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Whats the best fertiliser for your plants.... Should you use chemical or organic?
by Thomas Constant
Fertilisers are the key to growing large and healthy plants.
There are two categories: chemical and organic fertilisers. Chemical fertilisers are man-made mixes of the main nutrients needed for plant development. Organic fertilisers are natural by-products of farming, like manure or composted plant waste, containing nutrients and beneficial microorganisms.
Chicken manure is one of the most popular organic fertilisers in use, but an emerging organic fertiliser is the waste product of insect farming, frass. Frass has all the nutrients needed by plants, but also an extra compound, chitin, that activates plant defence systems and increases resilience.
So, which one should you pick? This article will compare the three.
1. Nutrient Composition
Expert advice is to first test your soil for the nutrient profile, as you don’t want to accidentally over fertilise and cause fertiliser burn. All fertilisers are sold with the composition of nitrogen (N), phosphorus (P) and potassium (K), the NPK. Chemical fertilisers can be made to any NPK but an equal ratio of all three is commonly used.
The NPK of organic fertilisers depends on the feed diet of the animal - chicken manure is normally sold as a ratio of 4 - 3 - 2 or 4.5 - 3.5 - 2.5. Insect frass is more variable (depending on the insect and feed) with research showing that nitrogen content of mealworm frass could range from 2.6 to 7.6% with different feed. Commercial frass fertiliser can be 2 - 2 - 2, 3 - 1 - 1 or 3 - 2 - 4.
2. Use in plants
Plants that produce fruit should be fertilised with a high P and K product, because nitrogen helps the leaves more. Commercially sold tomato fertiliser is often highest in potassium. Root vegetables like potatoes and onions benefit from high levels of phosphorus and potassium. For leafy vegetables like lettuce or sprouts, a balanced or high nitrogen fertiliser is recommended.
Perennial flowers are more sensitive and can be easily damaged by overfertilisation, so liquid chemical fertilisers are a no-go. Additionally, unless grown in unfertile soil, only some flowers (dubbed ‘Heavy-feeders’) need fertiliser, these include Chrysanthemum, Lupines, Lilies and Peonies. These flowers will love a fertiliser high in phosphorus and nitrogen. Bushes and trees, especially younger plants, benefit from higher nitrogen content fertilisers.
The cost of commercial synthetic fertiliser is around £1-5 per kilogram. While the cost of chemical fertilisers has greatly reduced over their century of production, according to the World Bank they are now at their least affordable levels since 2008, rising 30% since the start of 2022. In Sub-Saharan Africa the high cost of chemical fertilisers has meant land is underutilised and less food can be produced.
Chicken manure is more affordable, at £1-2 per kg commercially in the UK. Frass is more expensive (around £3–8) because it’s currently at an early stage of adoption and production, though it can be made much cheaper if you're willing to grow your own insects!
Synthetic fertilisers are designed with pure chemical nutrients, so they’re absorbed quickly by the plant and don’t need to be broken down in soil like organic fertilisers do. You can buy special ‘slow release’ fertilisers which are made to be broken down in the soil, or liquid fertilisers which are designed to be absorbed even faster. While some plants want a steady release of nutrients over time, some fast-growing and nutrient demanding plants might not get sufficient nitrogen without a quicker fertiliser.
Overuse of fertilisers damages the health of soil in the long term, removing organic material and minerals and acidifying the soil. Chemical nutrients run-off into water sources, leading to ‘dead-zones’ that starve habitats and kill fish. While much less impactful, chicken farming has its effects: soybean for livestock feed is contributing to deforestation to accommodate the growing global demand for meat. Additionally, chicken supply chains are estimated to emit 0.6 gigatonnes of CO2-equivalent, 8% of the livestock sector’s emissions.
In some cases, manure fertiliser can carry infectious bacteria that persist on crops and enter the human food chain. There have been cases of herbicides and pesticides entering manure through the animal food chain and reducing crop yields once applied to soil. Insect farming is much less impactful due to the smaller scale of everything involved. Insects produce around half as much CO2 as chickens, and around 10 times less than cows.
Using food waste as insect feed also makes insect farming a sustainable circular process for food production - insects convert food efficiently and can transform low-value products into high-quality food.
6. Pest resistance properties
As you may know, farmers use chemicals called pesticides on their crops to stop them from being eaten. You may also know that pesticides can linger in soil for years and enter the food chain. What if the fertiliser was able to make crops more resilient to pests? Insect frass is such a fertiliser.
Frass contains a molecule called chitin found in insect skin and the exoskeletons of many animals. It’s called a ‘microbe-associated molecular pattern’ which means that the plant thinks that the chitin is a microbe and reacts by strengthening its defences. Research has found that pure chitin prevents parasitic ‘root-knot’ nematode worms as good as banned pesticide oxamyl could. Researchers in India found that peanut plants applied with chitin had increased yields with a reduced nematode population. Chitin enhanced the soil microbial activity, showing that it acts differently to a chemical pesticide. A patent investigation into wireworm infestations found that pests caused a mortality rate of 90% in lettuce plants without frass, while plants that had frass application were healthy.
A study found that after a wilt infection of a cowpea field, plants without frass had died and those with frass were still healthy.
Comparison Table Overview
Cost (£ per kg)
1 – 5
1 – 2
3 – 8
Pellets, Granules, Powder, Liquid
Microorganisms and Chitin
Speed of absorption
Environmental impact of production
Environmental impact of application
*Chemical fertilisers can be NPK only.
The comparison table above summarises the topics covered here.
Chemical fertilisers are versatile and can be designed for any nutrient combination, allowing them to be used in many situations. They are bad for soil health in the long term, removing organic material from soil. Their production causes a great deal of pollution, with ammonia production estimated to produce 1.6% of all carbon emissions.
Chicken manure is cheap to buy and supplies all essential nutrients, however due to being an organic fertiliser it must be broken down in the soil to work, which may not be suitable for a nutrient-hungry plant. There is a chance that chicken manure contains pathogenic bacteria that could transfer to food. Chicken production also has an environmental impact due to the production of feed, and an ethical impact from battery farming.
Insect frass is more expensive due to being a newer product, but it improves plant resilience to natural stresses and diseases on top of supplying the essential nutrients and micronutrients needed by the plant. Like chicken manure it is organic so may not be suitable for hungry, fast absorbing plants.
In short, chemical fertilisers are more suited to nutrient demanding plants like large fruiting vegetables, however due to their drawbacks to soil health, organic fertilisers are the better choice for all other plants. In any case, they can be combined with chemical fertilisers to get the best of both worlds and reduce the impact on your garden and the environment.