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Fertilisers PDF Print E-mail

Fertilisers are used to:
1. Improve the structure and physical properties of the soil so that it holds more moisture, admits more air and allows roots to penetrate more easily. This often involves adjusting the acidity of the soil.
2. Supply plants with nutrients which may be lacking in the soil or have been exhausted by prolonged cropping.

Any enrichment of the soil will facilitate the growth of soil bacteria and other soil organisms which are essential for healthy plant growth, because they help to make nutrients available to the roots of the plants. The different functions of fertilisers are not independent because soil is a complex living system in which each part interacts with the others.

Many soils in Australia lack phosphorus which is essential for plant growth. Superphosphate (super), which is made by treating phosphate rock with sulphuric acid, is the most widely used fertiliser in agriculture. Only part of the phosphorus is taken up by the plants and if the fertiliser is applied every year it will gradually accumulate in the soil.

Some clay soils set very hard when they dry out and then do not absorb water readily. They can be modified by adding lime or dolomite (a mixed calcium-magnesium carbonate rock), but advice should be sought to determine how much has to be added.

Nitrogenous fertilisers are used on sugar cane and many non-cereal crops and in horticulture. They can range from purely chemical fertilisers like ammonia, urea and ammonium sulphate, to organic materials like blood meal or blood and bone. There are specially prepared fertiliser mixtures for particular types of plants, such as for azaleas, which need slightly acid soil, or for lawns, which require a lot of nitrogen. Other mixtures are available to supply necessary trace elements such as potassium, copper, manganese, molybdenum or boron which are lacking in some soils. It is important to know just which element is lacking before using these, as an excess can damage both the soil system and plants. Your state Department of Agriculture or an organic growers' association may be able to give you information about this.

An organic fertiliser consists of animal manure or of composted vegetable matter. It returns some nutrient to the soil where it is slowly released by bacterial action. It also improves the soil structure and encourages earthworms and useful soil microbes.

Health effects
The dust from some fertilisers may be irritating but is not a serious health hazard.

Environmental effects
There are some environmental costs to the use of fertilisers and it is important to know how much, when and which fertiliser to use. Many Australian plants are adapted to phosphate-poor soils and cannot tolerate high phosphate levels. Continued use of superphosphate can render the soil acid and reduce the action of nitrogen-fixing bacteria. Excessive use in agriculture can also lead to phosphate-loaded soil being carried into waterways where it will cause excessive algal growth and eutrophication. Nitrogenous fertilisers can have the same effect. Superphosphate commonly contains cadmium as an impurity coming from the original phosphate rock. Continued use can lead to cadmium contamination of the soil and of the crops and animals produced on it. Organic fertilisers derived from sewage sludge may also be contaminated with cadmium and other heavy metals (lead, mercury, chromium, copper). There are minimum standards for such fertilisers which are marketed commercially.
     Organic fertilisers, derived from animal manure, have no environmental disadvantages (except their smell sometimes) and are a good way of recycling resources. Composting and mulching can be used in gardens to good effect and there are several books which are helpful guides to using these methods (see Resource list.) Some local councils can provide composting bins for home use. Worm farms can be used to recycle household scraps and as a source of nutrients for the garden.

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