Soil - Agroecology: The Ecology of Sustainable Food Systems - page 102

where forests have been the dominant vegetative cover for
a long time. Laterization takes place on older and heavily
weathered soils of the humid subtropical and tropical for-
ested regions of the world, and gleization is most common
on soils where water stays at or near the surface for a good
part of the year. But depending on localized conditions of
slope, drainage, vegetation, depth to bedrock, etc., com-
binations of these processes can be found. On the whole,
soil formation and development is an ecological process,
where soil affects the vegetation, and the vegetation affects
the soil.

TABLE 8.2
Soil Texture Classifications

Category Diameter Range a (mm)

Very coarse sand 2.00-1.00

Coarse sand 1.00-0.50

Medium sand 0.50-0.25

Fine sand 0.25-0.10

Very fine sand 0.10-0.05

Silt 0.05-0.002

Clay <0.002

a According to the US Dept. of Agriculture system.

I MPORTANCE OF THE O RGANIC H ORIZON

In natural ecosystems, the O horizon is the most bio-
logically active part of the profile and the most important
ecologically. It plays a significant role in the life and
distribution of plants and animals, the maintenance of soil
fertility, and in many soil-development processes. Macro-
and microorganisms responsible for decomposition are
most active in this layer and in the upper part of the A
horizon. Significantly, the O horizon is usually greatly
reduced or even absent from cultivated soils.
The combination of local climate and vegetation type
contributes to the conditions that promote activity in this
layer; yet at the same time, the quality of the layer has
profound influence on what kinds of organisms prosper.
Bacteria, for example, favor nearly neutral or slightly alka-
line conditions, whereas fungi favor more acid conditions.
Soil-dwelling mites and collembola are more important
under acid conditions, whereas earthworms and termites
tend to predominate at or above neutrality.
The complex process of soil particle aggregation,
which creates what is called the crumb structure of the
soil, is greatly influenced by humus formed during decom-
position in the organic soil layer. In addition, many valu-
able soil fertility processes, discussed later in this chapter,
are related closely to the ecological characteristics of this
important layer.

eye, and feels gritty when rubbed between the fingers. Its
low surface-to-volume ratio makes it porous to water and
less able to adsorb and hold nutrient cations. Silt, although
finer than sand, still is grainy in appearance and feel, but
more actively holds water and nutrient ions. Clay particles
are difficult to see separately with the naked eye, and look
and feel like flour. Clay particles are colloidal in that they
can form a suspension in water and are active sites for the
adhesion of nutrient ions or water molecules. As a result,
clay controls the most important soil properties, including
plasticity and ion exchange between soil particles and
water in the soil. A soil very high in clay content, however,
can have problems with water drainage, and when dry can
exhibit cracking.
Most soils are a mixture of texture classes, and based
on the percentage of each class, soils are named as shown
in Figure 8.2. From an agricultural perspective, sand gives
a soil good drainage and contributes to ease of cultivation,
but a sandy soil also dries easily and looses nutrients to
leaching. Clay, at the other extreme, tends not to drain
well and can become easily compacted and difficult to
work, yet is good at holding soil moisture and nutrients.
What soil texture is best, depends on the crops grown
in it. Potatoes, for example, do best in a sandy, well-
drained soil, which helps prevent rotting of the tubers and
makes harvest easier. Paddy rice does best on heavy soils
high in clay content due to this crop's particular adapta-
tions to the wet environment. A clay loam soil may be
best overall in a drier environment, whereas a sandy loam
might be better in a wet one. The addition of organic
matter changes the relationships of the particles in mixtures,
as we will see below.

SOIL CHARACTERISTICS

In order to develop and maintain a healthy soil system,
as well as make sound judgments about particular soil
management strategies, it is important to understand some
of the most essential properties of soils as they affect crop
response.

S TRUCTURE

T EXTURE

In addition to the aspects of texture described above,
soils possess a macrostructure formed by the ways indi-
vidual particles are held together in clusters of different
shapes and sizes called aggregates (Figure 8.3). Soil
aggregates tend to become larger with increasing depth
in the soil. Soil texture is one important determinant of

Soil texture is defined as the percentage, by weight, of the
total mineral soil that falls into various particle size
classes. These size classes are gravel, sand, silt, and clay
(Table 8.2). Particles greater than 2.0 mm in diameter are
classified as gravel. Sand is easily visible by the naked