should be noted that the indicators should be used in 

 aqueous solution and not in alcoholic, since, according to 

 Prideaux,^ the addition of alcohol generally lessens the sensibility 

 of an indicator, that is, the indicator requires a higher value oi 

 [H-] or [OH'] to bring about its colour chanee. 



Work on South African Soils. 



A considerable amount of work on South African soils has 

 been done by Juritz,"- '*• '^ Marchand,"^ Williams, Watt,-" 

 Hall,^"'"'^* and others. The latest bibliography will be found 

 in the 1921 paper by the last-mentioned author." Hall's work, 

 while not primarily ecological, has constituted a notable advance 

 in our knowledge of nitrification and lime requirements in 

 numerous types of South African soils. He has shown that 

 nitrification in South African soils, when compared with that 

 in soils from many other countries, cannot be said to be excep- 

 tionally active, although it is good compared with the data from 

 other areas. He agrees with Watt that nitrification in Transvaal 

 soils is greater than at Rothamstead in England, but he points 

 out that field data from Ithaca, N.Y., record very much superior 

 amoimts of nitric nitrogen than were found here, and he cannot 

 endorse Watt's opinion that nitrification in Transvaal soils is, in 

 general, superior to that in soils of most temperate climates. 



In connection with the lime requirements of South African 

 soils, one of Hall's tables dealing with 54 representative soils 

 shows a variation from alkaline with 18'9 per cent, calcium 

 carbonate to soils with a " veitch lime requirement " of 14,400 

 pounds per acre foot. The relatively high lime requirement of 

 certain Natal High Veld soils agrees well with our pre- 

 liminary results obtained by the pH colorimetric method, though 

 it should be remembered that the lime requirement (a somewhat 

 ill-defined term) is probably a measure of total acidity and not 

 of real acidity. 



Hydrogen Ion Concentration in South African Soils. 



The results so far obtained by us are comparatively few in 

 number and must be looked upon as merely preliminary. We 



