Why do we need to fertilise our garden plants?

For a start, unless you are growing plants in their natural habitat and returning all waste matter to the soil, the soil will need to be fertilised to replace lost nutrients. As well, those garden plants that originate in soils with higher nutrient content they will need to be fertilised to grow happily. Another reason for fertilising is that, in time, established plants use up all the goodness in the surrounding soil.
  • Some plants, such as fruit trees, vegetables and roses, have been bred to be super productive and need extra. Lawns are other good examples. Every time the lawn is mowed and the clippings removed, the plants have to begin re-growing their leaves again. It is important to be aware that fertilisers are not ‘food’ for plants – plants manufacture their own sugars from the sun – but are necessary to enable plants to function effectively. However, generally, the term ‘plant food’ is commonly used when referring to fertiliser.

Elements Essential for Plant Growth

Carbon, oxygen and hydrogen can be obtained from the air and water but all other elements are dissolved in water and taken up by the plant’s roots or, to a limited extent, by its leaves.

1. Major Elements

  • Nitrogen (N) - Nitrogen is an essential part of the proteins in plant cells. It is most important for leaf growth and is a necessary part of the green pigment chlorophyll.
  • Phosphorus (P) - Phosphorus is imprtant in cell formation so is most needed by the growing parts of the plant. Phosphorus promotes the development of seedlings, root growth, flowering and formation of fruits and seeds.
  • Potassium (K) - Potassium assists in photosynthesis and is helpful to the plant’s ‘food factory’. It assists the plant’s overall strength, water uptake and disease resistance, and improves the quality of flowers, fruits and seeds.

2. Minor Elements

  • Calcium (Ca) - Calcium forms the cell wall structure. It is available in lime, superphosphate and gypsum but is quickly leached out by heavy rain. Watch out for blossom end rot (sunken areas on the base) on fruits, especially tomatoes. This is caused by lack of calcium or by poor uptake of calcium which may be a result of unreliable watering.
  • Sulphur (S) - Sulphur forms part of plant protein. Deficiency is relatively rare because sulphur is found in most plant foods.
  • Magnesium (Mg) - Magnesium is important in photosynthesis because it is present in chlorophyll. New leaves have first call on this nutrient and, because magnesium moves very readily through the plant’s system, deficiency is most often evident as yellowing of the older leaves. A magnesium boost is supplied by an application of Epsom Salts dissolved in water.
  • Trace Elements - Iron, Manganese, Zinc, Copper, Boron, Molybdenum.

Trace elements are essential to plant growth but are only needed in minute quantities. It is important to remember that the symptoms of excessive application may be as severe as the symptoms of deficiencies – in other words, use sparingly!

Types Of Fertilisers

Organic Fertilisers

Animal Manures – These are excellent for improving soil structure when used in relatively large quantities, but their nutrient value is relatively low and vary variable, depending on the type of manure and the animal’s diet.

Pelletised Poultry and Sheep Manures – Manure has been compressed into pellets and dried so that, as the pellets break down, the nutrients release gently over a long period. Yates Dynamic Lifter Soil Improver & Plant Fertiliser is a good example.

Blood and Bone – This is the original ‘slow release’ fertiliser. It is made from the waste products of abattoirs and provides a very gentle, long-term feeding. It does not contain potassium.

Green Manure Crops – A leguminous plant, such as peas, is grown (because it can trap atmospheric nitrogen) and dug into the soil after flowering. The use of green manure crops is limited by the space available.

Inorganic Fertilisers

Powdered and Granular NPK Fertilisers (such as Thrive granular), come in different formulations to suit different types of plants. These usually contain a high proportion of soluble nitrogen so can be very damaging to roots unless there is plenty of water available to assist the nitrogen to dissolve. Always supply to a moist soil and water well after application.
Examples: Thrive Granular Plant Foods.
Specific formulations are recommended for particular groups of plants (eg lawns, natives, camellias and azaleas).

Water Soluble and Liquid Fertilisers
These types of complete fertilisers are designed to dissolve rapidly in water and are applied directly to the plant by a watering can or a hose-spray attachment.
Examples: Thrive liquid concentrates,  Thrive Soluble (All Purpose and Flower & Fruit).

Controlled Release Fertilisers
These are relatively new developments in fertilisers and they have revolutionised fertiliser application in production nurseries. They consist of a soluble NPK fertiliser particle surrounded by a protective coating.

Tips on fertilising

Sometimes it would seem that the gardener’s life is complicated by a cacophony of advice. As regards fertilisers there are a few simple dont’s which are well worth bearing in mind.

  • Don’t apply any fertiliser until you have read the directions carefully.
  • Don’t try to apply one or two year’s supply of fertiliser at the one time. It is far better to provide little and often; trying to get fast growth by a heavy application is a recipe for plant failure.
  • Don’t fertilise into dry soils. To avoid damage to feeding roots, make sure that the soil is moist by soaking before and after the application.
  • Don’t apply fertiliser to a lawn and then neglect to water it in very thoroughly, especially in hot weather. It is inevitable that leaf burn will occur with careless applications of fertiliser on lawns.
  • Don’t fertilise ferns with anything except organic based fertilisers such as fish emulsion and blood and bone.
  • Don’t fertilise Australian native plants with fertiliser containing a lot of phosphorus. Whilst many Australian natives accept reasonable quantities of phosphorus, there are many that resent high phosphorus levels.
  • Don’t continually fertilise lawns with sulphate of ammonia, as it encourages excessive top growth but reduces root development and eventually makes for a weaker lawn. It can also alter the soil pH level towards acid conditions (which grasses don’t like). It is okay to apply the occasional feed with sulphate of ammonia providing there are follow up applications of a complete lawn food.

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