88 DRY-FARMING 
no determinations were made of the depth of the 
roots in later years, though it had undoubtedly 
increased. Alfalfa was the deepest rooted of all 
the crops studied by the American stations. Potato 
roots filled the soil fully to a depth of three feet; 
sugar beets to a depth of nearly four feet. 
In every case, under conditions prevailing in the 
experiments, and which did not have in mind the 
forcing of the roots down to extraordinary depths, 
it seemed that the normal depth of the roots of ordi- 
nary field crops was from three to eight feet. Sub- 
soiling and deep plowing enable the roots to go 
deeper into the soil. This work has been confirmed 
in ordinary experience until there can be little ques- 
tion about the accuracy of the results. 
Almost all of these results were obtained in humid 
climates on humid soils, somewhat shallow, and 
underlaid by a more or less infertile subsoil. In 
fact, they were obtained under conditions really 
unfavorable to plant growth. It has been explained 
in Chapter V that soils formed under arid or semi- 
arid conditions are uniformly deep and porous and 
that the fertility of the subsoil is, in most cases, 
practically as great as of the topsoil. There is, 
therefore, in arid soils, an excellent opportunity 
for a comparatively easy penetration of the roots 
to great depths and, because of the available fertility, 
a chance throughout the whole of the subsoil for 
ample root development. Moreover, the porous 
