102 NATURE TEACHING. 



VEGETABLE MATTER IN SOILS. 



1. In digging down through the soil it 

 was seen that the upper layers (sar face- 

 soil] were darker than the lower (sub-soil}. 

 This is due to the presence of decaying leaves, 

 roots and other vegetable matter, derived 

 from plants, previously growing on the spot, 

 or brought there as manure. 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 vegeta- 

 ble matter burns slowly away, it becomes 

 lighter in colour, and more like the sub-soil. 

 If the heat be great and long-continued the 

 soil undergoes still further changes of colour, 

 finally becoming red, like bricks. 



2. This decaying vegetable matter is 

 known as humus, and is essential to the pro- 

 duction of true soil. Mere crushed, powdered, 

 or disintegrated, rock does not constitute true 

 soil but requires the admixture of humus. 

 Humus plays several important parts. It in- 

 creases 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 maintaining 

 the friable condition known as tilth ; and, 



