Agriculture Reference
In-Depth Information
The result was that when, after a century had passed and the population started to ap-
proach former levels, there was a landbank of meadow and pasture which had accumulated
a large store of nitrogen and which, if ploughed up for arable, at first produced yields that
were double those of worn out arable land. The value of grassland was reflected in the say-
ing 'to break a pasture makes a man: to make a pasture breaks a man.' By the 1600s the
practice of ploughing up grassland, growing corn for some years, and then putting it back
to grass had become a long term rotational system, known as convertible or up-and-down
husbandry; and it is still reflected today in the common practice on mixed organic farms,
of alternating four or five years clover and ryegrass leys with two or three years arable.
But the rise of livestock-rearing after the Black Death, according to Allen, had another
effect. In the 15th and 16th centuries increasing numbers of arable farmers replaced spring
corn in their rotation with peas or beans, which contained higher levels of protein, and
whose 'appeal lay as a source of fodder to support the growing number of animals now be-
ing stocked'. This would have benefited the fertility of the soil insofar as the manure from
these animals was returned to the land, providing a small amount of readily available ni-
trogen. But Allen's point is that the continual cultivation of nitrogen-fixing peas and beans
and the incorporation of their residues into the soil gradually built up the reserves of organ-
ic nitrogen, resulting in a slow steady rise in the fertility of arable soils, and in the value of
the 1.5 per cent of the stored nitrogen released every year.
The introduction of clover with the Norfolk rotation in the 18th century, famously
brought immediate and obvious improvements in fertility from the additional livestock ma-
nure and (unlike peas and beans) from a certain amount of free nitrogen released from
ploughed in residues. But Allen estimates that 60 per cent of the nitrogen was ploughed in
and 'increased the stock of organic nitrogen and contributed to the long run rise in yields'.
It is, he claims, the slow rise in yields from the much earlier introduction of peas and beans
that has perplexed historians who:
have been trying to pin the revolution down to a half century some time between
1500 and 1800. But which? Was it during the parliamentary enclosures? the last half
of the 17th century? 1590-1660? The important point of the simulations reported here
is that the Agricultural Revolution took several centuries. 150-200 years is the time
frame for the stock of soil nitrogen to move from one equilibrium to another.
It is not only historians who were misled, but also the farmers themselves with all their
proverbs about the 'hooves that turned sand to gold' and 'muck is the mother of money'.
'Low medieval yields', concludes Allen, 'were not due to a deficiency of manure nor were
high 18th century yields due to its abundance. Livestock however were crucial to the yield
increases. It was not their dung that mattered, but what they ate, for the legumes fed to
them increased nitrogen stocks that ultimately benefited the corn.' By championing the vir-
