Using fertilisers
Soil testing
Soil testing can be used to work out nutrient levels in a soil. These missing nutrients can be supplied using organic or inorganic fertilisers.
Soil tests can be used to check that nutrient levels are at the recommended levels for a particular crop. The tests can show the level of the major nutrients nitrogen (N), phosphorus (P), potassium (K), sulfur (S), magnesium (Mg). It can also give information about the pH and the level of soil organic matter.
Several soil samples are taken as the soil can vary in a paddock. Results can then be averaged and compared. The results can be used to work out the types and amounts of fertilisers needed.
Applying too much fertiliser can cause as many problems as not applying enough. This is because adding fertilisers changes the chemical balance in the soil, and can make some elements less available to plants.
Too much of an element can be toxic to plants. Plants have bad reactions to excess minerals in soils.
Organic fertilisers
Organic fertilisers are made from plant or animal wastes.
Examples include:
- plant or animal products obtained from the meat industry. For example, blood and bone, fish, seaweed and poultry litter. They are relatively cheap but slow-acting, and their nutrient content is not high
- animal manure, compost, sewage sludge, sawdust, peat and bark. They are more useful for improving the physical condition of a soil than for supplying nutrients, as they are relatively low in nutrients compared to non-bulky or inorganic fertilisers.
Inorganic fertilisers
These are obtained from natural mineral deposits or they are manufactured from inorganic substances. Their nutrient content is high. They come in different forms.
| Type | Features | Examples |
|---|---|---|
| Solid | Sold as crystals or powders and are soluble in water. | Urea |
| Slow release fertilisers | Some fertilisers are made into pellets and coated with resin clay. The coating lets water in slowly so the elements are released slowly. Pellets are designed to last for varying amounts of time. | Osmocote |
| Granules | Some fertilisers are made into granules, which don't blow away like powders. They also last longer in soil, releasing their elements slowly. | Superphosphate |
| Liquid | Fertiliser is in a liquid form. Very beneficial to plants when they are about to produce their flowers. They are used mainly for greenhouse and indoor plants. | Liquid orchid food |
A simple fertiliser contains only one nutrient. Urea is an example of a simple fertiliser that only provides nitrogen.
Compound fertilisers
These contain at least two of the major nutrients, N, P or K. For example, nitrate of potash contains N and P. You can find the NPK ratio on fertiliser bags. This is a series of four or more numbers that show the amount of the major nutrients in the fertiliser.
The numbers are the percentages of nitrogen (N), phosphorus (P), potassium (K) sulfur (S) and sometimes magnesium (Mg).
The numbers are always in the same order, NPKS. This information is useful as it tells you what the fertiliser will supply. You can then match the soil needs to a fertiliser that can supply the nutrient required.
Balanced fertilisers
These contain approximately equal amounts of N, P and K and are made up of mixes of other fertilisers. Amounts of N, P and K are given as a ratio, N:P:K in this order. For example, Fulllfert has a ratio of 8:3:6. This means there are eight parts of nitrogen, three parts of phosphorous and six parts of potassium.
Complete fertilisers
These contain all nutrients needed by plants. An example is general garden fertiliser, which is a mixture of fertilisers, and is suitable for many plants.
Applying fertiliser
When and how fertilisers are applied depends on:
- soil conditions
- age of the plants
- stage of growth
- time of year (early spring is the time for most plants as they have a growth spurt. In autumn there is a smaller period of growth).
Solid fertiliser can be applied in the following ways:
Top dressing
The fertiliser is scattered uniformly over the surface of the ground. You can do this by hand in your garden.
Aerial topdressing is done by plane.
Fertiliser can be applied with a fertiliser spreader behind a tractor or four-wheel motorbike.
Band placement
Machinery is used to place fertiliser under the surface in bands beside plant rows.
Maintenance dressing
Containers
Liquid fertiliser is applied in these ways:
- directly to the soil (which should be watered first)
- added to irrigation water
- liquid fertiliser applied directly to the plant leaves.
Foliar feeding is used if:
- a quick response is needed
- there is a sudden deficiency of some nutrient
- nutrients are used up rapidly during a period of intense growth.
Drag each product to its correct fertiliser type.
Key points
- Soil management is what is done on a property to improve or change the soil condition. Nutrients are lost from the soil when products from plants are harvested and taken from a property. Some nutrients can be unavailable to plants or leach into deeper layers in the soil.
- Nutrients can be added to the soil from weathering parent rock, organic matter, legumes and fertilisers.
- Soil testing can be used to work out the nutrient levels of the soil.
Fertilisers come in these forms:
- organic
- inorganic
- solid
- liquid
- slow release.
Compound granular fertilisers containing mixtures of nutrients are commonly used. Each amount of nutrient in the fertiliser mixture is written as a ratio of N:P:K:S and sometimes Mg on the container.
What's next?
Go to: 4 Increasing organic matter.