228 SOILS. 



water table, however, is rarely level except in level and very 

 uniform ground, or after long periods of drought. The un- 

 dulations of its surface conform, in general, to that of the 

 ground surface, but are less abrupt; so that the water lies 

 nearer to the surface in low than in high ground, as is indi- 

 cated in the diagram above. 



King l has shown, moreover, that the level of the ground 

 water shows sensible variations due to increased or diminished 

 barometric pressure, as well as to variations of temperature in 

 the soil, which cause the air in the pores to expand or contract 

 to a degree sufficient to bring about variations in the flow of 

 springs and underdrains to the extent of 8 and 15% respect- 

 ively, in conformity with the daily changes of temperature 

 and pressure. 



The Depth of the Ground Water most Favorable to Crops 

 cannot be stated in a general manner, as it depends materially 

 upon the nature of the crop, its root habit, and the nature of the 

 soil. As has already been said, the amount of soil-moisture 

 most favorable to plant growth is about half of the maximum it 

 can hold ; and this condition, as is shown in the table in chapter 

 11, p. 208, is reached about the middle of the maximum height 

 to which the water can rise by capillarity from the water level. 

 Below this point the access of air to the roots becomes too 

 limited, and in case of continuous rains the root-ends would 

 soon begin to suffer from want of aeration. On " sub-irri- 

 gated " land, therefore, which is generally considered desirable, 

 crops must be carefully selected with respect to their root 

 habits. Thus while alfalfa needs considerable moisture to do 

 its best, its deep- rooting habit renders it undesirable when the 

 ground water is at less than five feet depth ; but red clover may 

 be grown even with the water level at three feet. 



In clayey soils root-penetration is always less than in sandy 

 lands; and although in the former the capillary ascent of water 

 goes higher than in the latter, yet its movement in clays is so 

 much slower than in sandy materials that unless water is within 

 comparatively easy reach, the plants may suffer from drought. 

 Experience has long ago fixed the proper depth at which to 

 lay underdrains limiting the rise of bottom water, at from 

 three to four and a half or even five feet in clay soils; greater 



1 Physics of Agriculture, p. 270. 



