Geoscience Reference
In-Depth Information
F
IG. 62
A cereal field after straw burning, showing the harrowed perimeter strip. (Photograph B.N.K.D.)
Ploughing the straw into the soil, on the other hand, has few advantages and sev-
eral disadvantages. In the first place, it requires more energy than burning and min-
imal cultivation, and is therefore more expensive; it could increase net costs on heavy
land by £20-50 a hectare. Straw incorporation probably improves structure and work-
ability but this is difficult to quantify. The roots, unburnt stubble and chaff contrib-
ute more organic matter than the cut straw, so ploughing it in produces only a small
increase; incorporating 5 tonnes/ha of straw into a silty clay loam for five years in-
creased the soil organic matter by 12 percent in one trial, while other, 17-year, trials
showed no significant gain. Straw contains small amounts of phosphate and potash
but very little nitrogen. The high carbon:nitrogen ratio of 80:1 means that the bac-
teria which attack the straw need extra nitrogen which has to be taken from the soil
reserves or from fertilizers applied - about 8 kg per tonne of straw or about 60 kg
per hectare. It was thought at one time that this nitrogen demand, and the phytotoxic
effects of breakdown products, posed serious problems for seedling development, but
it is generally agreed now that these fears were overplayed.
Despite the agricultural advantages of straw burning, they have been outweighed
by the environmental disadvantages. In the 1960s, much damage was done to
hedgerows and trees by uncontrolled straw burning, apart from the occasional damage
to neighbouring property. Bye-laws now require farmers to remove the straw from a
this, however, has become less acceptable to the public at large because of the pollu-
tion from smoke and smuts. If weed problems and soil compaction from direct drilling
