fertilizers on the market, the
farmer knows that the soil which is in a good state of fertility is
best for any of them, and if the soil is hard-run, it should have its
plant-food supply supplemented. The hard-run soil usually is lacking in
available supplies of all three plant-food constituents. If a
fertilizer containing 3 per cent of nitrogen, 10 per cent of phosphoric
acid, and 6 per cent of potash serves the wheat well, it will serve the
timothy that starts in the wheat. Likewise it will serve the corn,
although a heavier application will be needed because corn is a heavy
feeder. Experience has taught that it will serve the potato similarly,
and that the potato will repay the cost of free use of fertilizer. If
the soil is sandy and deficient in potash, the percentage of phosphoric
acid may be cut to 8, and the percentage of potash raised to 10, and
all these crops will profit thereby. If the nitrogen content in the
soil is high, none of these crops may need nitrogen in the fertilizer.
This is a general principle, and safe for guidance, though the best
profit will demand some modification that readily occurs to the farmer
as he studies his crops and their rotation. To illustrate: The corn is
given the clover sod or the manure partly because it requires more
plant-food than the wheat. It gets the best of the nitrogen, and may
need only a rock-and-potash fertilizer, while the wheat that follows
may need some available nitrogen to force growth in the fall. There is
no fixed formula for any field or crop, and the point to be made here
only is that the requirements of many standard crops do not have the
dissimilarity usually supposed, except in respect to quantity. A marked
exception is found in the oat crop, which does not bear the application
of much nitrogen, and often fares well on the remains of the manure
that fed the corn, if some phosphoric acid is added.

Maintaining Fertility.--A heavy clover sod gives assurance that a good
crop of corn or potatoes can be grown. If the amount of plant-food in
the sod is not excessive, a heavy crop of wheat can be produced. The
condition of the soil favors many crops. The clover has placed it upon
a productive basis for the time being.

The object that should be kept in view, when a scheme of soil
fertilization is worked out, is the maintenance of such a state of
fertility that the land can be depended upon for whatever crop comes
round in the rotation. When a 3-10-6 fertilize