108 NATURE TEACHING 



quantity of chalk over a field in order to observe at a 

 future date to what depth it had been buried. At the 

 end of twenty-nine years a trench was dug across the 

 field, when a line of white nodules was traced on both 

 sides of it at a depth of seven inches below the surface. 

 The mould, therefore (excluding turf), had been thrown 

 up at an average rate of -22 (or about |) inch per annum 

 through the agency of earth-worms. It was estimated 

 that the soil so brought up in this meadow weighed 

 about 73,000 Ib. From 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. 



Natural processes of decay lead to the steady dis- 

 appearance 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 pre- 

 sent, 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 

 annual addition of vegetable matter from the fallen leaves 

 being greater than the amount used up in a year. The 

 soil of such places, commonly called leaf-mould, is thus 

 usually very rich in humus, and much sought after for 

 purposes of cultivation, on account of its fertility. Un- 

 fortunately this fertility is often rapidly wasted because 

 the cultivator takes no pains to keep up the supply of 

 humus. 



In the cultivation of all soils it is necessary to add 



