io8 SOIL CONDITIONS AND PLANT GROWTH 



enough to retard, but not to prevent, percolation, and it 

 facilitates capillary movement of water. 



Fine sand (o"2 to 0*04 mm. in diameter) forms a consider- 

 able fraction — usually 10 to 30 per cent, or more — of nearly 

 all soils. Although its dimensions are relatively large, it still 

 possesses cohesiveness and a tendency 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^j 

 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 to 0"2 mm. in diameter) is perhaps the most 

 variable of all soil constituents in amount, and, as its pro- 

 perties 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 soils contain less coarse sand than one-half 

 the quantity of clay present. When the coarse sand exceeds 

 the clay in amount the soil becomes light, unless of course 

 the clay is above 20 per cent, when the soil must always 

 remain heavy. Not being a colloid, it possesses no power of 

 absorbing water or soluble salts. Soils containing 40 per 

 cent, or more of coarse sand and less than 3 per cent, of clay 



