228 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
inorganic. Some of these compounds are highly active even when 
present in tiny amounts, whereas others, like quartz, are relatively 
inactive even though their bulk is large. The total chemical analysis 
conceals these important variations. One must turn to the X-ray, the 
petrographic microscope, the electron microscope, the spectrograph, 
and similar devices to discover the relevant materials. The soil is 
much more than the surface film stirred in tillage; it is the group of 
layers that make up the whole volume of the landscape, extending 
down as deeply as the living organisms themselves. In fact, soil is 
the essential link between the lifeless mineral body of the earth and 
the biological kingdom, including man himself. 
THE SOIL PROFILE 
If, instead of looking only at the soil in our garden, we look at many 
gardens, all sorts of differences among these soils are evident. Then 
if the soils across a continent are examined a close relationship among 
vegetation, climate, and soil becomes apparent. Subhumid grasslands 
and black soils go together; cool, moist climates and evergreen forests 
with light-colored soils; tropical rain forests with red soils, and so on. 
Soils consist of several distinct sheets, one on top of the other. In 
an excavation or a cut these sheets appear as layers, or soil horizons. 
Of course, some layers in the soil are inherited from layers in the 
original rocks. But regardless of the original rocks, we shall find 
true soil horizons, resulting from the peculiar environmental conditions 
of the place. Taken together, from the surface down into the weath- 
ered rock beneath, a collection of horizons is called a soil profile. 
The soil profiles in different kinds of landscapes are strikingly different 
from one another, yet they are alike in the same kind of landscape 
wherever that landscape is found. 
This recognition, about 1870, of a unique soil profile for each kind of 
landscape was the greatest single advance ever made in fundamental 
soil science, analogous to the development of anatomy in medicine. 
One does not need to depend upon inference from the geological 
nature of the rock, or from climate, or from other environmental 
factors, considered singly or collectively; the soil scientist can go 
directly to the soil itself. J hasten to add that rather than making the 
other sciences useless in soil study, this step in fact made them more 
valuable. Through soil morphology, soil scientists found a basis 
on which to classify the results of observations, of experiments, and of 
practical experience. 
THE MATERIAL OF SOIL SCIENCE 
The material of soil science covers the land area of the world. 
Thousands of kinds of soils exist. Due to the complex nature of its 
