SOIL AND SUBSOIL. 183 



region, where the margin between adequate and inadequate 

 depth of soil and moisture-supply is much smaller. 



When farmers note such distress in the orchard, the first idea usually 

 is that fertilization is needed. This in the almost universally very rich 

 lands of the arid region is rarely the case until after many years of 

 exhaustive cultivation, and is scarcely ever of more than passing benefit 

 in such cases. The first suggestion should always be an examination oj 

 the substrata, and especially of the deeper roots ; in the diseased or 

 thirsty condition of which the cause of the "die-back" or yellowing 

 will commonly be found. Of course no amount of fertilization can 

 permanently remedy such a state of things, arising from impervious 

 substrata, coarse gravel, or shallow bottom water. 



Hardpan. By " hardpan " is understood a dense and more 

 or less hardened layer in the subsoil, which obstructs the pene- 

 tration of both roots and water, thus materially limiting the 

 range of the former both for plant-food and moisture, and 

 giving rise to the disadvantages following such limitation, as 

 described in the case of dense subsoils. The hardpans proper 

 differ from the latter, however, in being usually of limited 

 thickness only; the direct consequence of their mode of forma- 

 tion, which is not direct deposition by water or other agencies, 

 but the infiltration of cementing solutions into a pre-existing 

 material originally quite similar to that of the surface soil. 

 Such solutions usually come from above, more rarely from be- 

 low, and are of very various composition. The solutions of 

 lime carbonate in carbonated water have already been referred 

 to in this connection; as has also the fact that corresponding 

 solutions of silica, associated more or less with other products 

 of rock decomposition ( see chapters 2 and 4) are constantly 

 circulating in soils. The surface soil being the portion where 

 rock-weathering and other soil-forming processes are most 

 active, these solutions are chiefly formed there; and according 

 as their descent into the substrata is unchecked, or is liable to 

 be arrested at some particular level, whether by pre-existing 

 close-grained layers or by the cessation of rains, the subsequent 

 penetration of air, and evaporation of the water alone by shal- 

 low-rooted plants, may cause the accumulation of the dissolved 

 matter at a certain level, year after year. Fimllv there is 



