MODERN SOIL SCIENCE! 
By Cuaruses E. Keiioaa 
U. S. Department of Agriculture 
[With 2 plates] 
Most people probably think of soil as the loose surface material 
plowed in the field or spaded in the garden. At some time we have 
taken up a handful and let it run through our fingers, or squeezed it 
into a soft ball. We have seen sand grains, roots, and worms in it. 
Sometimes soil is hard, almost like rock; sometimes it is coarse and 
loose, like cinders and ashes; where most preductive, soil is soft and 
mellow. 
We cannot see or feel the chemical composition of the soil, but 
obviously it must contain a wide variety of elements and compounds. 
Some plants grow on every kind of soil, and we know that even the 
simplest plants require several nutrient materials besides the carbon, 
hydrogen, and oxygen they get from the air and water. 
Starting with this simple concept, the making of chemical analyses of 
soils to increase our knowledge is a natural step, and indeed even the 
early chemists did this. By analyzing the soils and the plants growing 
on them it seemed logical to work out a balance sheet of plant nutrients. 
If the nutrients in a soil and the amounts required by plants are known, 
it seems reasonable to predict how many crops could be grown before 
the soil would be exhausted, what crops would be best, and what 
fertilizers would be necessary. 
There were many difficulties with this simple concept of a balance 
sheet. Despite support from the great Justus von Liebig with his 
towering reputation, this theory failed rather badly on the practical 
side, and even worse on the scientific side. The fertilizer industry that 
grew up under the shadow of Liebig did develop useful fertilizers, and 
these increased yields, but the fertilizers were often inefficient and 
always unceriain. 
It really is too bad that the balance-sheet theory was so inadequate, 
since it was refreshingly simple. But the soil is anything but simple. 
First of all, actual soil in the woods or garden is partly alive. It is 
quite unlike the dead samples of soil stored in the glass jars of chemical 
laboratories. A soil consists of thousands of compounds, organic and 
1 Based upon material presented in Sigma Xi National Lectureships. Reprinted by permission from 
American Scientist, vol. 36, No. 4, autumn issue, 1948, and Science in Progress, Series VI, Yale Univ. Press. 
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