MINERAL CONSTITUENTS 139 



and may be easily reduced to a powder or dust made up of indi- 

 vidual particles. These run together badly with heavy rains. 



4. Sands and Sandy Loams. Sand is divided into three 

 groups, line, medium, coarse and sometimes very fine, varying 

 from 0.032 to 1 nun. in diameter ( Hopkins), ().().") to 1 mm. (Bureau 

 of Soils), or 0.12 to 1 mm. (Hilgard). Sand possesses very little 

 tenacity, hence little stability. There is usually great danger o[ 

 movement by the wind and in many cases sand soils are seriously 

 damaged in this way, as i.s seen in the " blow-outs " in sand areas 

 (see p. o!)). This movement may be prevented by incorporating 

 organic matter which imparts sufficient tenacity to hold the sand. 

 The fine and medium grades of sand allow fair moisture movement 

 both up and down, but the coarse allows too much percolation, while 

 capillary movement is exceedingly limited. It is generally believed 

 that sands are very deficient in moisture and that the "tiring" of 

 corn on sandy lands is always due to this cause. Often, however, 

 it is due to a lack of nitrogen, the drying of the lower leaves being 

 produced by translocation of nitrogen to carry on further growth in 

 other parts of the plant. This drying of the leaves may be almost 

 entirely prevented by supplying the crop with the necessarv food. 

 The fact that sands do not retain much moisture enables them to 

 warm up early in the spring. 



">. Gravel and Gravelly Loams. Many types of soil con- 

 tain considerable percentages of gravel. It is of verv little use 

 except that through its extremely slow decomposition it furnishes 

 a small amount of plant food. It may form a part of anv tvpe of 

 soil, but is more commonly associated with the coarser constituents. 



f>. Stones. Stones are quite common in many soils of the 

 glaciated and residual areas, but have very little value except to 

 modify temperature and conserve moisture to a slight extent. 

 Their slow decomposition may provide a small amount of plant 

 food. 



QUESTIONS 



1. Wlmt benefit is a knowledge of tlie physical composition of Boils'? 



2. What is meant by mechanical or physical analysis? 



.'?. Why is the Hopkins method considered superior to other-' 



4. Note the different factors or ratios between the grades. How much ,},, 



they vary? 



f>. Should these factors be constant ? Why? 



fi. Tlow is the sieve method used ? 



7. Explain how the separations are made in the subsidence method? 



8. What is the principle of Schone's elutriation method? 



