Fertilisers and the environment

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 and 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:

1. 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
2. production of organic acids from decomposing organic matter
3. 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.

For more information on this topic, contact your nearest NSW Agriculture horticulturist or agronomist.