Chemical vs organic fertilisers

From the desk of Neutrog’s resident microbiologist Dr. Uwe Stroeher


As a gardener, you may have a good understanding of how microbes benefit soil and plant health, but do you know how chemical fertilisers affect soil microbes?

Chemical fertilisers are good at making nutrients available to your plants as an instant hit, however it does come at an environmental cost, and can shut down some of the soil biology. A good example is a beneficial fungi in the soil that traps and kills nematodes – fertilisers high in nitrogen actually turn off the killing mechanism so the fungi can no longer do their job and kill the nematodes.

Furthermore, the use of a high-nitrogen fertiliser will also shut down the ability of free-living bacteria to fix nitrogen from the atmosphere, thereby depriving the garden the benefit of these microbes. High levels of phosphate in chemical fertilisers has been shown to reduce the interaction of plants with fungal species that would normally solubilise and take up phosphate for the plant to use.

Unlike the use of organic fertilisers, there are no long-term benefits gained by using chemicals, and in some cases can damage your soil microbiology. When I talk about long term benefits, I’m referring to soil structure improvement and fixing carbon in the soil – doing this improves your soils’ ability to hold onto nutrients, and opens up the soil to allow water and air to penetrate the roots. Achieving this is really only possible if you use an organic fertilising regime in combination with soil microbes…and some patience. 

Organic fertilisers (especially when pelletised) release nutrients slowly depending on the rate at which microbes break these pellets down. This microbial breakdown is generally matched to the plant’s requirements, i.e. faster when it is warm and moist, and slower in cold, very hot or dry conditions. This is not to say that at times a boost is not required – for example, some additional micronutrients or secondary nutrients such as sulphate or calcium can be applied in an organic base which have been enhanced by the addition of chemicals.

Of course, chemicals can be beneficial for some applications, especially for insect and pest control, but having said that, if you do have good soil biology, then more often than not, your plants are tougher and more resistant to insect and pests anyway. 

If you want to be more environmentally friendly, a great way to start is to apply some compost to your soil and supplement this with an organic fertiliser such as Neutrog’s Rapid Raiser or Seamungus – the reason being that most compost has low nitrogen levels, so it feeds the microbes but not your plants.

For that little bit of extra nitrogen and phosphate, I would suggest Rapid Raiser, which has a slightly higher nutrient content than Seamungus.  The additional phosphate in Rapid Raiser comes from rock phosphate, which has been shown not to interfere with the beneficial plant fungal interaction.  However, if you’re after a product which provides nutrients and also helps in water retention, then Seamungus is ideal (Seamungus contains composted seaweed so it acts as a fantastic soil conditioner as well). 

Both products contain millions of microbes so they help in nutrient cycling, feeding the bacteria and fungi as well as your plants. They are also certified organic by the ACO. As a rule, you only need to put down about 100 grams per square metre about every 6 to 8 weeks. Both Seamungus and Rapid Raiser are available from all good garden centres. 


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