Is compost an organic fertiliser?


Confusion on confusion which always happens when scientific terms and every day terms are mixed up. Even in science, the term organic means different things. In biology it means anything that is or has been alive and is related to the term organism. In chemistry it means chemicals that contain carbon. Organic chemistry is the study of carbon. What it means in everyday language is completely obscure.

Bottom line - compost is extremely good for plants. Put as much on your garden as is reasonable.

When talking about growing mediums Raviv, Wallach, Silber and Bar-Tal (2002) say, "[Growing media] are primarily divided into organic and inorganic materials. The organic materials comprise synthetic substrates (like phenolic resin and polyurethane) and natural organic matter (peat, coconut coir and composted organic wastes). Inorganic substrates can be classified as natural unmodified sources (sand, tuff, pumice), processed materials ( expanded clay perlite and vermiculite) and mineral wool (rockwool and glasswool).

Talk about mixing your meanings of organic!

Does compost provide food for the plant?

The term food is difficult one for a biochemist. If you look in a text book like ‘Biochemistry’ by Stryer the word food is not mentioned at all. However the whole book is about how chemicals are used to produce energy (catabolism) or make the building block molecules (anabolism) to allow an organism to live. Usually, if I have to, I define food as a substance that can be used in both catabolism and anabolism which does not help when looking at some substances like water and salts. In this case I hedge my bets by calling everything nutrients. (Remember there are lots of salts – not just sodium chloride.)

A biology professor I worked with used to say, “Trees don’t have great holes beneath them where they have eaten the soil!”

The amount of nutrients (nitrogen, phosphorus and potassium) that plants remove from the soil is tiny compared to the amount of carbon dioxide they take in to produce their bodies. Plants get all their carbon from the air. They get no carbon from the soil.

Fertiliser is just concentrated plant nutrient - for example nitrogen in the form of the salt ammonium nitrate. Fertiliser is analogous to refined sugar and compost is analogous to fruit, like an orange, which contains sugar.

The problem with adding fertilisers like ammonium nitrate to the soil is that bacteria can use the nitrogen to help to make their bodies removing carbon from the soil. In the same way, adding carbon to the soil reduces the available nitrogen. So adding nitrogen removes carbon from the soil; adding carbon removes nitrogen from the soil and locks them into the bodies of microorganisms. As this process is driven mostly by bacteria and fungi and aerobic microorganisms do this much quicker than anaerobic micro organisms, adding air by digging or hoeing reduces both nitrogen and carbon as the aerobic microorganisms reproduce and take up more nitrogen and carbon. However this can be replaced by adding lots and lots of compost. (Also carbon and nitrogen are returned to the soil when the microorganisms die.)

Animals are heterotrophs which get their nutrients from eating other organisms or parts or remains of organisms. Fungi are heterotrophs because they can get their nutrients from dead organic material. (I use organic here in both the chemical and biological meaning of the word because fungi are fantastic in decomposing pernicious polluting man made organic chemicals, which can add further confusion to both the word food and the word organic!).

 Fungi are not plants, which is why they are put into a new group of their own.

Plants are autotrophic, which means they produce chemicals for anabolism and catabolism using captured sunlight energy, carbon dioxide and water.

Plants, and by implication humans and other animals, are just wet wisps of air.

This is the process called photosynthesis which not only produces carbohydrates for the plants structure but also  energy that is needed to absorb tiny amounts of chemical nutrients in the soil by active transport. (However, most energy in the form of ATP is produced in the mitochondria from sugars made by photosynthesis, but that’s another story.) 

It’s not that easy to get things into and out of plant cells – or animal cells for that matter. These absorbed chemicals are then used – after photosynthesis in cascade reactions usually starting with glycolysis and the citric acid cycle – to produce chemicals that make up the structure of the plant.

So is there any plant ‘food’ in compost? As far as everyday language goes, definitely yes. It is much diluted but plants need such tiny amounts of nutrients like nitrogen, phosphorus and potassium that compost provides adequate amounts – unless you are part of a monocropping industry. Compost has a high cation exchange capacity meaning it captures nutrients and releases them slowly so that they can be used by the plant. It reduces bulk density, increases air filled and water filled porosity, improves the soil structure by forming aggregates, provides habitat for soil organisms and provides nutrients for plants. Some of the compost will eventually end up as organic matter that resists decomposition, which means that the soil is a carbon sink storing carbon so that it does not produce the greenhouse gas carbon dioxide.

There is nothing bad you can say about compost; put it on your soil.


  1. why is it that when i try grow plants in compost they are nitrogen deficient and require fertiliser but then if i plant in soil with the compost on top the plants are fine? is it because my compost isnt matured into worm castings and still probly has lots of little bits of bark in it? why do the plants underneath the compost do fine but the ones in it struggle? would plants not grow in compost in a forest? or was there already nitrogen in my soil and my compost just doesnt have any cause its all used up in the decomposition process?

    1. I don't know crouching tiger and I doubt whether anyone else knows either. It could be lots of things apart from the nutrient content. There may be nitrogen draw down or immobilisation by microbes in the compost. Leaving it to decompose a little longer may well help to alleviate this. Compost is sometimes very open depending on the materials that it is made from. I must admit that I find keeping commercial seed compost damp a challenge. This will lead to seedlings drying out unless the compost has been carefully sieved and only the sievings being used. The quality of the compost could also be a factor. The best composts are made from a diversity of different organic materials. If only one or two materials have been used there may be an absence of some micronutrient essential to the growth of the seedlings that you have used. If you compost manure then the quality of the manure depends on what the animals have been fed on. The microbe community associated with various compost materials may be altered by antibiotics given to animals. Other xenobiotic materials might be in the compost such as weed killer that has been applied to lawns and the mowings put onto the compost heap. The process of composting and the design of the compost heap might affect the compost. I compost anything that was once alive. The compost is left for four days and then turned over every two days for at least two months. This forms a very friable compost that can be passed through a one centimetre mesh giving a compost that I will use on the vegetable garden as a mulch. Anything that does not go through the sieve is put back into the compost bins. I do not use this material for seed sowing. I either use commercial compost or garden soil. At the moment I am composting raspberry roots, rhubarb roots, couch grass (Elymus repens), hedge bind weed ( Calystegia sepium), grass mowings, shirts, trousers, sweaters and kitchen scraps. Whether this is a wide enough range of materials to produce a good seed compost, I don't know but I would not use it for this job anyway.