Horticulture Principles and Practices

content. This section is sometimes called agricultural soil and is the portion
disturbed during tillage and other soil-preparation practices for planting crops.
The topsoil is also the part of the soil that experiences the most leaching and
weathering.
2. Subsoil (or B- and C-horizons). Subsoil is a transitional zone in the soil
profile, a “catch zone” for particles and mineral elements that have moved down
from the topsoil by water action. This horizon may accumulate clays, calcium
carbonates, and mineral oxides. Organic matter content is minimal in this zone;
some roots may occur.
3. Parent material (substratum or R-horizon). The R-horizon consists of the
primary parent material from which the soil above was formed. With time,
soil-forming factors will act on this material to produce more soil.
Properties of Soil
Some soils may consist almost entirely of organic matter, such as peat bogs, while others
may consist almost entirely of mineral elements, such as sandy soils in deserts. A good
agricultural soil has both mineral and organic constituents.A typical soil (mineral soil) has
four principal constituents—mineral, water, air, and organic matter (Figure 4–10).
The soil is truly a dynamic system in which three factors (physical, chemical, and
biological) interact to affect plant growth and development; soil is affected in turn by the
plants. Understanding the roles of each of these factors in the soil system and their
effects on plants is important in making an appropriate choice of soil for crop cultivation
and knowing the best way to amend or manipulate the soil for better crop production.
Aspects of these three soil properties that affect crop production are discussed next.
Physical Properties The physical properties of interest in soil include soil texture
and soil structure.
SOIL TEXTURE Soil may be physically separated on the basis of particle size. Three
basic particle size classes, called soil separates, are recognized; they include sand, silt,
and clay (Table 4–6). An agricultural soil normally contains all three soil textural classes
but in varying proportions. Soil texture may be defined as the proportions (percentages)
of sand, silt, and clay particles in a soil. When the three soil separates occur in equal
proportions, the substance is called loam. A perfect loam does not occur in the field.
Instead, one of the separates often predominates in the soil. Soils are therefore described
as, for example, sandy loam or sandy clay loam to indicate the predominant separate or
separates in the soil.
Soil texture affects soil drainage. Clay soils do not drain well and are easily waterlogged.
Clay soils are also described as heavy soils and are difficult to till. They often
crack upon drying. Because root growth is hampered in heavy soils, crops whose
Soil Texture
The relative percentages of
sand, silt, and clay in a soil.
Water
25%
Mineral
45%
FIGURE 4–10 The composition
of a typical mineral soil.
Air 25%
Organic matter
5%
4.3 Belowground (Soil) Environment 109