ARCHITECTURE OF THE SOIL WORLD - The Soil - page 20

still holds 20% or more. This is because there are more fine capillaries in the clay ex-
erting a suction force on the water.
Suction force can be expressed in various ways. The simplest is to equate it with
the pressure of a column of water; for example, field capacity in the laboratory is of-
ten taken to be equivalent to the pressure produced by a 50 cm column of water. If
one thinks of tyre pressures, then this would be equivalent to a very soft tyre with a
pressure of only 0.74 lb/sq in. or about 0.05 atmospheres. At the permanent wilting
point, however, the pressure is equivalent to 15,000 cm of water, 220 lb/sq in. or 14.8
atmospheres. A commonly used scale to calibrate suction force is the logarithmic one
introduced by R.K.Schofield in 1935. This he called the pF scale by analogy with the
soil acidity (pH) scale described above. One thousand centimetres of water thus be-
comes pF 3, the permanent wilting point occurs at pF 4.18, and so on. To take account
of metrification, modern texts often use Bar units (1 Bar = 1 million dynes/sq cm),
and for all practical purposes 1 Bar = 1 atmosphere.
Moisture always tends to move from a wetter to a drier soil until the forces on
both are equal, and the whole system is at the same 'moisture potential'. For the same
percentage moisture content, a sandy soil will have a lower moisture potential than a
clay soil, so that, if the two are brought into contact, moisture will be drawn out of
the sand into the clay. In practice, capillary movement of water is so slow that plants
cannot depend on this to provide adequate supplies. If a root uses up all the available
water in its immediate vicinity it will have to grow 'in search of' more water. This
slow capillary movement explains why, after a dry period, a shower of rain has such
little penetrative effect; the surface layer of soil absorbs all the water, up to field ca-
pacity, and holds it against mere gravitational downward pull. Of course, where there
is a high water table, as in a low lying meadow such as Wendlebury Meads ( see here ) ,
relatively little rain is needed to saturate the soil, and plants are liable to suffer from
too little air in the pore spaces.
By applying a series of suction pressures to a range of different soil types, as
defined in the triangular diagram shown earlier, one can map their relative water re-
tention properties ( Fig. 6 ) . Sands, loams and clays differ markedly not only in total
available water capacity but also in the proportions which are released under increas-
ing suction pressures. For example, the clay loam used in that trial held only half as
much available water altogether as the silt loam. Compared with the sandy clay loam,
on the other hand, the total available water was nearly the same but the clay loam held
much smaller proportions of this water at very low and very high suction pressures.
Many soil animals are rendered inactive long before the suction pressure reaches
the permanent wilting point. The interstitial fauna, such as protozoa, rotifers and eel-
worms, live in films of water in the soil pores, and it is known that most eelworms can