482 
Journal of Agricultural Research 
Vol. XIV, No. IX 
If the water stored within the zone of normal root development is not 
sufficient to meet the needs of the crop, the roots will continue during 
the life of the plants to penetrate deeper, provided the soil below is wet. 
Under such conditions the roots may successively occupy the fourth, 
fifth, and sixth foot. The roots of winter wheat have been traced to a 
depth of 8 feet. In this connection it should be noted that fertility of 
the subsoil is a general characteristic of the soils of semiarid regions. 
Roots do not penetrate dry soil, even though there may be wet soil be¬ 
neath it. Where shallowness of soil restricts root development to a depth 
less than normal, the plants may attain complete development, provided 
the water content of the zone occupied by the roots is maintained above 
the limit of availability. The shallower the soil or the smaller the quan¬ 
tity of available water it can retain the more dependent is the crop on 
rains that fall while it is growing. 
All field studies of root systems have been made on land given ordinary 
plowing, generally to a depth of about 6 inches. No comparative 
studies of root systems as developed in deep and in shallow plowing 
have been made. But studies that have been made on the quantity 
of water stored in the soil, the depth to which it is stored, the depth 
from which it is used and the degree to which it is exhausted, and the 
behavior and yield of the crop on land tilled to different depths all 
afford an abundance of indirect evidence that the form and extent of 
root systems are not primarily affected by the depth of tillage. 
Extensive soil-moisture studies that have been made in connection 
with the investigations reported in this paper indicate that the ability 
of the soil to take in or to retain water, or to give up water to the crop, 
is not determined by the depth of tillage. Sands and light sandy soils 
offer little resistance to the entry and downward passage of water. 
They are little changed and certainly not improved in this respect by 
cultivation. With the heavier clay soils in which penetration is slower 
and more difficult it would seem that there was more opportunity for 
improvement by a mechanical loosening. The result is not, however, 
what it might at first thought appear to be. The mechanical loosening 
that may be affected when such soils are dry enough to be loosened by 
tilling is of no consequence so long as the soil remains dry. When rains 
come and water enters the soil, it carries soil material with it in its down¬ 
ward passage through the loosened soil. The clay expands on becoming 
wet and the loosened and wetted area becomes an amorphous mass. On 
drying, the soil contracts. A part of the shrinkage is downward, and a 
part of it is lateral. The lateral shrinkage results in cracks that may 
open the soil as effectively as any tillage operation. Mathews (3) 1 has 
shbwn that when allowed opportunity for free expansion a soil when 
wet may occupy 2% times the volume it did when dry. 
1 Reference is made by number (italic) to “literature cited/' p. 521. 
