;h. i 



SOIL 11 



}f sandy soil from the Kordofan sand-dunes, when 

 analysed in the dry atmosphere of Khartoum, gave 

 from 0"02 to 0*10 per cent of moisture. 1 In England, 

 lanalyses carried on after drought gave higher results, 

 viz. in clay soil without organic manures 11*9 per 

 cent, and with annual dressings of manure 18*0 per 

 cent, in heavy clay 20 per cent, and in sandy soil 7 '6 

 per cent. 2 



According to the amount of moisture which is 

 present in the soil, whether it be due to the size of the 

 soil particles, their grain or texture, or to abundance 

 or otherwise of organic matter, or whether it be due to 

 flooding which may be caused by impermeable strata 

 below or from inrush of water from above or from the 

 sides, the soil may be classed as wet when all the avail- 

 able space not occupied by soil-particles is taken up by 

 water, or nearly so. Likewise, by gradually reducing 

 the amount of water in the soil, it passes through 

 different gradations known as moist, fresh, dry, and 

 arid. The most generally favourable to forest growth 

 is a fresh soil, which should only leave traces of moisture 

 on the hand when pressed. Such a soil contains an 

 adequate supply of water and air to suit the majority 

 of plants. Such as grow in wet soils must be able to 

 content themselves with a small or intermittent supply 

 of air, or they must furnish themselves with special 

 processes by which their roots or stems can take up 

 the necessary amount of air direct from the atmosphere. 

 Such are pneumatophores, which we shall refer to later. 

 In such soils, as well as in the driest, the plants have 

 foliage which reduces transpiration to a minimum. 



The cohesiveness of a soil, which denotes the amount 

 of resistance which it offers to penetration by roots and 

 also to air, moisture, and warmth, and the anchorage 

 which it may give to the plants, depends largely on 

 the mineralogical composition of the soil. Thus clays 



1 Third Report of the Wellcome Research Laboratories at the Gordon College, 

 pp. 416 and 418. 



2 " Science Progress," No. 21 ; The Soil and the Plant, by E. J. Russell, D.Sc. 



