THE NITROGEN QUESTION. 561 



Another theory is that the addition of readily soluble nitro- 

 genous fertilizers to a soil causes too great a concentration of salts 

 in the soil water, and I have no doubt but that this accounts in 

 some cases for the diminished yield sometimes obtained by the 

 use of soluble potassic as well as nitrogenous fertilizers. 



The real explanation, I think, is a two-fold one. In the first 

 place, the optimum percentage of available nitrogen (chiefly 

 nitrates) in the soil is lower the more arid the climatic conditions. 

 It is well known that one effect of excessive nitrogenous manuring 

 is greatly increased leaf development, whch means an unduly 

 large transpiring surface — a condition of things which is inimical 

 to successful crop production in a country with a limited rainfall and 

 subject to hot drying winds. 



In the second place the ratio of available nitrogen (nitrates) 

 to total nitrogen in the soils of semi-arid and semi-tropical 

 countries is greater than under more humid and temperate con- 

 ditions, for two reasons, namely : — 



(1) The diminution in the loss of soluble forms of nitrogen 



through drainage. 



(2) The increased bacterial activity of the soil. 



In a country like England, with a heavy rainfall and low 

 evaporation, the surplus water is mainly lost by percolation, i.e., 

 by natural or artificial drainage, with the result that there is a 

 considerable loss of soluble salts, especially nitrates, whereas in 

 such a district as the wheat belt of New South Wales, with a limited 

 rainfall and a high evaporation, the bulk of the water not used by 

 the crop is lost by evaporation, and the soluble salts are therefore 

 retained in the soil. 



That there should be increased bacterial activity in the soils of 

 warmer countries one would naturally anticipate, and that there is 

 such increased activity as far as the nitrifying bacteria are con- 

 cerned has been conclusively proved by some experiments carried 

 out by the writer in the Transvaal. 



The method adopted was as follows : — 



A solution was made up containing the following quantities 

 of the various salts per litre : — 



Potassium di-hydrogen phosphate . . 0'25 grs. 



Magnesium sulphate . . . . . , 0-125 grs. 



Ferrous sulphate . . . . . . 0"1 grs. 



Sodium chloride . . . . . . . . 0*5 grs. 



and Ammonium Sulphate . . . . . . 0'5 grs. 



This solution has been used by Ashby, Hall and others in 

 nitrification experiments, and has been found to be very readily 

 nitrifiable, when the necessary organisms are introduced. 200 cc. 



M2 



