rainfall often results in the fertiliser not reaching the root zone, because it remains too
near to the soil surface to be used by the plant.
the speed of the wind, reducing the water loss from both plants and soil. Appropriate
tree species must be chosen; ideally they should also produce something useful such as
fruit or timber as well as providing shelter and shade. The trees should also have a
modest water requirement and not be planted too close to crops, as they would remove
too much water and nutrients from the soil.
C Soil
Soil consists mainly of particles of sand, silt and clay in close association with organic
matter. The relative proportions of each of these soil components determines the soil
type, such as “sandy”, “clay”, “loam”, etc. The mineral content of soils is derived from
the erosion and weathering of rocks. Water percolates down through the soil, which
depletes the surface layers of soluble (and fine insoluble) substances in a process known
as leaching, or eluviation. This process affects the soil’s pH, discussed below in 1Cc.
Soil is an extremely complex medium; in addition to the constituents mentioned
above it also contains various amounts of bacteria, algae, fungi, viruses and
invertebrates such as worms and insects. The inter-relationships between these are
enormously complicated. Soil is thus filled with various forms of life from which it
produces other forms of life.
The Soil Profile
A vertical section, or profile, through a soil normally consists of three main layers:
- The surface soil (Horizon A) is the upper layer, containing relatively large amounts
of organic matter, which darkens the soil’s colour. It is normally called the topsoil. - The subsoil (Horizons B and C); Horizon B is a weathered mineral layer, often well
structured, almost free of organic matter and usually lighter in colour than A. It is
enriched with clay, silt, humus, iron and other nutrients leached from above.
Horizon C is also a mineral layer, though less weathered than B and sometimes
merging with the bedrock, which can be either shallow or deep, like people. - The bedrock (Horizon R) is usually the parent material of the overlying soil profile.
The Water Table
At a certain level below the soil surface most soils are full of water. The top of this
waterlogged soil is known as the water table, which rises and falls according to the
amount of rainfall or irrigation, the time of year, the soil type and the drainage capacity
of the soil.
If the water table rises too near to the soil surface for a period of time, the plant roots
cannot absorb oxygen and the plants suffer as a result. Some species such as wheat have
11
- Windbreaks. Trees planted around field borders (or hedges/slatted fences) reduce