Geoscience Reference
In-Depth Information
F IG. 61
A rain gun irrigating a potato field during the dry summer of 1989. (Photograph B.N.K.D.)
There are three main options for straw disposal open to farmers. First, the straw
can be baled and removed, as is usual with barley anyway, and common in the case of
wheat where it can be used for bedding for stock. Secondly, the straw can be chopped
up, spread and ploughed in. And thirdly, it can be burnt in swathes in the traditional
way or after spreading or collecting into heaps.
Where there is no demand for straw, burning has some obvious advantages for
soil cultivation: it is easy to do and cheap, it allows the soil surface to dry out quickly
after showers, and helps to produce a friable texture for direct drilling. Temperatures
at the soil surface reach a peak within about half a minute. They may exceed 100°C
for over a minute, but the soil cools down again quite quickly as the fire moves across
the field. Most of the heat is dissipated upwards, so, even with the largest amounts of
spread straw, the temperature a few millimetres below the soil surface remains below
60°C.
A good burn can also halve the number of black-grass seeds that germinate.
Apart from competing directly with the crop, grass weeds harbour aphids that carry
barley yellow dwarf virus which have to be controlled with pyre-throid or other in-
secticides. Unburnt straw, whether left on the surface or ploughed in, encourages
slugs which again require pesticides (molluscicides) to control them. (Fungal dis-
eases, such as eye-spot and take-all, are not significantly reduced by straw burning, as
had been thought, probably because it only needs a few live spores to start an infec-
tion if conditions favour their development.)
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