1840 the possibility of maintaining soil fertility by the application of
chemicals it seemed at first as though the question were practically
solved. Chemists assumed that all they had to do was to analyze the soil
and analyze the crop and from this figure out, as easily as balancing a
bank book, just how much of each ingredient would have to be restored to
the soil every year. But somehow it did not work out that way and the
practical agriculturist, finding that the formulas did not fit his farm,
sneered at the professors and whenever they cited Liebig to him he
irreverently transposed the syllables of the name. The chemist when he
went deeper into the subject saw that he had to deal with the colloids,
damp, unpleasant, gummy bodies that he had hitherto fought shy of
because they would not crystallize or filter. So the chemist called to
his aid the physicist on the one hand and the biologist on the other and
then they both had their hands full. The physicist found that he had to
deal with a polyvariant system of solids, liquids and gases mutually
miscible in phases too numerous to be handled by Gibbs's Rule. The
biologist found that he had to deal with the invisible flora and fauna
of a new world.

Plants obey the injunction of Tennyson and rise on the stepping stones
of their dead selves to higher things. Each successive generation lives
on what is left of the last in the soil plus what it adds from the air
and sunshine. As soon as a leaf or tree trunk falls to the ground it is
taken in charge by a wrecking crew composed of a myriad of microscopic
organisms who proceed to break it up into its component parts so these
can be used for building a new edifice. The process is called "rotting"
and the product, the black, gummy stuff of a fertile soil, is called
"humus." The plants, that is, the higher plants, are not able to live on
their own proteids as the animals are. But there are lower plants,
certain kinds of bacteria, that can break up the big complicated proteid