Introduction to Soil Chemistry

water 73
The soil atmosphere also contains water vapor and, in many cases, is at
100% relative humidity. Water vapor evaporating from the soil surface is one
mechanism by which water and dissolved components can move upward in a
soil profile.
These gases are interesting and important but do not represent all the gases
commonly found in analyses of the soil atmosphere. Even in aerobic soils, it
is common to find reduced species such as methane. In addition, if ammonium
is present in the soil solution, ammonia will be present in the soil atmosphere.
Under oxidizing conditions ammonium will be oxidized to nitrite and then to
nitrate, which under reducing conditions are converted to gaseous nitrogen
oxides, which will also occur in the soil atmosphere. Other gases, such as hydrogen
and helium, can be found in the soil under some conditions and at some
localities [1–4].
4.2. WATER
Water is a unique molecule, and when it is associated with soil, it is even more
unique. The most frequently cited unique characteristics of water are its high
melting and boiling points and its ability to dissolve a wide range of molecules
and ions. A less often appreciated characteristic of water is it density, which
decreases both above and below the freezing point of water with the maximum
density actually occurring at a temperature between 3 and 4°C.
These phenomena are related to hydrogen bonding in water, as was discussed
in Chapter 2, where it was pointed out that water is the prime example
of this phenomenon. In addition, the partially positive hydrogen of water is
attracted to electron pairs on any electronegative atom in any molecule, and
the partially negative oxygen in water is attracted to any positive atom in any
molecule. These attractions can occur between species, molecules, and ions in
solution and between water molecules and solid components in the soil. These
interactions are illustrated in structures (2.3) in Chapter 2.
One way of thinking about water in soil is to envision it as a layer around
and covering a soil particle (see Figure 4.2). As water is removed from outside
layers, the remaining molecules are held more strongly. The outermost layers
are held with a tension of 0 to -30kPa, 1 and are removed by the pull of gravity.
This is called gravitational water, and normally drains or percolates through
the soil and into the groundwater. Soil containing gravitational water contains
little or no air and because roots require air to function, this water is generally
said to be unavailable to plants.
The next layer of water held between -30 and -1500kPa, is available to
plants and is therefore called plant available water. The water present between
-1500 and -3100pKa is held in capillaries so tightly that it is not available to
1
One kilopascal (kPa) is equal to 1000 pascals, where a pascal is a unit of pressure defined as
1 newton per meter squared (N/m 2 ).