The function of the plant roots is to absorb water and nutrients from the soil, to anchor
the aerial (above ground) plant parts, and sometimes also to store food.
The young root that bursts out from the seed is called the radicle. Depending on the
species, this can either persist and become a deep growing primary root or tap root, or it
can be replaced by a more fibrous root system of secondary roots.
Adventitious roots are neither primary nor secondary roots, nor do they arise from
them, but are roots which develop in an abnormal position from stems or leaves.
Root hairs on the younger roots absorb water by osmosis and nutrients by active
selective absorption. This second process requires energy provided by root respiration,
which requires oxygen. If the soil is waterlogged, oxygen is unavailable and the roots
cannot respire, and so nutrients cannot be absorbed.
Plants need nitrogen (N) to make proteins and nucleic acids (DNA and RNA). However,
Nitrogen can only be used by plants when it is taken up by their roots, and since
Nitrogen is a stable and insoluble gas it has to be changed into soluble Nitrogen
compounds before it can be used by plants.
The nitrogen that is found naturally in the soil is mainly in the form of humus and
organic matter. Although almost 80% of the air is made up of nitrogen, plants cannot
use it (the nitrogen is said to be “unavailable” to them) until it has been broken down by
certain specialised soil bacteria, described below, or by lightning, Rhizobia etc.
Nitrogen is combined with other atoms to form molecules or ions, when it is said to be
Organic Nitrogen (N 2 )Î Ammonium (NH 4 +)Î
Nitrites (NO 2 )Î Nitrates (NO 3 – ) (& other soluble compounds)This process is represented in Figure 4 on the next page:
6 TONY WINCH
“fixed” ie converted from a gas to a solid: