So much is easy to see, but actually the question is extremely complicated.  When the German chemist, Justus von Liebig, pointed out in 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 molecules into their component parts and reduce the nitrogen in them to ammonia or ammonia-like compounds.  Having done this they stop and turn over the job to another set of bacteria to be carried through the next step.  For you must know that soil society is as complex and specialized as that above ground and the tiniest bacterium would die rather than violate the union rules.  The second set of bacteria change the ammonia over to nitrites and then a third set, the Amalgamated Union of Nitrate Workers, steps in and completes the process of oxidation with an efficiency that Ostwald might envy, for ninety-six per cent. of the ammonia of the soil is converted into nitrates.  But if the conditions are not just right, if the food is insufficient or unwholesome or if the air that circulates through the soil is contaminated with poison gases, the bacteria