Inorganic – or artificial fertilisers that have been synthesised through a human manufacturing process
Organic – or natural fertilisers that have been manufactured from one or a combination of organic ingredients
From a chemical point of view there is little difference between the quality of any nutrient in an inorganic fertiliser (e.g. nitrogen) and that in an organically derived fertiliser. However, there are differences in the quantities of nutrients supplied and how freely available they are to the plants for growth.
Try and match the need to feed with the plant’s need for growth. Over-fertilisation can cause plants to produce lush, sooky green growth that can lead to a serious infestation of pests and diseases. This in turn means you have to get out there and manage these damaged plant parts. All in all, not fun, and a serious waste of time and money!
Think about the growth needs of the plant before you decide how much it needs to be fertilised. If it is a vegetable or flowering annual or a tree coming into fruit, then it is likely to require regular feeding. However, if you have a garden bed full of indigenous or native plants growing in a well structured healthy soil within the correct pH range, the need to add additional fertilisers is probably not necessary. In fact, by feeding, you could very well be killing them with love!
Check out the N:P:K rating of the fertiliser you want to purchase. This shows the amount of macro elements contained in the product i.e. Nitrogen (N): Phosphorus (P): Potassium (K). For example, the amount of nitrogen in organic fertilisers generally ranges from 1% to 9% of the total content but in artificial fertilisers it can range from 10% to 50%. Truly organic fertilisers usually have a low nutrient rating, e.g. 3:2:1 whereas artificial fertilisers can be 24:7:18 depending on how they are formulated. There is no perfect formulation of nutrients that will cover all plants as different plants have different requirements. And even this will vary due to seasonal requirements such as flowering or fruiting. The trick is to ensure that adequate, but not excessive, levels of nutrients are available to maintain plant health and necessary vigor throughout the growing season and any periods of dormancy.
Organic fertilisers include manures and animal and vegetable byproducts, such as blood and bone and cow manure. These contain smaller amounts of the major plant foods, so they need to be added to the soil in greater quantities. However, as they very often contain a large proportion of fibrous material they are good for build improving soil structure and texture, especially in sandy soils. Because organic manures have to be broken down by bacteria they release their nutrients slowly over a long period. As with inorganic fertilisers, some, such as chicken manure, are high in nitrogen while others, like blood and bone, contain more phosphorus. The best results in the garden come from using a mixture of both organic and inorganic 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. Dynamic Lifter Organic Plant Food is an 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.
Powdered and Granular NPK Fertilisers 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.
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.
Controlled release fertiliser
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.
Supplementary fertilising will be required when plants are grown in containers or pots. Plants in pots are grown in a ‘soil-less’ medium or potting mix. In Australia this is generally composed of pine bark chips of various sizes. However, unlike natural soils, pine bark offers very little in the way of nutrients. Must modern potting mixes come with enough fertiliser to last for the first six months but thereafter, potted plants must be fertilised throughout the year to maintain plant growth.
It is important to only purchase fertiliser as needed as nutrients will be lost over time. Always store fertiliser in a cool, dry area, clearly labeled and dated. Small amounts of excess organic fertilisers can be added to compost heaps but adding artificial fertilisers is not recommended as they may damage composting micro-organisms.
Tips on fertilising
- 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 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 and other delicate plants with strong fertilisers. Use organic based fertilisers such as fish emulsion and blood and bone or a controlled release fertiliser.
- 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. Apply sulphate of ammonia occasionally, supplementing with follow up applications of a complete lawn food.
- Fertilise throughout spring, early summer and autumn.
- Water before and after fertilising.
- Apply fertiliser at the dripline and outwards. (The dripline is an imaginary line drawn from the outer edge of the canopy of leaves to the ground.)
- Don’t over-fertilise. Use fertiliser at the recommended rate.
- Combine your controlled release fertiliser with a soluble fertiliser occasionally to give your plants an extra boost.