94 BRITISH PLANTS 
aérobic forms leads to the formation of mild humus, 
which forms a slightly alkaline, well-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 aérobic 
bacteria—e.g. : 
1. Deficiency of oxygen. 
2. Excess of water excluding air. 
3. Too low a temperature. 
4, Deficiency of lime. 
The acidity of raw peat is due to the fact that in the 
absence of sufficient 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 aérobic 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. Leat- 
mould is forming in forests, mossy humus in bogs, fibrous 
peat on heaths. 
The Living Populations of the Soil. 
1. The Surface-Covering.—This consists chiefly of grass, 
mosses, etc., which form a protective covering, keeping the 
soil beneath moist by intercepting the sun’s rays and 
checking evaporation. A soil bare of such a garment 
dries quickly, and most of the rain, instead of soaking 
in, finds its way by runnels and gutters into the water- 
courses. When a sward is present, most of the water 
in the ground reaches the air only after passing through 
the bodies of the plants. 
