94 BRITISH PLANTS 



aerobic forms leads to the formation of mild hiunus, 

 which forms a shghtly alkaliae, weU-aerated fertile soil. 

 Mixed with earth, it forms mould, the most valuable of 

 all soils — e.g., leaf-mould, peat-mould, etc., according to 

 the origin of the humus. 



When decomposition is checked, raw or acid humus is 

 produced. This takes place when any cause is present 

 which renders the humus unfit for the existence of aerobic 

 bacteria, — e.g. : 



1. Deficiency of oxygen. 



2. Excess of water excluding aip. 



3. Too low a temperature. 



4. Deficiency of lime. 



The acidity of raw peat is due to the fact that in the 

 absence of sufiicient oxygen and lime free " earth "-acids 

 are produced which sour the soil. 



If the causes which exclude the bacteria be removed by 

 drainage, etc., the aerobic bacteria resume their sway, 

 the humous acids disappear, and a mild humus is pro- 

 duced. 



The presence of vegetable fibre in soil increases its 

 capacity for water. Peat itself is a perfect sponge ; it 

 has a greater water-capacity than any other soil. Sour 

 peat, whether wet or dry, is poor in nutriment, for the 

 form in which the nutriment exists is not one available 

 to plants ; it contains many fungi, but few bacteria and 

 no earthworms. Peaty matter is accumulating wherever 

 vegetation is flourishing and the soil is cold or wet. Leaf- 

 mould is forming in forests, mossy humus in bogs, fibrous 

 peat on heaths. 



The Living Populations of the Soil. 



1. Soil Bacteria. — Countless swarms of bacteria — a few 

 malignant, but the most so beneficent as to be indispens- 

 able to plant life — inhabit the upper layers of the soil. 

 It seems likely that most of the chemical changes taking 

 place in the soil, especially in its organic constituents, 

 are due to their activities. The most important are the 

 following : 



(a) Nitrate - Bacteria. — The nitrogenous compounds 

 present in decaying organic matter are decomposed in 



