EXTENSION" COURSE IN SOILS. 81 
paratively level alluvial soils, which are not so fine in texture as to 
make tillage for the development of good tilth and conservation of 
moisture impossible. Sandy loams, loams, and silt loams are there- 
fore better than heavy clay soils. The large amount of nourishment 
required by this crop can be supplied only by soils having high natural 
fertility or by the use of fertilizers. The virgin fertility of the rich 
black prairie soils has proved sufficient to meet the demands of this 
crop for a number of years after being first broken, but in no case can 
undiminished yields be expected to continue indefinitely without the 
application of fertilizers. 
The study of the root system of corn is interesting. (Ref. No, 2, 
pp. 215). As ordinarily planted in rows 3| feet apart in a deep 
permeable soil, the roots extend to a depth of 18 inches by the time 
the crop is 1| feet high and is about 6 weeks old. Even at this stage 
the roots meet between the rows so that the entire subsoil is occupied. 
When the corn has reached a height of 3 feet the roots often extend to 
a depth of about 24 inches. 
Cotton (Ref. No. 7, pp. 695, 696). — Cotton requires approximately 
130 days to reach maturity and so is confined practically to the 
region south of a line running from southern Virginia to northern 
Oklahoma. The lowland varieties of cotton require a longer season 
than do the upland varieties. The requirements of cotton for water 
and fertility are very similar to those of corn, and this crop gives 
good yields on heavy soils well supplied with organic matter in sections 
where the rainfall is not too large. This is especially true in Texas. 
In the Southeastern States, however, the most widely grown varieties 
give best results on sandy loam soils. 
Tobacco (Ref. No. 7, pp. 699-701). — Tobacco is similar to corn and 
cotton in its fertility requirements, except that it uses somewhat less 
phosphorus than these crops. It requires large amounts of nitrogen 
and potassium and must grow rapidly and thoroughly cover the 
ground in order to develop the self -shading which is necessary to the 
fine texture Of the leaf essential to the production of a good smoking 
flavor. For this reason the soil must be kept in the highest state of 
fertility, and there must always be an excess of the essential elements 
in available form beyond that needed to supply the actual require- 
ments of the growing crop. 
The texture of the soil also has an important influence on the quality 
of the tobacco leaf. The finer textured wrappers are grown only on 
loams and sandy loams, while the coarser textured fillers may be 
grown on heavier soils, which produce larger yields, though of a lower 
grade. Topography has an important bearing on the growth of 
tobacco, since it influences humidity and danger of storms to which 
this crop is especially subject. Shallow-dipping valleys in which the 
21862°— Bull. 355—16 6 
