32 SOILS AND MANURES 



an inch) diameter. Such material would be considered as 

 merging into clay on the one hand, and into gravel on the 

 other. In ordinary sands the grains are not of uniform 

 size, and,, though the extremes mentioned above are, per- 

 haps, not often reached, they may vary from about 0*5 

 to 0'05 m.m., or say, roughly, from ^ to ^J a ^ an ^ nc ^ 

 in diameter. Sand is often described as fine, medium or 

 coarse, according to the size of the grains ; the terms are 

 merely relative and are self-explanatory. Nearly all the 

 more important physical properties of sand can be traced 

 to the size and hardness of the particles. 



In the sense of mass, sand is a heavy substance, i.e., it 

 weighs more than an equal volume of more finely pulverised 

 material of similar density. Thus a cubic foot of sand 

 weighs about 90'3 Ibs., whereas a cubic foot of clay weighs 

 only 68 Ibs. 



In the agricultural sense sand is " light." In this case the 

 expression refers to the " tenacity " or lack of it which sand 

 exhibits, and which makes sand light or easy to cultivate. 



Corresponding to the size of the grains, the spaces 

 between them are also large and cannot therefore retain 

 much water. When the water drains away its place is 

 taken by air, and consequently sands are usually in a high 

 state of oxidation. But if the individual spaces are large 

 there are not so many of them, and the total amount of 

 unoccupied space is not so great as in substances which 

 consist of smaller particles. 



Sands are not usually associated with large amounts 

 of plant food. Quartz itself contains none, and whatever 

 plant foods are present are derived from other minerals. 

 Some deposits of sand, however, contain constituents of 

 plant food in quantities so large that they can be usefully 

 employed as dressings for cultivated soils. Conspicuous 

 amongst such are the previously mentioned micaceous 

 sand, which has been used as a source of potash; shell 



