INTUOUrCTORY. 



105 



The Cencral Fiiuctious 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 tlie most direct manner 

 with tlie nutrition of the plant. The substances which 

 the plant acquires from the soil, so far as they are nutri- 

 tive, may be collectively termed soil-food. 



2. Tlie 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 fi-om external sources, form at any one time but a mi- 

 nute fraction of its mass, and in various modes ensuring the 

 co-operation of the conditions which must unite to produce 

 the perfect plant. 



Variety of Soils. — In nature we observe a vast variety 

 of soils, which difier as much in their agricultural valuf 

 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 sliriveled 

 grasses find nourishment. Again there occur in the high- 

 lands of Scotland and Bavaria, as well as in Prussia, and 

 other tem))erate 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 tsrhornosem, or black earth, 

 5* 



