MODERN SOIL SCIENCE—KELLOGG 233 
several years ago that the average rate of denudation for the United 
States as a whole was about 1 foot in 8,760 years, with suspended 
matter accounting for about 65 percent and dissolved matter 35 per- 
cent. Solution progresses much more rapidly than this in areas of 
soft limestone and high rainfall, and, of course, the process is almost 
infinitely slow on the hard rocks of steep slopes. 
Other landscapes receive part of the erosion products and part of 
the solution products from the upland. It is probable that a third of 
the population of the world get their major food supply from alluvial 
souls recently rejuvenated by additions of fresh rock minerals to their 
surface. The Nile is a famous example. In flood stage the water of 
the Nile contains over 1,000 parts per million of suspended matter 
relatively rich in phosphorus, potassium, nitrogen, and other plant 
nutrients. Part of this covers the soil in the flood plain and part of it 
moves out into the sea. Barrell (1) estimated that the Nile Delta 
alone contains the equivalent of nearly 12,000 cubic miles of rock, to 
say nothing of the soluble material contributed to the sea water. 
According to Barrell’s figures the rock material in the delta of the 
Niger River is equivalent to a wedge-shaped mountain range some 
18 miles wide at the base, 3 miles high at the top, and 1,000 miles long. 
This gives some idea of the enormous movement of surface soil 
material as a natural process. In addition, in the Tropics especially, 
volcanoes often shower the landscape with fresh rock or ash. <A layer 
of ash only an inch thick amounts to 200 to 300 tons per acre of fresh 
fine rock material. Usually such ash contains significant amounts of 
calcium, potassium, magnesium, phosphorus, and other elements essen- 
tial to plant and animal growth. Even nitrogen is sometimes present 
in important amounts. An acre-inch of the ash from Paricutin, the 
new volcano in Mexico, contains the equivalent of over 20 tons of 
ground limestone. Sometimes productive soils are covered with a 
rather sterile ash, but more often old leached soils are rejuvenated. In 
the humid Tropics productive soils are generally those that are young, 
recently developed from volcanic lava, or soils that are kept reju- 
venated by the relatively rapid addition of fresh minerals from beneath 
as the surface erodes away, or by additions of alluvium or volcanic 
ash to the surface. 
The most nearly dead soils are those on flat landscapes of high rain- 
fall that are not rejuvenated by erosion, by new minerals entering the 
soil from beneath following natural erosion, or by sediments from 
above. These dead areas must await a new cycle of uplift and erosion 
before they become productive naturally. 
THE MINERAL-ORGANIC CYCLE 
We are interested in soil chiefly because it supports green plants, and 
from green plants all other plants and animals, including man himself, 
