Agriculture Reference
In-Depth Information
Inorganic
Organic
Air
Water
Figure 1.8.
Idealized general composition of a soil sample.
cooling, freezing and thawing, and wetting and drying. It might also be argued
that cultivation forces particles together and thus tends to improve structure.
However, it is equally true that cultivation breaks down structure. Because of
these two competing effects, the net effect of cultivation on soil structure can
be either positive or negative depending on the condition of the soil, particu-
larly wetness, when cultivated.
Ideally a well-aerated soil is considered to be half-solid and half-void space.
The void space is half-filled with air and half with water. This idealized con-
dition is illustrated in Figure 1.8. Such a soil is under general oxidizing condi-
tions, oxidation of all components, particularly organic matter, is expected. In
real soil samples the total amount of void space depends on soil texture and
structure, and the amount of airspace is inversely related to the water content.
When the void space becomes filled with water, the soil becomes reducing.
This takes a little time because dissolved and trapped oxygen must be used up
before full reducing conditions are reached. Reducing conditions are accom-
panied by the production of methane and, if sulfur is present, hydrogen and
other sulfides.
At this point oxidation and reduction in soil seem simple. When the soil is
not saturated with water, it is oxidizing and when saturated, it is reducing.
However, even under oxidizing conditions reduced compounds and species are
produced. How can methane be produced even in a soil under oxidizing con-
ditions? The answer is that, in addition to simple pores, which are large in
diameter and open at both ends, there are pores that have restricted openings,
have only one opening, or have only one restricted opening as illustrated in
Figure 1.9.
Where the pores have only one opening or restricted openings, they do not
drain and the interiors are reducing even when the soil in well aerated. The
reducing conditions lead to the production of species such as methane and
Fe
2+
. Although methane and iron are two of the most easily found and iden-
tified species, other reduced species, both organic and inorganic, are commonly
present.
Two conditions exist where the soil separates are not aggregated to form
secondary particles or peds. One is where the individual separates act inde-
