CORK OAK—RYAN AND COOKE 
367 
Tas ie 4.—Soil distribution of natural cork oak 
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in the surface that it forms a tough 
hardpan layer. Despite this fact, the 
bulk of the world’s cork is growing in 
these types of soils. 
The gray-brown forest soils are situ- 
ated in regions of more abundant 
rainfall and are considerably more 
moist than the steppe soils. The 
forest soils are well adapted to the 
support of both deciduous and conif- 
erous forests and have been supporting 
cork oak growth for centuries. 
The tropical and subtropical red 
and yellow soils are on the wetter 
side of the forest soils and are situated 
in warm regions of high rainfall. 
These soils are characteristic of the 
rainy Tropics and the humid sub- 
Tropics, such as the ‘‘South” of the 
United States. The tropical soils are 
well adapted to the support of all 
types of forest vegetation and have 
been supporting cork oak growth for 
hundreds of years. 
The complex soils of mountains and 
included valleys are mixtures of many 
kinds of soils, intermingled in such a 
way that no particular type _pre- 
dominates. They are situated in 
mountainous regions where they re- 
ceive rainfall in amounts adequate for 
the support of forest vegetation. Since 
time immemorial these complex soils 
have been supporting cork oak growth 
in the Mediterranean region. 
A study of the soil characteristics of 
Percent in soil region of— 
Steppe Forest Tropical | Complex 
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the United States potential cork area 
shows that there are three soil regions 
in addition to the above-mentioned 
four Mediterranean groups. They 
are the desert, chernozem, and prairie 
soils. 
In the potential area of the United 
States old cork oaks are growing in 
the desert soils of southern California 
and Arizona. The greatest numbers 
of trees are in the regions which are 
under irrigation and in which the 
soils are receiving a moisture supply 
equivalent to approximately 30 inches 
of annual rainfall. However, there 
are sufficient numbers in the normal 
arid regions of low rainfall to confirm 
the fact that desert soils, as a primary 
group, are satisfactory mediums for 
the growth of cork oaks. 
The chernozem soils are usually 
adjacent to, but on the wetter side of, 
the steppe soils, while the prairie soils 
are adjacent to, but on the drier side 
of, the forest soils. Both are situated 
in regions where the rainfall is ample 
for crops but not sufficient for the 
support of forest vegetation. They 
are considered to be among the most 
productive agricultural soils in the 
world. From the point of view of tree 
growth, however, these soils are less 
fertile than forest and more fertile than 
steppe soils. 
Although there are no old cork oaks 
growing in the chernozem and prairie 
