270 ROCKS AND SOIL 



give as great a present yield as possible, without losing its 

 fertility for the future. And because all of us, no matter 

 where we live, depend upon soil products, we ought all to 

 be interested in the soil and its welfare. 



295. Structure of Soil. We can classify soils as to the 

 way in which they were formed, and also as to their 

 structure. A sedentary, or residual, soil is one that was 

 formed where we find it (cf. 291). An alluvial soil is 

 one formed by deposit from water. Such is the soil of 

 the Gulf Region and of the Nile Valley. Glacial soils are 

 made up of glacial drift. 



Soils that are very sandy are called light soils. They are easy to 

 cultivate, and they absorb water easily. But they also lose it easily, 

 so that their plants cannot live long in dry weather. Soils with an 

 excess of clay are called heavy soils. They take up water slowly; but 

 when they have taken it up, they hold it very firmly. A heavy soil, 

 on level ground, and in a region of good rainfall, is likely to be swampy. 

 Such soils often need to be drained by the digging of ditches and the 

 laying of "tiles," or pipes. If heavy soils are ploughed while too wet, 

 they cake, forming large, hard lumps separated by cracks. 



The plant gets its food from that part of the soil that is 

 in solution; hence a soil must contain water. It must also 

 contain air for the respiration (cf. 52) of the roots and 

 underground stems. For these and other reasons the 

 size of the soil particles, and the distance they are apart, 

 are of great importance for the growing of crops. Be- 

 ginning with the largest, we may classify soil particles as 

 gravel, sand, silt, and clay. Dry clay particles form a 

 dusty powder, like flour. From a soil of the right degree 

 of fineness a plant can draw water even in dry weather, 

 because the water can be transferred, by capillary action 



