nitrogen was thrown away in some indecisive battle on the Aisne to save
India from a famine. The population of Europe as a whole has not been
lessened by the war, but the soil has been robbed of its power to
support the population. A plant requires certain chemical elements for
its growth and all of these must be within reach of its rootlets, for it
will accept no substitutes. A wheat stalk in France before the war had
placed at its feet nitrates from Chile, phosphates from Florida and
potash from Germany. All these were shut off by the firing line and the
shortage of shipping.

Out of the eighty elements only thirteen are necessary for crops. Four
of these are gases: hydrogen, oxygen, nitrogen and chlorine. 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