n soils.
Wherever the cropping has been hard, and manure has not gone back to
the land, the growth in stalk and leaves of the plant is deficient. The
color is light. Inability of a soil to produce a strong growth of corn,
a large amount of straw, or a heavy hay crop, is indicative of lack of
nitrogen in nearly every instance.

The legumes, such as clover, and the stable manures are rich in
nitrogen, and when the scheme of farming involves their use on all the
land of the farm, no need of purchased nitrogen may arise in the
production of staple crops. In the black corn soils the nitrogen
content originally was high.

Lands that naturally are not very fertile rarely have enough available
nitrogen. Where timothy is a leading crop, the demand for nitrogen is
heavy. A cold spring or summer, checking nature's processes in the
soil, may cause a temporary deficiency in available nitrogen in land
that usually has a sufficient supply. Associating a rank growth of
stalk and leaf with an abundance of nitrogen, the experienced man can
form a pretty safe opinion regarding the probable profitableness of an
investment in this element. It costs nearly four times as much per
pound as either of the two other constituents of a fertilizer, and so
far as is feasible it should be obtained through the legumes and stable
manure.

Phosphoric-acid Requirements.--Soil analyses show that the content of
phosphoric acid in most soils of this country is relatively small. The
results of experiments with the various constituents of fertilizers are
in accord with this fact. Fertilizer experiments at the various
stations and on farms are nearly a unit in showing that if any need in
plant-food exists, phosphoric acid is deficient. When crop-producing
power decreases, and the farmer begins to seek a commercial fertilizer
to repair the loss, he finds that bone-dust or acid phosphate is
serviceable. The resulting increase in yield often leads to such sole
dependence upon this fertilizer that clover and manure are disregarded,
the percentage of humus is allowed to drop, and finally the fertilizer
is brought into disrepute. The need of phosphoric acid is so common
that it is the sole plant-food in much fertilizer, and the dominant
element in practically all the remainder on the market.

[Illustration: Plat experiments.]

The Need of Potash.--Land which is deficient in organic matter
ordinarily is lacking in available potash, and responds with profit to
a