60 SOIL CONDITIONS AND PLANT GROWTH 



chalk (which become converted into calcium bicarbonate 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 car- 

 bonate in the soil. Further, as pointed out above, clay " fixes " and 

 retains the ammonia and potash supplied as manure. In general 8 to 

 1 6 per cent, is a satisfactory proportion of clay in a soil where the rain- 

 fall is 20 to 30 inches per annum. 



Fine silt (croi 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 pro- 

 perties of clay and is less altered by lime ; indeed no method is known 

 for making it tractable. It is usually less in amount than the clay ; cer- 

 tain peculiarities in cultivation are manifested where the reverse obtains, 

 e.g., in the Lower Wealden strata, the Upper Greensand and the Lincoln- 

 shire warp lands. 



The coarser grade of silt (0*04 to 0*01 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. 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 im- 

 portant part in maintaining the even conditions of moisture so desirable 

 for plant growth. It is fine enough to retard, but not to prevent, per- 

 colation, and it facilitates capillary movement of water. 



Fine sand(O'2 to 0*04 mm. in diameter) forms a considerable fraction 

 usually 10 to 30 percent, or more of nearly all soils. Although its 

 dimensions are relatively large, it still possesses cohesiveness and a ten- 

 dency to cake together ; it has not, however, so great an effect as silt 

 in maintaining a good moist condition. Soils containing 40 per cent, 

 or more of fine sand tend to form, after rain, a hard crust on the surface, 

 through which young plants can only make their way with difficulty 

 until it has been broken by a roller. But they have no great water- 

 holding capacity or retentive power, and are not infrequently described 

 by their cultivators as hungry soils that cannot stand drought The 

 notoriously infertile Bagshot sands and the barren Hythe beds in 

 West Surrey are largely composed of this fraction, as much as 70 per 

 cent, being sometimes present. In all these cases, however, clay is 

 deficient and the situation is dry ; better results are obtained when 



