INTRODUCTORY. 105 
The General Functions of the Soil are of three kinds: 
1. The ashes of the plant whose nature and variations 
have been the subject of study in a former volume (H. 
C. G., pp. 111-201,) are exclusively derived from the soil. 
The latter is then concerned in the most direct manner 
with the nutrition of the p!ant. The substances which 
the plant acquires from the soil, so far as they are nutri- 
tive, may be collectively termed so/l-food. 
2. The soil is a mechanical support to vegetation. The 
roots of the plant penetrate the pores of the soil in all 
directions sidewise and downward from the point of their 
junction with the stem, and thus the latter is firmly 
braced to its upright position if that be natural to it, and 
in all cases is fixed to the source of its supplies of ash-in- 
gredients. 
3. By virtue of certain special (physical) qualities to be 
hereafter enumerated, the soil otherwise contributes to 
the well-being of the plant, tempering and storing the 
heat of the sun which is essential to the vital processes ; 
regulating the supplies of food, which, coming from itself 
or from external sources, form at any one time but a mi- 
nute fraction of its mass, and in various medes ensuring the 
co-operation of the conditions which must unite to produce 
the perfect plant. 
Variety of Soils.—In nature we observe a vast varicty 
of soils, which differ as much in their agricultural value 
as they do in their external appearance. We find large 
tracts of country covered with barren, drifting sands, on 
whose arid bosom only a few stunted pines or shriveled 
grasses find nourishment. Again there occur in the high- 
lands of Scotland and Bavaria, as well as in Prussia, and 
other temperate countries, enormous stretches of moor 
land, bearing a nearly useless growth of heath or moss. 
In Southern Russia occurs a vast tract, two hundred mil- 
lions of acres in extent, of the tschornosem, or black earth, 
5 
