THE RCOT 79 
The chief minerals present in granite are potash- 
felspar and mica, while those found in basalt are lime- 
soda-felspar, augite and iron oxide. These minerals 
supply the bulk of the potassium, calcium, magnesium 
and iron essential to the growth of plants. Associated 
with the foregoing substances are the small quantities of 
phosphates and sulphates that provide the necessary 
phosphorus and sulphur. The decomposition of these 
rocks is brought about chiefly by the action of carbonic 
acid on the potash, lime and magnesia of the constituent 
minerals. 
It will be noted that so far there is no mention of 
nitrogen. This element is in most cases derived from 
the humus and is organie in its origin. How then did 
plants get a footing on the earth before there were 
living things to produce organic matter for the forma- 
tion of humus? It would appear that certain bacteria 
differ from green plants in having the power to assimi- 
late the free nitrogen of the air. These probably 
existed in the early stages of the earth’s history, and, 
as they died and decomposed, gradually formed a 
humus that prepared the way for the vast variety of 
oereen plants we see to-day. It is elear then, that in 
soil and air are contained all the elements necessary to 
build the plant body. 
Soils are classified in accordance with the amount of 
sand, clay, lime and humus they happen to contain. A 
sandy soil is more than three-quarters sand, and a clay 
soil more than three-quarters clay. The former, 
though self-draining and easy to work, does not hold 
the necessary moisture, while the latter, though 
tenacious of water, is hard to work, apt to become 
sticky, and, above all, does not contain the air neces- 
sary to the work of roots. The ideal soil is a loam, 
or a mixture of about equal parts of sand and clay 
with the addition of a fair amount of humus. In lime- 
stone country, marl, a soil of which at least a quarter 
is limestone, may be found. Peaty soils are those which 
