THE WATER OF SOILS. 



229 



depths are only exceptionally used, partly because the laying 

 of drains then becomes too expensive. 



A mass of four feet of clay-loam soil is commonly, then, con- 

 sidered as sufficient to supply the needs of a crop ; it being 

 understood that in the humid region at least, such soils are 

 usually the richest in plant food, so that a deeper range of the 

 root system is not called for. It is quite otherwise in the sandy 

 soils of the same region, which being usually poor in plant 

 food, must afford a deeper penetration in order that an ade- 

 quate amount of the same shall be within reach of the roots. 

 Sandy lands, then, should be deep in order to repay cultivation ; 

 and fortunately this is usually the case. But when this is 

 otherwise ; when for instance a sandy soil four feet in depth is 

 underlaid by impervious clay, underdrains may be quite as nec- 

 essary as in the clay lands; since the depth of actually available 

 soil mass would otherwise be reduced to two or two and a half 

 feet only, by the water stagnating on the clay surface and rising 

 from 1 6 to 24 inches in the sand. Soils thus shallowed can 

 with difficulty be maintained in good productive condition even 

 by the most energetic fertilization. 



Moisture supplied by tap roots. In most cases, sandy lands 

 do not require underdraining; and in them, root-penetration 

 may reach to extraordinary depths in the case of certain plants, 

 especially when tap-rooted. Thus the roots of alfalfa (lucern) 

 are very commonly found to reach depths of twenty to twenty- 

 five feet, and even sixty feet has been credibly reported for the 

 same plant in the arid region. It is obvious that for such 

 plants, a high level of l>ottom water is wholly undesirable, since 

 they are enabled to obtain their moisture supply from great 

 depths, and can thus utilize for their nutrition much larger soil- 

 masses than can shallow-rooted plants. 



Reserve of Capillary ll'atcr. It must be remembered that it 

 is not only, nor usually, the bottom water that supplies moisture 

 to plant growth; for all soils of proper texture for cultivation 

 retain within them a certain amount of capillary moisture after 

 the ground water has reached its permanent level ( see this 

 chap. p. 226), and when the tap or main roots are plentifully 

 supplied with water, the upper and chief feeding roots draw 

 but lightly upon the moisture within their immediate reach for 

 the purpose of leaf evaporation. This fact can be plainly ob- 



