646 Journal of Agriculture. [10 Oct., 1910. 



THE INFLUENCE OF STUBBLE BURNING ON THE 

 FERTILITY OF THE SOIL. 



Alfred J. Eivart, D.Sc.. Ph.D., F.L.S.. Government Botanist and 

 Profcssor of Botany in the Melbourne University. 



The statement is frequently made that burning off the stubble after a 

 grain crop improves the fertility of the soil and favours the growth of 

 the succeeding crop. This effect is generally ascribed to the alkaline 

 ashes left behind after burning, but since these ashes are no greater in 

 amount than what is returned to the soil when the stubble is ploughed in, 

 the only difference could lie in the fact of their being immediately available 

 for absorption by the plant, instead of being slowly set free as the stubble 

 decomposed in the soil. 



The experience of others has led theni to deny either that stubble 

 burning does improve the fertility of the soil, or that if there is any effect, 

 it can be due to the ashes of the crop. Mr. Herbert {Journal of Agricul- 

 ture of South Australia, 1910, page 791) compared the effects of scatter- 

 ing ashes over one plot and burning rubbish on another. The experiment, 

 though a very crude one, showed in favour of the latter plot. In the 

 same Journal, page 967, the explanation is suggested that where stubble 

 burning exercises a beneficial effect on the succeeding crop, this may be 

 due to the effect of the partial sterilization of the soil by heat. Recent 

 investigations have shown that steaming soil, or heating it to 180° Fahren- 

 heit, temporarily destroys the minute animal organisms which feed ui)on 

 the nitrate-producing and nitrogen-fixing organisms in the soil. As the 

 result, the bacteria increase in numbers and make more nitrogen available 

 for the use of the crop. 



As against this explanation, we have the fact that burning off stubble 

 only heats the immediate surface of the soil and does not appreciably 

 affect the temperature of the deeper layers below the surface inch or two. 

 Even in burning off dense scrub, it is surprising to how small an extent, 

 the .soil below the surface layers becomes heated. It is only when thick 

 roots smoulder away underground, or when the soil contains so much peat 

 or humus as to burn itself, that it becomes strongly heated to any depth. 

 The explanation therefore, though ingenious, cannot be regarded as 

 definitely established without further proof. 



It should be remembered that three classes of bacteria, which render 

 jjUiogen available for the use of the plant, exist in the soil. The first 

 group, which we mav generally term Nitrate bacteria, are concerned in 

 converting the organic nitrogen of the humus in the soil (which ordinary 

 plants cannot use), into nitrates, chiefly of Calcium and Potassium, which 

 they can freely ab.sorb. This action can only go on when alkaline or 

 alkaline earth bases are present such as Calcium, Potassium, Magnesium, 

 &c. The addition of such substances in the form of ashes, even if the 

 amount was slight, might temporarily increase the production of nitrates, 

 particularly in acid soils. At the same time, this means that more humus 

 is oxidized and the nitrogen capital of the soil reduced. Further, it is 

 only where the soil contains nitrogenous humus, that any such action is 

 possible. Hence, stubble burning not only decreases the amount of humus 

 returned to the soil, but also accelerates the exhaustion of that already 

 present in it. Owing to their high mean temperature, the oxidation of 

 humus is already sufficiently rapid in most Victorian soils. 



