Manufacturers of commercial fertilizer make use of
other materials, some of which, like manufactured nitrogen, are
excellent, and others are low in quality and slow in action. The
sources of plant-food that have been described form the great bulk of
all fertilizers on the market, and from them may be selected all the
materials a farmer needs to use on his land, either singly or
home-mixed. In most instances the selection will embrace only four or
five of these fertilizing materials.