Agriculture Reference
In-Depth Information
X
silt particles is called '
a silt soil
' and feels 'silky' (or
'soapy') when wetted
X
clay particles is called '
a clay
' and feels 'sticky'
when wetted.
A loam is an idealized soil for growing because its
proportions of sand, silt and clay particles are such
that none of their individual properties (grittiness,
soapiness and stickiness) is evident. Soils like this are
relatively easy to manage. In practice, when the soil is
wetted and moulded between the fingers, it is usually
possible to detect at least a little
X
grittiness, making it a
sandy loam
X
soapiness, indicating a
silt loam
X
stickiness, indicating a
clay loam
.
Soil texture can be considered to be a fixed character-
istic and provides a useful guide to a soil's potential.
requirement to cultivate a clay soil is much greater
than that for a sandy soil. The expression 'light' and
'heavy' reflects the working properties rather than
the actual weight of the soil, that is, for sandy soil
requiring only one horse to pull a plough in sandy soil
(light) but four horses needed on clay (heavy). The
high water content of clays does make them heavier
than sands when both are wet.
The texture of a soil also influences the soil structure
and cultivations.
12
Soil structure
In order to provide a suitable root environment for
cultivated plants, the soil must be constructed in such
a way as to ensure:
X
gaseous exchange (oxygen in, carbon dioxide out)
X
adequate reserves of water available to plants.
Soil water-holding properties
In general, fine-textured soils such as clays, clay
loam, silts, silty loam and fine sands have good
water-holding properties (see p. 152) but poor water
movement unless improved by cultivation and/or the
addition of organic matter. In contrast, coarse-textured
soils (coarse sands and coarse sandy loams) have
good water movement but low water-holding capacity
unless improved by addition of organic matter.
Soil temperatures are closely related to soil texture
because water has a much higher specific heat
value than soil minerals, i.e. it takes more energy to
heat up wet soil than the same volume of dry soil.
Consequently, well-drained coarse sands warm up
more quickly in the spring compared with other soils.
Darker soils (e.g. those enriched with organic matter)
also warm up quicker than light ones. Conversely,
plants growing on darker and drier soils are more
vulnerable to frost damage.
Soil structure
is the arrangement of particles in
the soil.
There should be:
X
a high water infi ltration rate
X
free downward movement of water ('drainage')
X
an interconnected network of spaces allowing
roots to fi nd water and nutrients without hindrance.
There should be no large cavities that:
X
prevent good contact between soil and seeds or
roots
X
dry out seeds or roots.
Soil crumbs
The plant roots and soil organisms live in the spaces
(pores) between the solid components of the growing
medium (sand, silt, clay, organic matter). In the
same way that a house is mainly judged by the living
accommodation created by the solid material (bricks,
wood, plaster, mortar), so a soil is evaluated by
examining the spaces created between the particles.
By aggregating the particles correctly, a mixture of
pore sizes, allowing free water movement, gaseous
exchange and thorough root exploration can be
created. This is achieved with a 'crumb soil', which
is ideal for most horticultural purposes. The best
arrangement of small and large pores for establishing
plants is illustrated in Figure 12.8 alongside a 'dusty'
tilth with too few large pores and a 'cloddy' tilth that
has too many large pores. The interior of a soil crumb
is made up of many small pores holding water against
the pull of gravity, whereas the bigger gaps between
Nutrient levels
Soils with high clay content continue to release nutrients
as they weather and they have good nutrient retention.
In contrast, nutrients are not released from sand
because most of it is inert and those that are present
in sandy soils are readily lost by leaching. This constant
need for nutrients is often met by adding manures (see
p. 169), which need frequent replenishment because
they decompose rapidly in the well-aerated sandy soils.
Hence sandy soils are often referred to as 'hungry soils'.
Ease of cultivation
The differences in the effort required to dig various
soil is familiar to most gardeners. The power
