(_d_) the subsoil soon
settles together because it contains little organic matter.
Subsoiling is generally approved and little practiced. Land at
plow-depth becomes packed by the tramping of horses upon it and the
pressure of the plow, when the plowing is done at the same depth
year after year, and in some soils subsoiling has been found
distinctly valuable.

Time of Plowing.--In great measure the time of plowing is determined by
the effect upon soil moisture, and is discussed in the next chapter.

Method of Plowing.--The depth of plowing should be fixed largely by the
amount of organic matter in the soil. It is essential that a good
percentage of this material should be mixed throughout the soil, and
when it is in scant supply, the depth of plowing usually should not be
great. Fertile soils should be plowed deep for their own good, and thin
soils should be deepened gradually, as sods and manures afford a supply
of humus-making material. Even when manure is used liberally in a
single application on a poor soil, a large amount of inert subsoil
should not be thrown upon the surface. The manure goes out of reach of
the greatest need, which is in the surface soil where plant-life
starts. A gradual process of deepening the soil is to be preferred, but
such deepening should not be neglected. The subsoil is a store of inert
fertility that should not remain dormant.

It may not do to say that the success of the best farmers is due to
thoroughness in plowing, but it is true that the more successful ones
are insistent that the plowing be absolutely thorough. Every inch of
the soil should be stirred to a certain depth, and that requires a plow
so set that it does not turn a furrow-slice much wider than the point
can cut. Evenness in depth and width of furrow is seen in good plowing.

The Disk Harrow.--The purpose of the plow is to break up the soil so
that it will be crumbly and mellow. The frequency with which land
should be thoroughly stirred to full plow-depth depends upon the
condition of the soil and the character of the crops. Oftentimes a disk
or cutaway harrow may replace the plow. Its action is the same as that
of the plow, loosening and turning the soil over. When land has had a
good plowing within the year, and has not become compact, stirring to a
depth of four inches may give a better seed-bed for some crops than
could be made by use of a plow. This is true of land that has produced
a culti