666 
Journal of Agricultural Research v 0 i. xxvii, No. 9 
those soluble substances that cause what is known as the alkali problem. 
Were it possible to obtain even approximately uniform penetration of 
water the soluble material would be carried downward with each suc¬ 
cessive irrigation, and the subsoil accumulations could be removed by 
artificial drainage where the natural drainage conditions were inadequate. 
With conditions as they are, such that the water penetrates readily in 
some parts of the field or check and very slowly and only to a short dis¬ 
tance in other parts, there is a tendency for initial diversities to be ex¬ 
aggerated with the passage of time and with repeated applications of 
water. The water that is applied to the , field carries some dissolved 
material, often substantial quantities. Some of this water is used by the 
growing crops, but a large part of it is lost by evaporation from the soil. 
In either event the dissolved material is deposited in the soil. Where the 
conditions of permeability are good and an adequate quantity of water 
is applied there is at least a small surplus to percolate downward beyond 
the root zone and beyond the range of evaporation. In those spots in 
the field where the soil is less permeable, the soluble material accumulates 
in the surface soil or at the lower limit of water penetration, with the result 
that these spots may finally become unproductive. 
' The recognition of the existence of such conditions of diversity in the 
permeability of the soil is essential to an understanding of how irrigated 
fields become unproductive and why it is often found to be difficult to 
reclaim such fields even when irrigation water is used in excess of crop 
requirements and evaporation losses. A field of saline soil may be 
provided with an adequate drainage system and may be heavily irri¬ 
gated. There may be a free discharge of water from the drains and yet 
a large part of the field may remain unproductive. (See PI. 2, B.) 
An investigation of such a situation is likely to show that the leaching 
effect of the irrigation is confined to those spots where the soil is per¬ 
meable. In the remainder of the field the water is not moving down¬ 
ward but is held in the soil except to the extent that it is lost by evapora¬ 
tion between irrigations. In fields where the differences in the perme¬ 
ability of the soil are not very pronounced, a protracted period of leaching 
may be effective in removing the soluble material from all parts of the 
field. This reclamation may be hastened wherever it is possible to hold 
the water in place by a system of interior borders that will tend to equalize 
penetration by holding the water longer on those spots where it soaks in 
more slowly. 
It is well to recognize that there are at least two different sets of con¬ 
ditions involved in differences in permeability. One of these has to do 
with the texture of the soil, and the other has to do with its physical con¬ 
dition which is largely influenced by the character of its combined bases; 
that is to say, a soil that is made up largely of clay and fine silt is less per¬ 
meable than a soil composed largely of sand, providing both types are in 
the same condition as regards their combined bases. When a check or 
border includes both types of soil it is well worth while to subdivide it 
by interior borders which conform to these differences of soil type. Such 
subdivision makes for economy in the use of water whether for leaching 
the soil or for irrigating crops. 
Where the differences in permeability within a border are due chiefly 
to the physical condition of the soil resulting from the character of the 
combined bsfses it is possible to obtain more uniform penetration of 
the water by making local applications of salts that will improve the 
