THE FORMATION OF SOIL 319 
mixed with leaves, twigs, and rootlets, gradually obtains 
a supply of decaying vegetation, which eventually leads 
to the formation of humus. 
The work of the Earthworm, in the formation and 
improvement of soil is deserving of special mention ; for an 
account of this see Part IT., page 104, “The Earthworm.” 
The soil, then, consists of mineral or inorganic matter, 
and humus or organic matter ; the former is derived from 
the decomposition of rocks, and the latter from the decay 
of animal and vegetable substances. Heat a small quantity 
of garden mould over a hot flame. The soil should first be 
dried to remove as much of the moisture as possible, and 
the flame should be strong enough to raise it to a red heat. 
Notice the strong smelling smoke being given off, and 
continue to heat until this ceases to appear. If the dry 
mould be weighed before and after being “burned,” it will 
be found to have lost considerably in weight, the loss being 
due to the material which formed the smoke: this is the 
organic matter or humus ; that which remains is the 
inorganic or mineral matter. 
The most abundant mineral ingredients in the soil are 
sand and clay. Limestone to a less extent is also some- 
times present. Sand consists almost entirely of silica, so 
that a soil composed of sand would be useless to the 
cultivator, for it is lacking in plant food-— moreover, it 1s so 
loose and porous that plants would be unable to get 
sufficient hold for their roots, and would pine away from 
the absence of moisture, which sand is unable to retain. 
It is, however, of considerable use when mixed with clay 
and other constituents, for it tends to make the soil light 
and porous, thus admitting moisture, air, and warmth. The 
presence of sand is helpful in another way ; the particles 
of which it is composed become heated by the rays of the 
sun and so tend to raise the temperature of the soil. 
