_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-irrigated” 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 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, considered 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 adequate 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 necessary 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 16 to 24 inches in the sand. Soils thus shallowed can with difficulty be maintained in good productive condition even by the most energetic fertilization.