Agriculture Reference
In-Depth Information
13
Pimentel, D
et al
(2005),
Organic and Conventional Farming Systems: Environmental and Economic Issues
,
Cornell University.
http://dspace.library.cornell.edu/bitstream/1813/2101/1/pimentel_report_05-1.pdf
One of these experiments stands out from all the rest. The Rodale Farming Institute's
three and five course rotations have produced a crop every year, and derive all their ni-
trogen from catch crops of green manure - either red clover crops undersown with the
main crop, and left in the ground over winter, or hairy vetch planted in the autumn. Crop
yields are comparable with respectable yields achieved on the same site through chemic-
al agriculture, and through a closed organic livestock mixed farming system. The results
are impressive. The trouble is that this performance has not been replicated elsewhere and
others who have tried similar rotations have failed. David Pimentel's 2005 study of the
Rodale rotations observes: 'Other researchers have been less successful in maintaining and
improving soil fertility levels in organic systems. Rodale's results could also be influen-
pity, because if Rodale's results were universally applicable, we would not need to resort to
agrochemicals to feed everyone, we would have enough spare land to allow ourselves the
luxury of meat, and the world's troubles would be over.
However, the reliable average seems to be about 2:1 - one acre of green manure for every
two in crops. Dennis Avery reports: 'The average US organic farm has about one third of
its land in green manure crops and fallow to make up for the lack of chemical nitrogen that
very light soils stockless rotations would only work with a very high percentage of fertility
writer Jenny Hall, in her article 'Stockfree Britain', states:
The green manuring proportion of a farm rotation would be 25 to 40 per cent, but on sandy soils more consid-
eration would need to be given to fertility. Therefore in our calculations we have opted for 40 per cent to err on the
side of caution but would expect this to lower as a percentage as stockfree fertility farming techniques improve.
34
These figures mirror the two and three course rotations used in the medieval open
fields, before the development of the Norfolk four course system. Fifty per cent green ma-
nure is the equivalent of a two course rotation in which half the fields at any one time are
fallow, and 33 per cent green manure is the equivalent of a three course rotation.
If we accept a 2:1 ratio, this means that for every hectare of arable stockfree cultivation,
another half hectare of arable land is 'fallow'. This effectively reduces the land use effi-
ciency of stockfree crops by a third, which in turn, affects the conversion ratios we have
been examining above, in respect of organic husbandry. So for example if a crop of grain
is fed to beef cows at a feed to meat conversion rate of 10:1, the livestock system will be
using 6.66 times as much land as an equivalent stockfree system. If the crop is fed to dairy
animals at a conversion rate of 3:1, then the livestock system will be using twice as much
land as a stockfree system.
