Soil - Agroecology: The Ecology of Sustainable Food Systems - page 104

From an agroecological perspective, good crumb
structure is of considerable significance. Soil particles that
are bound together resist wind and water erosion, espe-
cially during any time of the year when vegetative cover
is minimal. Good structure also helps maintain low bulk
density , defined as the weight of solids per unit volume
of soil. Soil with a low bulk density has a higher percent-
age of pore space (higher porosity), more aeration, better
water percolation (permeability), and more water storage
capacity. Obviously, such a soil is easier to till and allows
plant roots to penetrate more easily. Excessive cultivation
accelerates breakdown of soil organic matter and increases
the potential for compaction, causing bulk density to go
up and many of the advantages of good crumb structure
to be lost.
The formation of soil aggregates has essentially two
components: the attraction between individual soil parti-
cles, the degree of which is very dependent on soil texture,
and the cementing of these attracted groups of particles
by organic matter. The first component cannot be very
easily manipulated by the farmer, at least in any practical
manner, but the second can be very much impacted by
farming practices. Thus good crumb structure can be
maintained, degraded, or improved.
For example, excessive tillage with heavy equipment
while the soil is too wet can lead to the formation of large
blocky clods of soil that can dry on the surface and later
be broken apart only with great difficulty. Compaction, or
the loss of pore spaces and a rise in bulk density, is an
indication of the loss of crumb structure, and can be
caused by the weight of farm machinery, by the loss of
organic matter from excessive tillage, or by a combination
of the two.

structure and chemistry is necessary to complete the pic-
ture, but color is a good beginning. In addition, soil color
can influence the interaction of the soil with other factors
of the environment. For example, it may be an advantage
to have a lighter-colored, sandy soil on the surface in some
tropical farming systems in order to reflect the sun's rays
and keep the soil cooler; conversely, a darker soil surface
in areas with cold winters will help the soil temperature
rise earlier in the spring, dry the soil sooner, and permit
soil preparation for planting at an earlier date.

C ATION -E XCHANGE C APACITY (CEC)

Plants obtain the mineral nutrients described in Chapter 2
and Chapter 3 from the soil in the form of dissolved ions,
whose solubility is determined by their electrostatic
attraction to molecules of water. Some important mineral
nutrients, such as potassium and calcium, are in the form
of positively charged ions; others, such as nitrate and
phosphate, are in the form of negatively charged ions. If
these dissolved ions are not taken up immediately through
plant roots or fungi, they risk being leached out of the soil
solution.
Clay and humus particles, separately or in aggregates
that form platelike structures known as micelles, have
negatively charged surfaces that hold the smaller, more
mobile positively charged ions in the soil. The number of
sites on the micelles available for binding positively
charged ions (cations) determines what is called soil CEC,
which is measured in milliequivalents of cations per 100
g of dry soil. The higher the CEC the better the soil's
ability to hold and exchange cations, prevent leaching of
nutrients, and provide plants with adequate nutrition.
CEC varies from soil to soil, depending on the struc-
ture of the clay/humus complex, the type of micelle
present, and the amount of organic matter incorporated
into the soil. Multisided polyhedrons form lattices that
vary in their sites of attraction and flexibility in relation
to moisture content. Cations cling to the negatively
charged outer surfaces of the micelles and humates with
differing degrees of attraction. The most tenacious cations
— such as hydrogen ions added by rain, positively
charged acids from decomposing organic matter, and
acids given off by root metabolism — can displace other
important nutrient cations such as K + or Ca 2+ . Organic
matter in the form of humus is many times more effective
than clay in increasing CEC since it has a much more
extensive surface area-to-volume ratio (hence more
adsorption sites) and because it is colloidal in nature.
Farming practices that reduce soil organic matter content
can also reduce this important component of soil fertility
maintenance.
Negative ions that are important for plant growth and
development, such as nitrate, phosphate, and sulfate, are
more commonly adsorbed to clay micelles by means of

C OLOR

Soil color plays its most important role in the identification
of soil types, but at the same time it can tell us much about
the history of a soil's development and management.
Dark-colored soils are generally an indication of high
organic matter content, especially in temperate regions.
Red and yellow soils generally indicate high levels of iron
oxides, formed under conditions of good aeration and
drainage, but these colors can also be derived directly from
the parent material. Gray or yellow-brown colors can be
indicators of poor drainage; these colors form when iron
is reduced to a ferrous form rather than oxidized to the
ferric form in the presence of abundant oxygen. Whitish
light-colored soils often indicate the presence of quartz,
carbonates, or gypsum. Standardized color charts are used
to determine a soil's color.
Hence, a soil's color can be an indicator of certain
kinds of soil conditions that a farmer might want to look
for or avoid, depending on the kinds of crops or cropping
systems that might be used. More specific analysis of soil