bstances are dissipated as gas by the decomposition
bacteria, others are washed away in the drainage water. These
losses are small in poor soils, but they become greater in rich
soils, and they set a limit beyond which accumulation of material
cannot go. Thus a virgin soil does not become indefinitely rich
in nitrogenous and other organic compounds, but reaches an
equilibrium level where the annual gains are offset by the
annual losses so that no net change results. This equilibrium
level depends on the composition of the soil, its position, the
climate, etc, and it undergoes a change if any of these factors
alter. But for practical purposes it may be regarded as fairly
stationary.

When, however, the virgin soil is broken up by the plough and
brought into cultivation the native vegetation and the crop are
alike removed, and therefore the sources of gain are considerably
reduced. The losses, on the other hand, are much intensified.
Rain water more readily penetrates, carrying dissolved substances
with it: biochemical decompositions also proceed. In consequence
the soil becomes poorer, and finally it is reduced to the same
level as the rate of gain of nitrogenous matter. A new and lower
equilibrium level is now reached about which the composition of
the soil remains fairly constant; this is determined by the same
factors as the first, _i. e._ the composition of the soil,
climate, etc.

Thus each soil may vary in composition and therefore in fertility
between two limits: a higher limit if it is kept permanently
covered with vegetation such as grass, and a lower limit if it is
kept permanently under the plough. These limits are set by the
nature of the soil and the climate, but the cultivator can attain
any level he likes between them simply by changing his mode of
husbandry. The lower equilibrium level is spoken of as the
inherent fertility of the soil because it represents the part of
the fertility due to the soil and its surroundings, whilst the
level actually reached in any particular case is called its
condition or "heart", the land being in "good heart "or "bad
heart", according as the cultivator has pushed the actual level
up or not; this part of the fertility is due to the cultivator's
efforts.

The difference between the hi