Soil Particles, Water, and Air
The physical condition of soil has a lot to do with its ability to produce crops. A degraded soil usually
has reduced water infiltration and percolation (drainage into the subsoil),
aeration, and root growth. These conditions reduce the ability of the soil to
supply nutrients, render harmless many hazardous compounds (such as pesticides),
and maintain a wide diversity of soil organisms. Small changes in a soil’s
physical conditions can have
a large impact on these
essential processes. Creating a good physical environment, which is a critical
part of building and maintaining healthy soils, requires attention and care.
Let’s first
consider the physical nature of a typical mineral soil. It usually contains
about 50% solid particles and 50% pores on a volume basis (figure 5.1). We
discussed earlier how organic matter is only a small, but very important,
component of the soil. The rest of a soil’s particles are a mixture of
variously sized minerals that define its texture. A soil’s
textural class—such
as a
clay, clay loam, loam, sandy loam, or sand—is perhaps its most
fundamental inherent characteristic, as it affects many of the important
physical, biological, and chemical processes in soil and changes little over
time
The textural class (figure 5.2) is defined by
the relative amounts of sand (0.05 to 2 mm particle size), silt (0.002 to 0.05
mm), and clay (less than 0.002 mm). Particles that are larger than 2 mm are
rock fragments (pebbles, cobbles, stones, and boulders), which are not
considered in the textural class because they are relatively inert.
Soil particles
are the building blocks of the soil skeleton. But the spaces (pores) between
the particles and between aggregates are just as important as the sizes of the
particles themselves. The total amount of pore space and the relative quantity
of variously sized pores—large, medium, small, and very small—govern the
important processes of water and air movement. Soil organisms live and function
in pores, which is also where plant roots grow. Most pores in clay are small
(generally less than 0.002 mm), whereas most pores in sandy soil are large (but
generally still smaller than 2 mm).
The pore sizes
are affected not only by the relative amounts of sand, silt, and clay in a
soil but also by the amount of aggregation. On the one extreme, we see that
beach sands have large particles (in relative terms, at
least—they’re visible) and
no aggregation due to a lack of organic matter or clay to help bind the sand
grains. A good loam or clay soil, on the other hand, has smaller particles, but
they tend to be aggregated into crumbs that have larger pores between them and
small pores within. Although soil texture doesn’t change over time, the total
amount of pore space and the relative amount of variously sized pores are
strongly affected by management practices—aggregation and structure may be
destroyed or improved.
WATER AND
AERATION
Soil pore space can be filled with either
water or air, and their relative amounts change as the soil wets and dries
(figures 5.1, 5.3). When all pores are filled with water, the soil is
saturated, and the exchange of soil gases with atmospheric gases is very slow.
During these conditions, carbon dioxide produced by respiring roots and soil
organisms can’t escape from the soil and atmospheric oxygen can’t enter,
leading to undesirable anaerobic (no oxygen) conditions. On the other extreme,
a soil with little water may have good gas exchange but be unable to supply
sufficient water to plants and soil organisms.
Water in soil is
mostly affected by two opposing forces that basically perform a tug of war:
Gravity pulls water down and makes it flow to deeper layers, but water also has
a tendency to stay in a soil pore because it is attracted to a solid surface
and has a strong affinity for other water molecules. The latter is the same
forces that keep water drops adhering to glass surfaces,
AVAILABLE
WATER AND ROOTING
There is an additional
dimension to plant-available water capacity of soils: The water in the soil may
be available, but roots also need to be able to access it, along with the
nutrients contained in the water. Consider the soil from the compacted surface
horizon in figure 5.6 (left), which was penetrated only by a single corn root
with few fine lateral rootlets. The soil volume held sufficient water, which
was in principle available to the corn plant, but the roots were unable to
penetrate most of the hard soil. The corn plant, therefore, could
not obtain the moisture it
needed. The corn roots on the right (figure 5.6) were able to fully explore the
soil volume with many roots, fine laterals, and root hairs, allowing for better
water and nutrient uptake.
Similarly, the
depth of rooting can be limited by compaction. Figure 5.7 shows, on the right,
corn roots from moldboard-plowed soil with a severe plow pan. The roots could
not penetrate into the subsoil and were therefore limited to water and
nutrients in the plow layer. The corn on the left was grown in soil that had
been subsoil, and the roots were able to reach about twice the depth.
Subsoiling opened up more soil for
