conservation planter—with rugged coulter blades for
Figure 15.3. Corn roots
in-row soil loosening, tine wheels to remove surface
from a compacted
plow layer are thick,
residue from the row, and accurate seed placement—can
show crooked growth
be a highly effective implement because it can suc-
patterns, and lack fine
laterals and root hairs.
cessfully establish crops without intensive tillage (see
chapter 16). Reducing tillage and maintaining significant
amounts of surface residues not only prevent crusting,
but also rebuild the soil by increasing organic matter
and aggregation. Soils with very low aggregate stability—
especially those high in sodium—may sometimes benefit
from surface applications of gypsum (calcium sulfate).
The added calcium and the effect of the greater salt
provide good insights on the quality of the soil. This is
concentration in the soil water as the gypsum dissolves
best done when the crop is in an early stage of develop-
promotes aggregation.
ment but after the rooting system has had a chance to
get established. If you find a dense rooting system with
PlOW lAYER AND SUbSOIl cOMPAcTION
many fine roots that protrude well into the subsoil, you
Deep wheel tracks, extended periods of saturation, or
probably do not have a compaction problem. Well-
even standing water following a rain or irrigation may
structured soil shows good aggregation, is easy to dig,
indicate plow layer compaction. Compacted plow layers
and will fall apart into granules when you throw a
also tend to be extremely cloddy when tilled (figure
shovelful of soil on the ground. Compare the difference
15.2). A field penetrometer, which we will discuss in
between soil and roots in wheel tracks and nearby areas
greater detail in chapter 22, is an excellent tool to assess
to observe compaction effects on soil structure and plant
soil compaction. A simple shovel can be used to visually
growth behavior.
evaluate soil structure and rooting, and digging can
Roots in a compacted plow layer are usually stubby
and have few root hairs (figure 15.3). The roots often fol-
low crooked paths as they try to find zones of weakness
in the soil. Rooting density below the plow layer is a good
indicator for subsoil compaction. Roots are almost com-
pletely absent from the subsoil below severe plow pans
and often move horizontally above the pan (see figure 6.6,
p. 61). Keep in mind, however, that shallow-rooted crops,
such as spinach and some grasses, may not necessarily
experience problems from subsoil compaction.
Compaction may also be recognized by observing
crop growth. A poorly structured plow layer will settle
into a dense mass after heavy rains, leaving few large
pores for air exchange. If soil wetness persists, anaero-
bic conditions may occur, causing reduced growth and
Figure 15.2. Large soil clods after tillage are indicative of compaction and poor aggregation.
denitrification (exhibited by leaf yellowing), especially
163
Building SoilS for Better CropS: SuStainaBle Soil ManageMent