ine. Five are
metals: potassium, magnesium, calcium, iron and sodium. Four are
non-metallic solids: carbon, sulfur, phosphorus and silicon. Three of
these, hydrogen, oxygen and carbon, making up the bulk of the plant, are
obtainable _ad libitum_ from the air and water. The other ten in the
form of salts are dissolved in the water that is sucked up from the
soil. The quantity needed by the plant is so small and the quantity
contained in the soil is so great that ordinarily we need not bother
about the supply except in case of three of them. They are nitrogen,
potassium and phosphorus. These would be useless or fatal to plant life
in the elemental form, but fixed in neutral salt they are essential
plant foods. A ton of wheat takes away from the soil about 47 pounds of
nitrogen, 18 pounds of phosphoric acid and 12 pounds of potash. If then
the farmer does not restore this much to his field every year he is
drawing upon his capital and this must lead to bankruptcy in the long
run.

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