Agriculture Reference
In-Depth Information
free water in wide vertical capillary (80)
solid soil particles (2 - 7)
free water (80)
bound water (ca. 35)
air (1)
Fig. 5.15
Constituents of a soil in a horizontal cross-section and -
in brackets
- its real relative
permittivities (From Scheuermann et al.
2002
, altered)
Water is held in soils either as “
free water
” or as “
bound water
”. Free water
drains readily and is easily available for uptake by crops. Contrary to this, bound
water molecules are attached to soil particles by means of capillary- or colloidal
forces. The finer the soil particles are and thus the higher the clay content is, the
more water is bound. And since at least some of the bound water might not be avail-
able for uptake by crops, agronomists preferably use the tension that is needed to
extract water from the soil as a criterion for the supply of plants. This tension can be
expressed in units of Pascal or in mm of water column.
The problem is that for direct sensing of the
water tension
, presently no on-the-
go methods are available or in sight, neither for operating from farm vehicles, nor
from aerial platforms or from satellites. However, in an indirect way it is possible to
sense whether moisture can be drawn from the soil, namely during the growing
season by means of site-specific signals about the water-transpiration of crops. This
Yet fortunately the sensing of soil moisture by way of real permittivity too takes
into account mainly free water and leaves out bound water - at least approximately.
This is because bound water has lower permittivities than free water as a result of
surface tensions that act on it (Fig.
5.15
). However, a standard method for defining
the bound water by means of permittivities does not exist. In a laboratory, bound
water can be determined by first removing it in a drying oven using a temperature of
105 °C and 24 h time. After this, the soil is subjected to air with 50-60 % relative
humidity. It takes up moisture from this air because of its hygroscopic properties.
The bound water is equivalent to this hygrocopic equilibrium moisture that can eas-
ily be recorded by weighing before and after uptake (Robinson et al.
2002
).
