gher and lower fertility level is
not wholly a question of percentage of nitrogen, carbon, etc. At
its highest level the soil possesses a good physical texture
owing to the flocculation of the clay and the arrangement of the
particles: it can readily be got into the fine tilth needed for a
seed bed. But when it has run down the texture becomes very
unsatisfactory. Much calcium carbonate is also lost during the
process: and when this constituent falls too low, the soil
becomes "sour" and unsuited for crops.

The simplest system of husbandry is that of continuous wheat
cultivation, practiced under modern conditions in new countries.
When the virgin land is first broken up its fertility is high; so
long as it remains under cultivation this level can no longer be
maintained, but rapidly runs down. During this degradation
process considerable quantities of plant food become available
and a succession of crops can be raised without any substitution
of manure ... After a time the unstable period is over and the
new equilibrium level is reached at which the soil will stop if
the old husbandry continues. In this final state the soil is
often not fertile enough to allow of the profitable raising of
crops; it is now starving for want of those very nutrients that
were so prodigally dissipated in the first days of its
cultivation, and the cultivator starves with it or moves on.

Fortunately recovery is by no means impossible, though it may be
prolonged. It is only necessary to leave the land covered with
vegetation for a period of years when it will once again regain
much of the nitrogenous organic matter it has lost.[39]

Dr. Russell adds that soil-exhaustion is essentially a modern
phenomenon, however, and gives the following reasons for supposing
that the medieval system conserved the fertility of the soil. First,
the cattle grazed over a wide area and the arable land all received
some dung. Thus elements of fertility were transferred from the
pasture land to the smaller area of tilled land. This process, he
admits, involved the impoverishment of the pasture land, but only very
slowly, and the fertility of the arable was in the meanwhile
maintained. Secondly, the processes of liming and marling the soil
were known, and by these means the necessary calcium carbonate was
supplied. Thirdly, although