Geoscience Reference
In-Depth Information
F
IG. 60
Example of changes in soil moisture deficit and irrigation needed for a potato crop over a period of 9
weeks. The arrows indicate the times when the soil moisture deficit exceeds the laid down threshold value
of 30 mm, and 25 mm of irrigation is therefore applied. (Data from MAFF, ADAS Cambridge.)
During this period, the estimated daily water loss ranged from 2.7mm in May to
3.1mm in June, while the average daily rainfall for the first seven weeks was less than
0.7mm. The crop therefore had to be irrigated five times as the soil moisture deficit
kept climbing remorselessly back to the threshold value. During 16-18 July, however,
in the eighth week, there was 29.6mm of rain and so it was a fortnight before any
more irrigation was needed. Notice that the rainfall was enough to cancel out the ac-
cumulated soil moisture deficit almost exactly, and return the soil to field capacity.
Any more rain would have been assumed to be surplus and to drain away or run off,
i.e. soil moisture deficit cannot be negative.
Irrigation is most valuable for potatoes, especially early potatoes, secondly for
most vegetables, thirdly for sugar-beet, and lastly for cereals. Large-scale sprinklers,
S
TRAW DISPOSAL
Often again it profits to burn the barren fields, firing their light stubble with crack-
ling flame. It is whether the earth conceives a mysterious strength and sustenance
thereby, or whether the fire burns out her bad humours
Virgil (37
BC
),
The Georgics Book 1
Straw burning was introduced in the 1960s as a result of research by ADAS and
caught on, dare one say it, like wildfire. The benefits and drawbacks have been stud-
ied ever since, and, as the implications for soil management are so great, they are
worth considering in some detail.
As yields of corn have increased dramatically in the past 20 years, so have the
quantities of straw produced, largely in parts of the country where the number of live-
stock, and therefore the demand for straw, has decreased. In the early 1980s, the total
amount of straw produced in the United Kingdom was over 12 million tonnes, of
which more than 5 million were surplus to agricultural requirements. On individu-
al farms, the amount of baled straw probably averaged about 4 tonnes a hectare but
measurements have shown that an equal quantity is left behind. A heavy crop, there-
fore, could mean that there were 10 tonnes a hectare to dispose of one way or another.
