146 Report S.A.A. Advancement of Science. 



examined in our laboratories. I do not propose to give a long array 

 of figures, but will be content to deal with averages as far as possible. 

 In last year's paper, above referred to, will be found details of the 

 methods employed in the analyses, and a discussion of the deductions 

 which may be drawn from the various determinations. 



It must suffice on the present occasion to point out that although 

 plants require from the soil a large number of substances, the actual 

 fertility generally depends upon the soil's capability of supplying a 

 sufficiency of four materials, viz., combined nitrogen, potash com- 

 pounds, phosphates, and lime. Other constituents of plants, e.g., 

 sulphur, chlorides, iron, magnesia, etc., though indispensable, are so 

 widely distributed that it is rare to find soils lacking in them. 



For the present, then, only the four first-mentioned constituents 

 will be considered. 



Combined Nitrogen. 



In most soils this is present almost entirely in the form of 

 complex organic compounds, resulting from the decay of vegetable or 

 animal matter. The organic matter, in addition to yielding supplies 

 of nitrogen, has an important influence on the water-holding capacity 

 of the soil. 



In this state, nitrogen is not available to plants, but becomes so 

 by processes of decomposition, resulting in the formation of 

 ammonium compounds, nitrites, and, lastly, nitrates. It is in this 

 form that practically all the nitrogen which the plants require is 

 assimilated. The successive changes which lead to the formation of 

 nitrates from complex organic nitrogenous compounds are known as 

 nitrification, and are accomplished under the influence of at least 

 three species of micro-organisms. In order, therefore, for a soil to 

 be well adapted for supplying nitrogen to plants, it is necessary for it 

 to possess a sufficient store of organic, nitrogenous matter, suitable 

 micro-organisms, and the necessary conditions for their activity. 

 Among the latter, some of the most important are sufficient moisture, 

 the presence of some basic material, suitable for the formation of 

 nitrates, and a moderately high temperature. The first and last of 

 these are mainly dependent upon climatic conditions, the only 

 modification which is practicable for man to introduce is by means of 

 irrigation. The basic material most suited for the purpose is 

 carbonate of lime, and the process of nitrification leads to a consump- 

 tion of this substance. 



Most of our Transvaal soils are low in lime, and the process 

 of nitrification is probably limited in many cases by the deficiency of 

 basic material. On the other hand, other conditions for rapid 

 nitrification, e.g., high temperature, porosity, and, in the summer, 

 moisture, are favourable in this country. 



The amount of combined nitrogen in most of our soils is very 

 low, compared with ordinary English soils. There are, of course, 

 many exceptions, for soils from marshes, or " vleis," are often rich 

 in organic matter, and also in nitrogen. 



