220 VEGETABLE SOIL DIAGRAM 19. 



organic matters is requisite to form a good vegetable soil. Dead 

 leaves, the fragments of plants which fall on the ground, and soil 

 drifted by the wind, all contribute to increase continually the 

 thickness of this vegetable layer. 



Many rivers overflow their banks at certain seasons of the 

 year, and cover broad plains with their waters, which there 

 deposit the earth which they carry with them. Soil, thus formed 

 of earth or sand brought down by the agency of water, is called 

 an alluvial deposit. 



Vegetable earths may be classed in four principal groups : 

 1st. Sandy soils. 



2nd. Clayey soils. 

 3rd. Calcareous soils. 

 4th. Peaty soils. 



1st. Sandy soils are chiefly composed of gravel or of sand, 

 which is sometimes very fine, and does not retain water ; these 

 soils are liable to drought. Such soils are generally found on 

 the shores of the sea, or of rivers. When mixed with a large 

 proportion of decaying vegetable matter, the sandy soils form 

 heath-soil, which may be rendered very productive by abundant 

 watering or manuring. 



2nd. The clayey earths are those where clay predominates, and 

 agriculturists generally call them stiff, or heavy soils. They are 

 often of a reddish colour, and may even be recognized at a distance 

 by their appearance. When mixed with water, they form a kind 

 of stiff paste. Clayey soils are nearly impervious to water ; if 

 sloping, they are easily cultivated, and yield large returns, but 

 if they cover a flat country, the water cannot run off, but ac- 

 cumulates at the surface, and the vegetation is to some extent 

 drowned. Then, when warm weather returns, the soil hardens, 

 cracks, and thus tears the roots of the plants. 



On the other hand, when clayey suils contain a proper propor- 

 tion of sand or lime, they are excellent for agricultural purposes. 



3rd. Calcareous soils are those which are formed by the decom- 

 position of calcareous strata. When there is too much lime, the 



