DECOJIPOSITIOX OF HTSITS. 71 



5. Ascertain the quaatity of clay by the operation described. 



6. By sifting the residuiim througii 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 compirisoa of the results will show the difference in the com- 

 position of the two Sjils — -and in none of tae cDnstitaents wiU the 

 diflFerence bs more mirkei than in the orgiaio constit'ieats and in 

 the sani aai gravel; ta: former, composel mainly of humus or the 

 constituents of humas, having a great affiaity for moisture, being 

 greatly in excess in the forest sail, 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 first editor of the Quarterly Journal of Agri- 

 culture, issued by the Highland and Agricultural Society of Scotland, 

 and who subsequently resided in Ceylon, where he had opporttmitiea 

 for prosecuting researches upon which he had previously entered. 

 The writer remarks, in regard to effects foUowing 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 reiy unproductive. Had 

 occasional trees in the forest been left to give shale 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 succe^ive crops growing on the soU, as they required it. 

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



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



