OTHER SOIL ANIMALS - The Soil - page 104

These changes had no discernible effects on productivity in fertile orchard soils
because the tree roots were already well established and able to exploit deeper soil
layers. One may wonder, however, whether they might affect the establishment of
seedlings. In less fertile soils, the effects of of worms may be much more important
for tree growth. In Gisburn Forest in North Yorkshire, for example, there is a 30
year old experiment on the effects of growing oak, alder, Scots pine and Norway
spruce separately and in pairs. In the spruce monoculture, the worm population was
extremely low and consisted mainly of one small species Lumbricus eiseni. Mixtures
of alder or Scots pine with spruce, on the other hand, gave a more palatable leaf litter
which attracted more worms including the red worm Lumbricus rubellus. Not only
was there a marked increase in the weight of worms per square metre here compared
with the monoculture, but this was associated with an increased mineralization of ni-
trogen and phosphorus, and also with an improved growth of the spruce.
The highest populations of worms occur in permanent pastures with estimated
weights of between 456 and 646 lb an acre - 511 to 724 kg per hectare. The often
quoted comparison, attributed to the Danish observer Hansen, is that this weight ex-
ceeds that of the livestock grazing the land. However, this was thirty years ago, and
the agricultural improvement of old pastures with fertilizers, and modern grazing
practices, make this comparison less realistic and meaningful today.
Though there are no precise measurements of the total amount of soil passing
through the bodies of earthworms, A.C.Evans at Rothamsted suggested that this could
be up to 36 tons an acre a year (about 90,000 kg per ha) in grassland. At this rate, the
fine soil in the top four inches of an old pasture at Rothamsted could all pass through
the guts of the worms living there in 11 years. Arable fields have much lower worm
populations, and the comparable calculation for a nearby field was 80 years.
A good way of seeing worm burrows and soil mixing is to set up a small chamber
consisting of two glass plates held apart by strips of wood 1-2 cm thick with an elastic
band around them. A few small worms, such as the grey worm Aporrectodea cali-
ginosa , are placed inside, and the chamber is filled with different coloured layers of
soil and put in a cool place for a week or two. This species has been used in exper-
iments to show that tunnels can be made in densely compacted blocks of soil by in-
gesting soil particles, and not just by pushing the soil aside. This fact is important for
it means that worms can recolonize arable soils that have been compacted by the pas-
sage of heavy machinery.
The deeply burrowing species can significantly improve drainage in certain in-
stances. Some interesting observations were made by W. Ehlers in Germany by irrig-
ating plots with water containing a blue dye, and then finding stained worm channels
down to 180 cm. These established channels, in a silty loam soil with poor natural