VEGETABLE MATTER IN SOILS. 103 



moreover, soils rich in humus do not become 

 hard and compact. It is worth noting that 

 the common expressions * poor land ' and 4 rich 

 land ' usually refer respectively to soils with 

 little humus and soils with much humus in 

 them. 



3. Earth-worms are very active agents in 

 distributing humus through the soil. They 

 carry leaves down into their burrows and bring 

 to the surface, and deposit there, large quanti- 

 ties of earth in the form of castings. Darwin 

 estimated that in an English meadow the 

 earth-worms brought to the surface upwards of 

 15 tons of earth per acre per year. Owing to 

 this action of the earth-worms, objects lying on 

 the surface are slowly buried or appear to sink 

 into the ground. In 1842 Darwin spread a 

 quantity of chalk over a field in order to ob- 

 serve 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 were 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 "2*2 inches per 

 annum through the agency of earth-worms 

 It is estimated that the soil so brought up in 

 this meadow weighed about 73,000lbs. From 



