there was insufficient replacement of the
phosphates taken from the soil, the yield of wheat was so low that the
amount of phosphoric acid removed was small, and the system was
permanent for all practical purposes. One of the facts given in
substantiation of this view is that the yield after enclosure
increased considerably.[40]

In discussing these points, it will be well to begin with the evidence
as to exhaustion afforded by the increased yield under enclosure. The
improvement in yield took place because of the long period of fallow
obtained when the land was used as pasture; or, in the eighteenth
century, with the increase in nitrogenous organic matter made possible
when hay and turnips were introduced as field forage crops. That is,
the increase in yield depended either upon that prolonged period of
recuperation which will _restore fertility_, or upon an actual
increase in the amount of manure used. Apparently, then, open-field
land had become exhausted, since an increase in yield could be
obtained by giving it a rest, without improving the methods of
cultivation, etc., or by adding more manure.

There was not, as Dr. Russell supposes, enough manure under the
medieval system of husbandry to maintain the fertility of the soil. It
is true that the husbandman understood the value of manure, and took
care that the land should receive as much as possible, and that he
knew also of the value of lime and marl. But, as Dr. Simkhovitch says:

It is not within our province to go into agrotechnical details
and describe what the medieval farmer knew, but seldom practiced
for lack of time and poor means of communication, in the way of
liming sour clay ground, etc. Plant production is determined by
the one of the necessary elements that is available in the least
quantity. It is a matter of record that the medieval farmer had
not enough and could not have quite enough manure, to maintain
the productivity of the soil.[41]

The knowledge of the means of maintaining and increasing the
productivity of the soil is one thing, but the ability to use this
knowledge is another. The very origin and persistence of the
cumbersome common-field system in so many parts of the world is
sufficient testimony as to the impossibility of improving the quality
of the soil in the Middle Ages. The only way in which these men could
divide the land into portions of equal value was to divide it first
into plots