THE damage heavy tractors inflict on soil can reduce crop yields by up to 80
per cent, according to trials by German scientists. The researchers also say
that ploughing only makes matters worse.
The average weight of tractors and other agricultural machinery used in the
West has tripled since 1970, says Rainer Horn of Kiel University. In joint
experiments with the universities of Göttingen and Braunschweig and the
Jena Agricultural Institute, Horn’s team found that after six passes with a
tractor exerting a force of five tonnes at each wheel, the density of small
invertebrates such as worms and arachnids, which contribute to soil fertility,
fell by up to 80 per cent at depths of around a metre. This caused crop yields
to fall by around 80 per cent.
Ploughing breaks up the water-filled channels that give soil its springiness
and mechanical strength, down to a depth of 30 centimetres. This allows more of
the pressure exerted by a tractor to be transmitted to lower layers. These
layers are not broken up by ploughing, and so become more and more compacted,
according to Horn and his colleagues.
The researchers found that plants were unable to push roots into this
compacted subsoil, and suffered more in dry weather because of this. Water could
not penetrate the subsoil, and more ran off the surface, increasing erosion. The
damage to the subsoil could be cut by about a third by replacing ploughing with
a system known as low tillage, in which the soil is broken up to a depth of only
Horn says farmers have tried to compensate for the effects of soil compaction
by using increasing amounts of fertiliser. It is only when there is more
fertiliser in the soil than plants can absorb that the effects of compaction
become visible in reduced yields. Horn says that some wheat fields in Bavaria
have lost 20 per cent of their yields from compaction, while 30 million hectares
of soil in Europe is “completely degraded by soil compaction”.
In February, Germany passed a law that obliges farmers to protect the soil,
and some researchers are suggesting that farmers should be paid to switch to low
tillage. Another idea is to use the giant Dowler tractor, which is wider than a
tennis court and according to its makers can increase yields by 25 per cent
(Technology, 3 February 1996, p 20).