What Is Present - Field Sampling: Principles and Practices in Environmental Analysis - page 241

source. They may also be present in combined form as a result of microbial activity. Cell
walls, compounds with long hydrocarbon moieties such as lignin, and other structures
resistant to decomposition may persist in the soil for significant amounts of time [25].
9.5. THE LIVING COMPONENT
All soil has organisms living in it, including animals, plants, and microorganisms. The
number of microbes varies with the counting method, and is as high as 10 9 per g of soil.
The activities of these organisms will affect the sampling plan and analytical procedures.
One affect is soil component decomposition, and another is their movement through soil.
Such organisms as ants and termites can move large amounts of soil. Often this means
moving soil from some depth to the soil surface. This is important because it might be
assumed that the A horizon is being sampled when in reality it is the C or B horizon that
is really being sampled and analyzed.
In some cases the reverse may be true. Groundhogs dig sizable tunnels down and
through the soil profile. When the animals abandon these holes they may fill with soil
washed into them from the surface. In this case samples assumed to be from the B or
lower horizons may actually be from the surface. This may not be evident by observing a
single sample, but would be obvious when observing a soil profile. The same type of
thing can happen with other animals and animal holes in soil.
Where a spill has happened in this type of environment, contaminants can move deep
into the soil through these types of holes. In this case sampling must be deeper than the
holes if an accurate determination of the extent of contamination is to be known. In both
of these cases knowing that borrowing animals are common and that burrows have been
filled in is essential. Such information is available in a soil survey.
The above examples are for groundhogs, ants, and termites; however, in different parts
of the world different animals will be living in soil and making it their home. It should
not be assumed that because groundhogs do not live in a particular area this type of soil
mixing cannot happen. It can.
Plants change the soil in a more dramatic way. As roots grow through the soil they
change its physical and chemical properties. Roots push soil particles together, allowing
them to be held together by cations, clay, organic matter, and microbial gums to form
secondary particles, or peds. Secondary particles have distinct areas of weakness and void
volumes between them that increase infiltration and percolation. This in turn will increase
contaminant movement through the soil. Sampling will need to take these changes into
consideration.
Roots exude acids and give off carbon dioxide. Both of these make the soil more acid.
The roots also deplete the soil around them of water and base cations. As it moves toward
the roots, water can carry components, including contaminants, to plants that either
collect at the root surface or are taken into the roots and transported to the plant leaves.
Soils with a dense plant cover such as sod * will have characteristics that are different
* Sod refers to any situation in which soil has a continuous grass cover (e.g. lawns, golf courses,
hay fields).