Geoscience Reference
In-Depth Information
crops could feed. The first point was quite right but the second was a misconception.
The idea that a fine tilth at the soil surface was intrinsically good for crops persisted
for 200 years until E.W.Russell's experiments in the 1930s established the real pur-
poses and advantages of cultivation. We now know that good soil aeration is import-
ant for root growth but that this is achieved by good drainage below rather than by
pulverization at the surface.
The free movement of water and air through the soil depends on good pore struc-
ture and is greatly reduced by compaction. Tull noted the bad effects of compaction,
remarking that the frequent treading of moist soils by horses' hoofs made the ground
like a highway. He suggested fitting subsoil tines to ploughs to break up the pans
so caused. The problem of compaction has always haunted farmers, and has merely
taken on a new dimension as horses have given way to machinery.
In 1918, A.Amos expressed the view that heavy tractors “may do untold dam-
age” to soils especially when they were wet. Heavy steam tractors of his day weighed
3-5 tons and could indeed make bad ruts with their metal wheels. Modern tractors
are much more powerful for a relatively small increase in gross weight, but anything
above 12 tons is likely to cause appreciable compaction. Laden trailers and equipment
for spreading lime or fertilizer may weigh up to 15 tons and pose special risks of com-
pacting the subsoil (below the normal depth of cultivation) if they bear on it directly
through ruts. Whereas inadequate compaction in a sandy soil can be easily rectified
by rolling, too much compaction in a heavy soil can take years to correct.
There are many different factors that determine soil compaction by agricultural
machinery which can only briefly be mentioned here; the Scottish Institute of Agri-
cultural Engineering has made a special study of this topic. One set of variables
relates to the vehicle and one set to the soil. For a given load, one can reduce the con-
tact pressure on the ground either by increasing the width of the tyre or by decreasing
its inflation pressure. However, if the load is increased then the tyre pressure needs to
be increased to support it, and this will extend the depth to which the stress is com-
municated in the soil. Increasing the speed can also reduce compaction because a soil
can spring back through natural resilience if it has to carry a load for only a short
time. Here again, though, higher speeds for a given load may require greater infla-
tion pressures which will counteract the advantage. An institute can set guide lines for
the best combinations of load, tyre width, inflation pressure and speed, but translat-
ing these into practice is another matter. The fitting of dual or cage wheels is a partial
solution, and another is to use tracked vehicles. These are popular in Russia but not in
this country because they are relatively slow and cannot be used on roads.
It may be necessary to traverse a field several times in a season to plough, cul-
tivate and drill, to spread fertilizers or lime, and to spray with weedkillers, fungicides
