234 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
get their food supply, either directly or from other plants and animals 
that live on the green plants. Of equal importance to this basic fact is 
the one that plants, and all the other living matter associated with 
plants, are chiefly responsible for the kind of soil developed. It can be 
stated that there is no life without soil, and no soil without life. Life 
and soil seem to have evolved together in a mineral-organic cycle, or 
rather in many cycles, because there are many kinds of plant associa- 
tions, many kinds of landscapes, and many kinds of soils. 
After the earth cooled into a solid surface, and before vegetation 
covered the land, it probably looked something like an extreme desert 
region looks today, with jagged hills, sharp angles, and deep, irregular 
stream courses. With the coming of vegetation, soils began to form. 
Landscapes became more stable; jagged peaks softened into round hills 
with gentle slopes. Vegetation not only changes the rock material, but 
holds it in place, allowing it to slip away only gradually and harm- 
lessly. 
The plants select the essential elements from the mass of rock ma-~ 
terial into which they extend their roots. The great bulk of most soils 
consists of various combinations of silicon, aluminum, and oxygen 
along with significant, but often tiny, amounts of the other 92 elements. 
Plants take in phosphorus, calcium, potassium, manganese, copper, 
and other elements which they require, and build them into character- 
istic fruits and leaves, bark and wood. As the leaves and fruits and 
other parts fall, and as plants die, this organic matter becomes in turn 
the food of bacteria and other decay organisms. With their help, the 
chemical decomposition releases the nutrients to the soil for other 
plants. 
Estimates have been made of the annual production of organic mat- 
ter in the natural landscape (10); unfortunately few data exist for the 
roots. For tall-grass prairie the figures for annual production are from 
around 1 ton to over 2 tons per acre. In temperate forests, values 
of around 3 tons per acre have been reported, with about one-half 
wood and one-half needles or leaves. Figures for tropical vegetation 
are much harder to determine, but estimates run up to 90 tons per acre 
per year. Of course, one year is but a moment in the life of a soil. But 
in 1,000 years a truly enormous influence is exerted on the soil by this 
root-sorting and the biological and chemical reactions associated with 
the production and decomposition of such vast quantities of plant ma- 
terial—quantities ranging from perhaps a low of 100 tons per acre 
through a medium of 3,000 tons, to nearly 90,000 tons, depending upon 
the kind of landscape. 
To have some notion of the magnitude of this organic-mineral cycle 
in terms of a few specific elements, we might look at data for the annual 
leaf fall in ordinary hardwood and evergreen forests in northern 
United States (4, 5). 
