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Horticulture 1000
HT1033 - Soils 3 - Using fertilisers
Soil management and fertiliser Increasing organic matter

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.

Compost is an example of an organic fertiliser

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.

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.

The N:P:K:S ratio for the fertiliser in our photo is 15–10–10–8.
The N:P:K:S ratio for the fertiliser in our photo is 15–10–10–8.

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.

This rose fertiliser is a complete fertiliser.
This rose fertiliser is a complete fertiliser. The N:P:K ratio is 5–4–8.

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

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

Fertiliser is placed around trees and shrubs to drip  line and watered in.
Fertiliser is placed around trees and shrubs to drip  line and watered in.


Fertiliser is mixed into container media before planting, or added to the surface and watered in as maintenance dressing
Fertiliser is mixed into    container media before planting, or added to the surface and watered in as    maintenance dressing.

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.

Activity 3 Drag each product to its correct fertiliser type.


Key points   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.

Soil management and fertiliser Increasing organic matter