V. THE ANALYSIS AND COMPOSITION OF SOILS 97 



duced during the first fourteen days' incubation and rarely before the 

 end of twenty -one days. With Transvaal soils, however, Experiments 

 made under the writer's direction, in Pretoria, showed, under conditions 

 parallel to those in Ashby's experiments, formation of nitrites in five 

 or six days. As a rule, the nitrifying power of a soil is greater, the 

 larger the amount of nitrogenous organic matter present, but in some 

 of the vlei soils of the Transvaal, very rich in organic matter, the nitri- 

 fying organisms are apparently absent, for after addition of calcium 

 carbonate, no nitrification of ammonium sulphate occurred, even after 

 sixty days' incubation at 30 C. 



The writer is of opinion that in soils of temperate climates the pro- 

 gress of nitrification is generally limited by the temperature being too 

 low, while in hot countries it is rather the absence of basic material 

 (lime or magnesia) and perhaps, often of moisture, that limits the 

 rapidity of nitrification. 



For another method of determining the nitrifying efficiency of soils 

 vide Stevens & Withers, U.S. Dept. of Agric., Bull. 132, 1910. 1 



Interpretation of the Results of Analyses of Soils. The results 

 of an analysis of a soil are usually expressed in the following 

 manner : 



1. Stones retained by 3 millimetre sieve .... per cent. 



The air-dry " fine-soil " contains : 



2. Moisture expelled at 100 C 



3.* Loss on ignition . . . 



4. Insoluble matter ........ 



5. Iron oxide and alumina ....... 



6. Lime .......... 



7. Magnesia . 



8. Potash 



9. Phosphorus pentoxidc 



10. Total 



11.* Containing nitrogen ,, 



12. " Available " potash ........ ,, 



13. ,, phosphorus pentoxide ..... ,, 



The meaning of these terms has already been explained as well as 

 the methods of which their amounts are determined. A few words 

 may, however, be said as to the deductions which may be made from 

 the results expressed in this manner. 



1. Stones retained by 3 millimetre sieve. This item is subject to 

 enormous variation. Except in tenacious, heavy soils, as a rule, the 

 smaller the percentages of stones present in a soil the better, though 

 their presence in certain cases is useful in rendering the soil more 

 porous and open. In the Transvaal the soils are, as a rule, remark- 

 ably free from stones. In some gravelly soils, on the other hand, 

 stones may be very numerous. As sources of plant food they are 

 practically useless. 



2. Moisture. This, determined on the air-dried sample, varies 

 greatly. With sandy soils it is usually small, while the presence of 

 large amounts of humus or clay tends to cause it to be higher. As a 



1 Abstract in Jour. Soc. Chem. Ind., 1910, 1262. 



7 



