DECOMPOSITION OF HUMUS. 71 



6. Ascertain the quantity of clay by the operation described. 



6. By sifting the residuum through a piece of fine muslin, the sand 

 may be separated from the gravel, and the quantity of each deter- 

 mined by weighing them apart. 



A comparison of the results will show the difference in the com- 

 position of the two soils — and in none of the constituents will the 

 difference be more marked than in the organic constituents and in 

 the sand and gravel ; the former, composed mainly of humus or the 

 constituents of humus, having a great affinity for moisture, being 

 greatly in excess in the forest soil, and the latter, having little or no 

 affinity for moisture, being almost awanting. 



This humus is a product of vegetation, inasmuch as it is a result 

 of the decay and decomposition of vegetable products ; and it is not 

 only not decreased but it is constantly being increased by vegetation. 

 Under no crop is this more manifestly the case than under a crop of 

 deciduous trees. For this the annual fall of the leaves enables us 

 without difficulty to account. The shelter afforded by a wood pre- 

 vents both the fallen leaves, and the dust to which they may be 

 reduced, from being borne away by the wind, and the shade prevents 

 the humus from being so decomposed as to be carried off by the air. 



With the smaller quantity of humus in the soil not protected by 

 shelter and shade, it is otherwise. What there occurs is thus stated 

 in a paper I had occasion to cite in treating of the effect of de- 

 nudation of the country on the hydroscopicity of the soil,* a paper 

 on the philosophy of arboriculture, by the Rev. Dr Macvicar, of 

 Moffat, who was the iirst editor of the Quarterly Journal of Agri- 

 cidture, issued by the Highland and Agricultui-al Society of Scotland, 

 and who subsequently resided in Ceylon, where he had opportunities 

 for prosecuting researches upon which he had previously entered. 

 The writer remarks, in regard to effects following the destruction of 

 forests, — " The soil when stript of the clothing which the forest 

 afforded, and exposed naked to the heat of the sunbeam, changes very 

 rapidly from the rich mould which the long-continued fall of the leaf 

 in the forest had made it, and becomes very unproductive. Had 

 occasional trees in the forest been left to give shade during part of 

 the day, the destination of the carbon in the mould would have been 

 to have been slowly converted into carbonic acid, and so to supply 

 food to the successive crops growing on the soil, as they required it. 

 But when the sunbeam is left to break in its fall force on the soil all 



• " Hydrology of South Africa " (p. 213). 



