Agriculture Reference
In-Depth Information
may exist between the soil pH and that at which the CEC was measured, often pH 7
or higher. In terms of understanding the field behaviour of soils, it seems appropriate to
measure the cation exchange capacity under conditions approximating those pertaining
in their environment.
1.1.2
THE SOIL SOLUTION
The water that occurs in the pore space of soils and moves through it contains a wide
diversity of dissolved and suspended materials: organic, inorganic and organo-mineral.
The soil solution is important in plant nutrition since the net flow of water to plant roots
also moves a range of dissolved elements in the same direction. In pedogenetic terms,
the soil solution mediates the vertical (in both directions) movement of dissolved and
suspended materials both within the profile, and laterally from one part of the landscape
to others downslope. Dissolved gases, including oxygen and carbon dioxide are also
present; the latter gas occurs in the highest concentrations due to its greater solubility.
The soil solution can be sampled using lysimeters (with or without an applied suction)
to collect the water draining through the soil. Alternatively, it may be displaced from
soil samples by centrifugation, application of a suction or by infiltrating samples with
fluids that displace the soil solution (Vedy and Bruckert, 1979). A problem with this
methodology is the difficulty of deciding whether some of the material present in
the sample is there because of the disturbance involved in installing the lysimeters,
or because of the energy applied to extract the soil solution (McKeague
et al
., 1986).
Nutrients dissolved in soil water can also be extracted using other methods (Arnold,
1958; Gibson
et al
., 1985). Bags containing synthetic ion-exchange resins may be buried
in the soil for periods; following collection, the adsorbed elements may be quantitatively
leached to give satisfactory indexes of the concentrations of selected ions in the soil
solution. Campbell
et al.
(1989) used an immiscible liquid to expel the soil solution from
soils sampled in a largely agricultural environment. They found considerable seasonal
variation in soil solution composition between different soil taxa, and between land use
categories within such taxa.
1.1.2.1
Biological nutrient and other elemental concentrations
As shown below, the concentrations of many elements in soil solutions extracted under
suction are normally higher than those draining freely into lysimeters. Lysimeter water
is that held at high matric potentials in the larger pores while that extracted using
suction, centrifugation or other methods is water that was previously held at lower matric
potentials in the smaller pores. The soil solution extracted from the smaller pores has
been in more intimate contact with the surrounding soil materials; residence times are
also longer in these smaller pores.
Table I.7 presents the concentrations of a range of organic and inorganic solutes both
displaced and freely-draining from the E horizon of an acid French spodosol supporting
a pine forest (Bonne
et al
., 1982). Several points emerge from this table. The first is
the notably higher concentrations of many solutes in the displaced than in the lysimeter
solutions, although this was not invariably so for Al, Si and N. Secondly, there is clear
