THE SOIL. 21 



the dead and weathered bodies of plants, and possesses in 

 itself all the ash elements required to build up new plants. 

 The weathering of humus takes place, in the presence of 

 air and moisture, under the action of bacteria which 

 oxidize it to nitric acid and other bodies available as plant 

 foods. The organic part of the soil contains practically 

 all the nitrogenous plant food of the soil. Besides yielding 

 most important plant foods, a supply of humus in the 

 soil is valuable in that it feeds the organisms in the soil 

 and thereby aids in the weathering of itself and the 

 mineral part of the soil. Its decomposition results in the 

 formation of acids which, dissolved in the water, increase 

 its weathering power. Humus improves the physical 

 properties of the soil by increasing its power of retaining 

 and raising water, by forming a binding material for 

 sandy soils, and loosening and opening up clayey soils. 



The organised part of the soil consists of living things, 

 such as plant roots, worms, insects and bacteria. As far 

 as plants are concerned, the bacteria are the most active 

 and most important of the organised matter. They assist 

 or cause most of the weathering processes, and are therefore 

 indispensable in the preparation of food for crops. The 

 plant roots are themselves, to a slight degree, weathering 

 agents, owing to the acid substances they exude and the 

 plants are thus able to assist in the preparation of their 

 own food. Of the insects living in the soil probably more 

 do harm than good. 



A supply of air in the soil is necessary to permit of 

 the breathing of the plant roots and of the other living 

 bodies in the organised part of the soil. 



