The low down on fertiliser


A fertiliser is anything that supplies nutrients to plants with the intention of promoting vigorous growth.

 Fertilisers come in two broad categories – Complete or Compound fertilisers, and Straight fertilisers.  Compound fertilisers are those that contain a number of different elements as opposed to the Straights which provide primarily one nutrient, or are used for one main nutrient.  Incitec’s Pasture Boosta range is an example of Compound fertilisers as they contain a blend of nutrients such as nitrogen (N), phosphorus (P) and potassium (K).  Some blends also contain sulphur and trace elements.  Superphosphate is often thought of as a Straight because it is used to supply phosphorus, but it actually contains higher levels of calcium and sulphur!  Urea is a true Straight as all it contains is nitrogen.

 There is a lot of interest in organic fertilisers both from the organic and commercial sectors.  The latter group sells a lot of product to the domestic consumer where there has always been a strong market for organic inputs.

 Organic fertilisers are usually dried and / or heated to kill pathogens.  We are most familiar with dynamic lifter and blood and bone, but there is a new wave of organic fertilisers emerging which includes waste products from anaerobic digestion and dehydrated food.  SESL Australia has been working closely with a number of these producers in recent times to evaluate the efficacy of these fertilisers.  This work has included incubation studies to see how quickly nutrients are released, and more ‘real world’ studies including pot trials.

 A principal consideration when applying fertilisers is the rate at which they release their nutrients for plant uptake.  Many of the common manufactured fertilisers are very soluble so the nutrients are quickly available to plants.  In contrast, many organic fertilisers need the mediation of soil microorganisms to breakdown the organic material before nutrients are available.

 Fertiliser image

The main issue with soluble fertilisers is our tendency to use more than is required.   We see this most commonly when testing soil from private gardens.  Nutrient levels are very often ‘off the charts’ as a result of gardeners’ regular attentions.  It is important to remember that there is very little nutrient removal from gardens.  Apart from annual pruning, or perhaps lawn clipping removal, most of the nutrients in a garden stay there and cycle between living plant parts, the soil’s organic matter pool and back into the plants again.

 Soil testing is therefore critical so that we understand what we are managing.  SESL’s soil tests come with full interpretation and recommendations for additional fertilisers if required.  In this way, it is better to understand what we are managing rather than inadvertently killing our plants with kindness.


Featured Image: Pot trial fertilised by dehydrated food (rates are equivalent nitrogen application rates)