180 The Principles of Fruit-growing. 
The actual fertility of the soil depends, therefore, upon 
the plant as well as upon the land. The better and more 
comfortable the plant, the more food it can appropriate from 
a given soil; hence that soil is practically the richer. The 
chemist does not determine the physical conditions which 
make the plant comfortable and active. In other words, the 
amount of plant-food in the soil is only one of the ele- 
ments in the fertility of the land. 
In most instances as much depends upon the physical 
condition of the soil as upon its chemical constitution, and 
in many cases even more depends upon it. 
Soil is derived from two sources—rock and organic mat- 
ter. Each is essential to it. Without the rock matter it 
would lose body and staying qualities. Without the organic 
matter it would lose life, or “heart” and activity. 
Nature adds the organic matter to the soil by growing 
plants upon it and then incorporating their remains with it. 
Everywhere the process of soil-building is now going on. 
The longer the soil is in crops the richer it becomes, al- 
though the relative amount of mineral matter which it con- 
tains may be decreasing at the same time. 
Nature makes the soil richer, then, both by fining and 
digesting the mineral matter and by ameliorating its physical 
condition through the incorporation of humus or organic 
matter. 
This fining process must ultimately cease, but the addi- 
tion of humus never ceases. The final and complete en- 
richment of the soil, therefore, must come largely as_ the 
result of the ineorporation of humus with it. 
The chief value of this humus is not to directly afford 
plant-food, but to improve the conditions of temperature, 
moisture, aération and the like. 
b. Man’s treatment of the land.—Man’s chief desire is to 
use the organic products of the land. He consumes the 
plant product. As a consequence, cultivated soils soon tend 
to become hard, dense, heavy and lifeless, and the more 
elay-like the land the more pronounced is the result. 
