THE SOIL 107 



in the formation of peat, which is almost pure vegetable 

 matter. If a little of the surface-soil is burned, by 

 placing it on a sheet of iron over a fire, it will be seen 

 that it first becomes dark, owing to the charring of the 

 vegetable matter ; then, as the vegetable matter burns 

 slowly away, it becomes lighter in colour, and more like 

 the subsoil. If the heat be great and long-continued, 

 the soil undergoes still further changes of colour, often 

 finally becoming red, like bricks. 



This decaying vegetable matter is known as humus, 

 and is essential to the production of true soil. Mere 

 crushed, powdered, or disintegrated rock does not con- 

 stitute true soil, but requires the admixture of humus. 

 Humus plays several important parts. It increases the 

 amount of water which sandy soils can retain ; it tends 

 to preserve the porous nature of stiff clays, facilitating 

 drainage and admitting more air ; it assists in maintain- 

 ing the friable condition known as tilth ; and, moreover, 

 soils rich in humus do not become hard and compact. 

 It is worth noting that the common expressions " poor 

 land " and " rich land " usually refer respectively to soils 

 with little humus and soils with much humus in them. 



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 quantities 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 



