Horticulture Principles and Practices

4.3 BELOWGROUND (SOIL) ENVIRONMENT
The soil is the primary medium for crop growth, although modern technology allows
crops to be grown in other media. The role of climate in determining crop adaptation was
discussed earlier in this chapter but was limited to the aboveground environment.
Climate plays a significant role in determining the types of soils in which crops may be
grown. This role comes from the fact that climate is a primary factor in the dynamic
process of soil formation called weathering, the process by which parent material (the
rocks from which soils are formed) is broken down into small particles. The type of soil
formed affects the kind of vegetation it can support, which in turn further impacts the
process of soil formation by influencing the organic matter and nutrient content of
the soil. Soil formation is a continuous process.
4.3.1 SOIL
Role of Soil in Horticulture
The role of soil in horticultural crop production is to provide physical support and a
reservoir of nutrients and moisture for growing plants. In terms of nutrition, soils may be
described as fertile, marginal, or infertile. Soil nutrients are depleted with years of use
and need to be replenished periodically. The soil may not be rich in native nutrients, but
to be useful for crop production, it should at least be capable of holding water and nutrients
for some time. If this condition does not exist, the grower should make provisions
to supply supplemental nutrition to prevent deficiency problems. To be of any use for
crop production, the soil should be deep enough to permit root development for good
anchorage while supplying adequate nutrition.
Soil Profile
A vertical cross section of
the soil showing the
various layers or horizons
that have developed over
the period of soil
formation.
Soil Profile
When a deep trench is dug to expose a vertical cross section of the soil, one usually can
observe different layers called soil horizons. These layers together constitute the soil
profile. The degree of profile development depends on the age of the soil, young soils
showing less development than older ones.
A soil profile can be described in great detail, but for our purposes, a profile is
considered to consist of three general sections (Figure 4–9):
1. Topsoil (or A-horizon). Topsoil is the upper layer of soil, where most plant
roots are found. It is usually darker in color because of the high organic matter
FIGURE 4–9 (a) A simplified
illustration of a typical soil profile.
(b) An actual profile of a Mollisol
showing a dark surface horizon
and white subsoil.
(Source: For (b), USDA)
Topsoil
Subsoil
A-horizon
B-horizon
(High in organic matter;
highly weathered soil)
(Leached materials from A
accumulate here; soil less
weathered)
C-horizon
(Weathering just starting)
Parent material
(Material from which soil
is formed)
(a)
(b)
108 Chapter 4 Plant Growth Environment