updated 10 July 2008
Nitrogen is the nutrient
most needed by plants, it helps them produce bulky growth and is the most
likely to be deficient from soils if not added as fertiliser because as
nitrate it leaches quickly from soils. Nitrogen in the form of organic
matter and ammonium ions has a positive charge and stick to clay
particles, clay particles have a negative charge. When soil bacteria break
them down into nitrates they have a negative charge and repel from the
clay particles and then become freely available to plants. Because
nitrates are mobile in the soil they easily reach roots and are quickly
taken in by roots, they also are easily leached from soils.
Other matters affecting the
availability of Nitrogen (N)
At a pH of 6 to 7.5 nitrogen is freely
available, when the pH drops below this figure nitrogen fixing bacteria do
not thrive and therefore cannot convert organic nitrogen to ammonium and
ammonium to nitrates.
Flooded or saturated
soils hold little oxygen, oxygen is necessary for all aerobic
bacteria to function. Soils that dry out and rewetted have a
tremendous bacterial activity, therefore this is the sort of soil
condition we should encourage to get good nitrogen conversion.
Temperature is another
factor, the metabolism of micro organisms increase in warm soils. So
organic nitrogen does not convert to nitrates in cold soils.
(winter) neither does ammonium nitrogen convert into nitrate
As an example the
results of a soil test in an organic rich soil will vary depending
on the conditions available to break the organic matter down. If you
think about it, organic matter breaks down more efficiently in top
soil where oxygen is present than in subsoil where there is very
much less oxygen. Therefore the correct amount of air to water ratio
is important for the conversion of nitrogen. For plant roots to take
in ions they gain their energy from sugars and other compounds
supplied by the phloem system, this is made in the leaves as a
result of photosynthesis.
They also need Oxygen from the soil air
so that the roots can respire. Therefore non-compacted soil is
essential. During periods of very wet weather leaves appear pale, the
absence of soil air and the reduction in transpiration reduces the
amount of nitrogen ions (amongst others) that are taken in by the
plant and therefore the plant appears pale in colour.
N & P are used to synthesise
proteins and DNA that the cells need to survive. They are needed in
large quantities to produce good plant growth.
deficiency can cause leaves to be pale green and growth to be
checked. A severe deficiency will result in stunted growth and pale
yellow-green foliage. Leave size is reduced and youngest leaves
remain erect. Only the youngest leaves darken when buds become
visible. Flowering is delayed, bloom size reduced and sprays develop
fewer blooms than normal.
A mild excess of nitrogen causes very
dark green leaves and delayed flowering. Severe nitrogen excess may
cause root damage and subsequent poor growth. Leaves are very dark
green and plants wilt easily. In extreme cases the symptoms are very
similar to iron deficiency.
To resolve nitrogen deficiency feed with a soluble high nitrogen
fertilser such as sulphate of ammonia.
Excess nitrogen can be reduced by
flooding in order to leach away the excess, however be aware that
waterlogging in a retentive medium may make matters worse.