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soil may be decreased by a diminished rainfall and by the 

 more or less complete failure of the monsoon, by draining, 

 by the removal of litter, or by interrupting the leaf canopy, 

 and this may cause trees to become stag-headed. The same 

 result may be caused by underplanting trees with a species 

 which absorbs large quantities of moisture fronT the soil. 

 In an ordinary well-drained soil oxygen is always present 

 as gas in the interspaces and also dissolved in the water. If 

 this oxygen is removed the root-hairs are killed and the roots 

 rot, the trees becoming stag-headed. This may be caused 

 by water being allowed to stagnate around the roots as hap- 

 pens in swampy ground, and it commonly occurs also in dense 

 soils and in soils with an impermeable substratum. The 

 available oxygen in such cases is gradually exhausted, that 

 utilized by the roots being not replaced sufficiently quickly 

 by the admission of fresh air. The same result may be caused 

 by other factors which interfere with the free aeration of the 

 soil, such as heavy grazing which hardens and increases the 

 density of the soil. The roots of plants grown in glazed 

 pots are unable to obtain a sufficient supply of oxygen and 

 often rot in consequence. Similarly trees growing in 

 towns are often unhealthy, owing to the buildings and 

 close pavements preventing sufficient water and air entering 

 the soil. 



The free aeration of the soil is also important for another 

 reason. We have seen above that organic substances, such as 

 humus, cannot be directly used by the higher plants as food, 

 although such substances always contain a certain quantity 

 of the elements, such as phosphorus, potassium, nitrogen, 

 etc., which are essential for the existence of such plants. With- 

 out the aid therefore of fungi and bacteria which break down 

 this organic material and liberate these elements in a suitable 

 condition, they could not be utilized by the higher plants 

 and consequently humus would accumulate in our forests 

 and the soil would gradually become poorer in the essential 

 food materials, until eventually no trees could grow in it. 

 Now a sufficient supply of oxygen is absolutely necessary 

 for the existence of a large proportion of these useful fungi 

 and bacteria, and they are in consequence found in the upper 

 layers of the soil,* where, moreover, their presence is most 



* Experiments have shown that when impure sewage water containing large 

 numbers of bacteria is passed through sandy soil most of the bacteria are retained 

 in the superficial well-aerated layers of soil, on which fact the system of filtering 

 water throug'i sandy soil is chiefly based. 



