Agriculture Reference
In-Depth Information
Plant requirements
As gardeners, most of us are familiar with plants,
our specially selected ones and the weeds, doing
well in one place but not in another. Indeed, in some
situations, our preferred plants perform very badly.
Often the difference in their performance can be
attributed to the soil. It can be because of the nutrient
problems and these are dealt with in Chapter 14.
Quite often it is simply the physical properties of the
soil that provide the challenge. Although out of sight,
roots play a vital role in supplying water and nutrients
to the plant. They must access a large enough
volume of soil to supply the plant's needs and reach
a depth that helps to maintain a water supply as the
surface layers dry out. Within a full season, a single
plant growing well in open ground develops some
500-1,000 km of root, with most plants penetrating
at least half a metre below the surface. This vast root
system is usually more than is required to supply the
plant in times of plenty, but the extent of the network
is indicative of what is needed in unfavourable
conditions. It also serves to remind us of what we
undertake to provide when we restrict root growth
either accidentally in soils or deliberately when we
grow plants in containers (see Chapter 11).
Plants do not grow until the growing medium is warm
enough: usually above 5°C for most temperate plants
and 10°C for those from tropical areas; optimum
temperatures are in Table 5.1. Growing media also
need to be free of harmful substances, which can
include some nutrients applied in excess quantities.
The growing tip of the root wriggles through the
growing medium following the line of least resistance.
Roots are able to enter cracks that are, or can be
readily opened up to, about 0.2 mm in diameter, which
is about the thickness of a pencil line. Once into these
narrow channels, the root is able to overcome great
resistance to increase its diameter. Compacted soils
severely restrict root exploration, which in turn limits
plant growth. When this happens, action should be
taken to remove the obstruction to root growth or to
supply adequate air, water and nutrients through the
restricted root volume.
The root ball normally provides the anchorage needed
to secure the plant in the soil. Plants, notably trees
with a full leaf canopy, become vulnerable to the
effect of the wind if their roots are in loose material, in
soil made fluid by high water content or are shallow,
such as roots over rock strata close to the surface.
Until their roots have penetrated extensively into the
surrounding soil, transplants are very susceptible
to wind rocking; the plant may be left less upright
unless secured. Furthermore, water uptake remains
limited after transplanting until the delicate root hairs
damaged in the process are replaced and 'plugged in'
to the soil.
In order to grow and take up water and nutrients, the
root must have an energy supply. Efficient energy
production is only possible if oxygen is brought to the
site of uptake (see respiration, p. 116). Consequently,
the soil around the root must contain air as well as
water. To ensure the supply of oxygen is constantly
replenished, and for the carbon dioxide to be taken
away, there needs to be good gaseous exchange
between the atmosphere around the root and the soil
surface. This is usually achieved by managing the soil
particles in the rooting zone, so the spaces between
them contain air as well as water. A lack of oxygen
or a build-up of carbon dioxide will reduce the roots'
activity. Furthermore, in these conditions anaerobic
bacteria will proliferate and produce toxins such as
alcohols. In warm summer conditions, roots can be
killed after just a few days in waterlogged soils. There
are plants that can grow successfully in waterlogged
soil, or in water, because they have adaptions that
make that possible, such as aerenchyma (see p. 79).
Plants can also grow very successfully in water if
the water around the roots is kept oxygenated (see
hydroponics p. 183). A naturally fertile soil or a well-
managed one provides the plant with:
X water
X air (oxygen)
X nutrients
X anchorage.
Origin of soil
Most soils in Britain and Ireland are mineral soils,
that is, derived from rocks (Figure 12.1). Many of
the remainder are peats which are made solely from
organic matter, that is, dead plants. The mineral soils
are the product of three types of weathering: physical,
chemical and biological activity.
Weathering of rocks
Soils form in the layers of rock fragments over the
Earth's surface. The parent rocks that provide the
mineral material for soil formation are weathered by
physical, chemical and biological forces.
Parent rock is the rock from which a soil is
made.
Weathering is the breakdown of rocks.
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