Robin Gale-Baker, from Sustainable Macleod, discusses the relative merits of organic and non-organic fertilisers. This is one of a series of articles she has written about growing techniques (see right hand sidebar). She has also written a number of articles about growing vegetables and fruit trees.
In this article, the word ‘organic’ is used to mean ‘in line with certified organic principles’, with the word ‘non-organic’ being used to mean ‘not in line with certified organic principles’.
The fundamental difference between organic and non-organic fertiliser is their source. Organic fertilisers are derived from living things, including plants, animals and manures, while non-organic are synthetically derived chemicals plus minerals from the earth.
The main active components of all fertilisers are the three macronutrients that all plants need (nitrogen, phosphorus and potassium or N:P:K) plus some micronutrients which are minerals, also known as trace elements. The table below sets out the sources for these components for both organic and non-organic fertilisers.
Sources for organic fertilisers
Sources for non-organic fertilisers
|Nitrogen||Mostly derived from animal manure||Mostly from petroleum|
|Phosphorus||Mineral sources such as rock dust, colloidal phosphate or bone or fish meal||Rock phosphate treated with sulphuric acid which then converts to other substances and finally to superphosphate or via other processes to triple superphosphate|
|Potassium (potash)||Wood ash, kelp, greensand (ancient seabeds) and compost||Potash ores such as kainit and sylvanite|
Non-organic fertilisers usually also include sulphur, while organic fertilisers typically have a wide range of other nutrients.
Those favouring organic fertilisers say that the advantages include:
- The slow release of nutrients.
- The addition of organic material to the soil.
- The resulting ability of the soil to retain moisture.
- Sequestering carbon into the soil.
- It is made from renewable, rather than finite, resources.
- The fact you can produce your own by making compost.
Proponents of non-organic fertilisers (which include many companies producing it and many broad acre farmers) argue that the advantages include:
- High in nutrients.
- Quick release and this results in quicker uptake and growth.
- The exact amount of nutrient needed can be measured and used (no under or over feeding).
- They are less bulky, saving money in transport and spreading costs.
Organic growers argue that, whilst non-organic fertilisers give plants nutrients, they do not build soil fertility, biomass or contribute to moisture retention. They also point to loss of soil through erosion by wind and water when soil is ploughed and bare, the build up of chemical toxicity in the soil from repeated applications, and the degradation of soil long term. Regenerative farmers – 12% of Australian farmers – argue that what makes their farms viable is that they do not have to spend anything on buying fertilisers. Keeping the soil under a cover crop throughout the year, as they do, also builds biomass in the soil and moisture retention for only the cost of the initial cover crop. (Regenerative farmers plug seedlings into the cover crop so that the cover crop roots are always left in while the above ground growth is slashed and pushed back down into the soil mechanically).
As I discuss in my article on seaweed tonics and liquid fertilisers, manufacturers often try to have it both ways – or both markets, you might say, i.e. organic and non-organic – so don’t be taken in by a fertiliser that has ‘organic’ incorporated into its name, e.g. ‘prorganics’ (a name I have made up), as this is to mislead. The packaging needs to state that an organic product is a ‘Certified organic’. There will always be some organic matter in any non-organic fertiliser but this does not make it ‘organic’. Strangely, it does not seem to work the other way: I’ve never seen an organic product presented as an non-organic one.