62 SOIL CONDITIONS AND PLANT GROWTH 



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 croi 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 (0*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 

 the clay exceeds 8 or 9 per cent, or when the water table is near the 

 surface, especially if the amounts of coarse sand and gravel are not 

 too high. 



Coarse sand (i 0-2 mm. diameter) is perhaps the most vari- 

 able of all soil constituents in amount, and, as its properties are in 

 many ways the reverse of those of clay, it exercises a very great effect 

 in determining fertility. Through its lack of cohesion it keeps the 

 soil open and friable ; in moderate amounts it facilitates working, but 

 in excess it increases drainage and evaporation so much as to interfere 

 seriously with the water-holding capacity of the soil. Many good 

 loams contain less than 4 per cent, and in general strong or tenacious 



