Agriculture and Soils of the Cape Province. 439 



depend largely on their size. The coarsest particles (viz., coarse 

 sand) are roughly the very opposite in properties to the finest (viz., 

 clay), as will be apparent from the following parallel comparison: — 



Clay. Coarse Sand. 



The individual particles are invisible The particles are plainly visible to- 



to the eye, even when aided by the unaided eye. 



the most powerful microscope. 



When rubbed with water clay be- Does not become sticky or impervi- 



comes very sticky and impervious. ous under similar treatment. 



Clay is very plastic when moist, that Coarse sand can be moulded into 



is, it can be moulded into shapes ; shapes when wet, but the particles 



these shapes are retained when fall apart as soon as the moisture 



the clay dries and even when it evaporates, viz., the particles have 



is fired. Clay particles therefore no cohesion except when wet» 



possess the property of cohesive- Thus coarse sand tends to keep 



ness to a high degree. Clay in the the soil open and powdery. 



soil binds the particles together — 



into clods if the soil has been 



badly worked, into granules or 



crumbs if properly tilled. 



When dry clay is wetted it swells No such changes take place with 



and gives out heat. On drying coarse sand when similarly 



again it contracts. [A soil which treated. 



contains much clay cracks open 



when it dries out.] 



Clay impedes the movements of air Coarse sand permits of the free 



and water. movement of air and water. 



Clay retains much water. Coarse sand retains very little water. 



. Fine sand unlike coarse possesses cohesiveness ; the particles of 

 this group tend to cake together. If a soil contains very much fine 

 sand it usually "crusts " after irrigation or rain. Its power to retain 

 water is only slightly better than that of coarse sand. 



Silt is a very valuable constituent of soils; while fine enough to 

 retard, it does not prevent the ready movement of water; it retains 

 a fair quantity of water and possesses the power of lifting water well 

 by the so-called capillary action. 



Fine silt has great water-retaining power ; it is, however, neither 

 so impervious nor so plastic as clay. Fine silt hinders the movement 

 of water and air in the soil and is a valuable constituent provided 

 the soil contains little clay; should, however, the soil contain much 

 clay, the presence of fine silt only serves to accentuate its undesirable 

 physical properties. 



Hiimus possesses many properties similar to those of clay"; for 

 example, it is highly retentive of moisture, it expands when wetted 

 and it is very efficacious in binding particles of sand together, also 

 particles of clay. Humus therefore improves the physical properties 

 of both sands and clays. 



Carhonates.— Under this head are included carbonate of lime 

 and carbonate of magnesia. Both these substances are grouped 

 together because their effect on the physical properties of the soil is 

 similar. These carbonates have a veiy beneficial effect in making clay 

 soils easier to work, owing to the fact that they dissolve in the soil 

 water to a sufficient extent to produce a granulating or flocculating 

 eft'ect on clay. Chemically these two carbonates act in the direction 



