12 TROPICAL AGRICULTURE chap. 



tion of surface soil. These most useful animals feed on the 

 leaves and other parts of plants, and they burrow into the 

 ground in all directions, sometimes even to the depth of five 

 They drag or six feet. In order partly to protect themselves from 

 leaves into their encmics and partly to supply themselves with food, 

 t eir o es. ^^^^^ drag half-decayed leaves and stalks into their burrows, 

 and so block them up to a depth of two or three inches. 

 Thus worms fertilize the soil by adding organic matter to it. 

 The little heaps of earth often seen at the mouths of the bur- 

 rows, more especially in wet weather, are called casti7igs^ 

 and they consist of very fine particles of mould mixed with 

 decayed vegetable matter. The worms make their burrows 

 by swallowing the soil and then bringing it to the surface 

 where it is got rid of in the form of these castings. In this 

 They turn way they continually turn over the soil, and besides, their 



over and mix ,, . , -i • i i 



the soil. burrows allow air and water to get easily mto the earth, 

 and also form channels for the roots of plants to grow along 

 without difficulty. It is seen, therefore, that the humble 

 earthworms are really working for the benefit of the planter ; 

 and so, on that account alone, none of them should be pur- 

 posely injured or killed. 



Constituents of Soils. — All sorts of soils are composed 

 of five substances, namely (i) Sand, (2) Clay^ (3) Lime, (4) 

 Vegetable Matter, (5) Stones. 



Sand. — This substance is well known to every one. It 

 consists of fine particles of hard stone, which do not cohere 

 when wet, and which fall quickly to the bottom when they 

 are thrown into a vessel containing water. If sand be mag- 

 Nature of nified it will be found to be made up of minute smooth 

 stones with rounded edges, or, in other words, of very small 

 water-worn pebbles. There are several kinds of sand com- 

 monly met with. Calcareous sands consist of grains of 

 limestone, or of broken shells, as is often seen on the sea 

 shore ; hence we have coral-sands and shcU-sands. Mica- 

 ceous sands are largely composed of a hard substance known 



sand. 



