Fertilisers provide nutrients for
plants. Nutrients needed in the largest quantities in agriculture are nitrogen,
phosphorus and potassium.
The adverse publicity given by the
media to agriculture's role in polluting the environment may make farmers feel
guilty about using fertiliser. However, reducing fertiliser input can lead to
reduced plant growth which can aggravate problems such as soil erosion.
It is important for you to be aware of
the effects of fertilisers and to use them carefully, but it is also important
that everyone realises that agricultural fertilisers are not the main sources
of environmental pollution. If you apply fertilisers sensibly, so that plants
use all the nutrients and none are leached, there is little opportunity for
pollution.
Nutrients
Nitrogen
On farmed land, most nitrogen is in
organic matter which must first be mineralised by soil microbes into ammonium
or nitrate to be used by plants. Nitrate is easily leached from soil ans so
presents the most opportunity for pollution.
Phosphorus
Phosphorus is a very stable element and
moves only 1–5 mm from where it is spread. It binds quickly with soil minerals,
so is unlikely to leach through soil except under high rainfall in very sandy
soils. It is mainly lost from the soil by erosion when soil particles holding
the phosphorus are blown or washed away. For this reason fertiliser phosphorus
is unlikely to be a major contributor to phosphate pollution of waterways,
unless erosion occurs.
Potassium
Potassium is taken up by plant roots
very rapidly and is not used in great quantities, so represents little
environmental threat. Bananas need large quantities of potassium and care needs
to be taken to apply it in small amounts, often, so that the plant can use all
of it.
Environmental
hazards
Groundwater
pollution
Nitrate leaching through the soil can
present a serious health hazard and contributes to soil acidification. When
high rates of nitrogen are used or where clover grass pastures fix substantial
nitrogen, especially on sandy or permeable soils, inevitably some nitrate is
leached and may enter groundwater if there is a watertable. If this groundwater
is used for domestic supplies, the leaching presents a serious health hazard.
Eutrophication
Eutrophication is the enrichment of
water by the addition of nutrients. The extra nutrients encourage the growth of
algal blooms, particularly in stagnant water. Blue–green algae may produce
toxins poisonous to animals, including humans. For this algae to grow,
phosphorus must be present in the water above a certain level.
Phosphorus may be introduced into
waterways in run-off from pasture, forests and fertilised land, and in drainage
from irrigated land and urban areas. These sources, representing most of the
total run-off, normally contribute low concentrations of phosphorus and are
referred to as diffuse or non-point sources. Point
sources, such as sewage effluent and drainage from dairies and feedlots,
contribute smaller flows but contain much higher concentrations of phosphorus.
These are frequently found to be the sources for most of the phosphorus found
in waterways.
Soil acidity
There are three major acidifying
processes in NSW agricultural systems:
- addition of nitrogen to the soil by fertiliser or
fixation of atmospheric nitrogen, followed by loss of nitrate from the
soil due to leaching or run-off
- production of organic acids from decomposing organic
matter
- removal of alkaline products such as hay from the
soil.
Contrary to popular belief,
superphosphate does not cause soil acidification.
You can take several actions to lower
acidification rates in your soil.
- Use less acidifying nitrogen fertilisers: for
example, use urea rather than ammonium sulfate.
- Incorporate stubbles into fallow to minimise net
nitrification.
- Sow early to maximise the opportunity of the crop to
recover soil nitrate.
- Use perennial deep-rooted plants able to rapidly
absorb mineralised nitrate at the start of the growing season and maintain
low soil nitrate levels throughout the year.
- Use deep-rooted crops.
- Minimise water percolation below the root zone.
- Avoid excessive irrigation.
- Minimise removal of product from the soil. To
prevent acidification you need to apply 55–60 kg of lime for every
tonne of lucerne or clover hay removed; 35 kg of lime per tonne of grass
hay removed; 22 kg of lime per tonne of cereal hay removed; and 3 kg
of lime per tonne of cereal grain removed.
- Minimise manure removal from pastures, preferably
leaving manure where the animals graze.
- Feed hay on the paddocks where it is cut.
- Use cropping rotations to minimise excessive
accumulations of soil organic matter under pasture.
Cadmium
Cadmium is present in tiny amounts
(less than 0.5 mg/L) in the soil, and in larger amounts in rock phosphate.
Plant uptake of cadmium is small, but when plants containing cadmium are grazed
by livestock, the cadmium accumulates in offal and may reach very high
concentrations. This is a severe problem on the sandy grazing soils of South
Australia and Western Australia, but is not a problem on soils with even a low
clay content.
Fertiliser
guidelines
Superphosphate
- Don't topdress dams, streams or swampy areas.
- Don't topdress bare ground.
- Maintain good groundcover around dams and streams.
- Avoid topdressing when heavy cyclonic rain is
expected.
Nitrogen
- Use the least acidifying fertiliser you can afford.
- Apply it in small amounts frequently rather than all
at once, to minimise nitrate leaching.
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