THE COMPOSITION OF THE SOIL 107 



effects are reduced by flocculation effected by dressings of 

 lime or chalk (which become converted into calcium bicarbon- 

 ate in the soil) and by organic matter ; on the other hand, 

 they are intensified by the deflocculation resulting from the 

 use of alkaline manures like liquid manure, or by sodium 

 nitrate, which leaves a residue of sodium carbonate in the soil. 

 Further, as pointed out above, clay "fixes" and retains the 

 ammonia and potash supplied as manure. In general 8 to 

 16 per cent, is a satisfactory proportion of clay in a soil where 

 the rainfall is 20 to 30 inches per annum. 



Fine silt (o-oi to 0*002 mm. in diameter) has also great 

 water-holding power, and in excessive amounts (above 10 to 

 15 per cent.) it increases the difficulty of working the soil, 

 especially if much clay is present. It does not possess the 

 marked plastic and colloidal properties of clay, it behaves 

 differently towards electrolytes (see p. 162) and is less altered 

 by lime ; indeed no method is known for making it tractable. 

 It is usually less in amount than the clay ; certain peculiarities 

 in cultivation are in some cases manifested where the reverse 

 obtains, e.g. in the Lower Wealden strata, the Upper Green- 

 sand and the Lincolnshire warp lands ; in North Wales, how- 

 ever, many soils possess more silt than clay without any 

 apparent disadvantage. 



The coarser grade of silt (0*04 to o-oi mm. in diameter) 

 appears to be very valuable, and constitutes 30 to 40 per 

 cent, of many of the loams most famous in the south-east of 

 England for carrying their crops well and not drying out. 

 The fertile loess soils of the United States are also rich in 

 silt, containing 55 or more per cent, of material of '05 to 

 •005 mm. diameter.^ Light, sandy loams, on the other hand, 

 may contain only 10 to 20 per cent. ; some of these are 

 highly fertile, but as a rule they require large dressings of 

 dung, or a situation favourable for water-supply. Probably 

 silt plays a very important part in maintaining the even con- 

 ditions of moisture so desirable for plant growth. It is fine 



*J. G. Mosier and A. F. Gustafson, Soil Physics and Management, 1917 

 p. 64. 



