104 NATURE TEACHING. 



these and similar facts it has of late years 

 been recognised that earth-worms exercise a 

 very considerable influence in keeping soils 

 in a fertile condition. 



4. Natural processes of decay lead to the 

 steady disappearance of humus ; so that if land 

 is cultivated and the crop steadily removed, 

 there is a tendency for the soil to become poorer 

 and poorer as the humus, originally present, 

 rots away and nothing is added to replace it. 

 When this happens we hear complaints about 

 the soil being ' worn out' * Wasted ' would be 

 a better expression. In places where no crop 

 is removed, as in woods and forests, there may 

 be a steady increase of humus owing to the 

 leaves and other parts of the trees falling to the 

 ground and accumulating there more rapidly 

 than they are used up. The soils of such places 

 are thus, usually, very rich in humus, and 

 much sought after, for purposes of cultivation 

 on account of their fertility. Unfortunately, 

 this fertility is often rapidly wasted because 

 the cultivator takes no pains to keep up the 

 supply of humus. 



5. In the cultivation of all soils it is neces- 

 sary to add supplies of vegetable matter from 

 time to time, so that it may decay and become 

 mingled with the soil as humus. Good agricul- 



