By Dave of Darlington

The importance of carbon-to-nitrogen ratios

The carbon-to-nitrogen ratio (usually abbreviated to C/N ratio) is one of the most important characteristics of soil and soil amendments. In Growing Green International magazine Num 14 p 14-15 I tried to show how the C/N ratio of a plant residue controls the way in which it decomposes in the soil. So knowledge of the C/N ratios of different materials can help us to decide which is the appropriate one to use.

In vegan-organic growing the soil amendments are nearly all of plant origin. But plant residues cannot feed the crops directly – they first need to be broken down into very simple inorganic substances before the roots of the crops can absorb them. The final stage of this breakdown is carried out by microorganisms – bacteria and fungi.

We can divide vegan-organic soil amendments into two distinct classes, according to where this microbial breakdown takes place. They may be either composts or mulches. In the case of composts most of the microbial breakdown takes place away from the soil in pile of plant material known as a compost heap or, on a larger scale, a windrow. With mulches, however, the breakdown takes place in, or in contact with, the soil where the crops are growing.

The pros and cons So what are the advantages and disadvantages of these two types of soil amendment? Composts have the advantage that they are lighter and easier to handle than mulches. They usually have a powdery or granular form, which makes it easy to spread them on the soil, even while the crops are growing. The bulkiness of most mulches makes this difficult. Composts can also be used advantageously in the case of materials that should not come into direct contact with the soil, such as human waste. The product of a compost toilet is a common example of this.

The main advantage of mulches is that, weight for weight, they do more good to the soil. This stems from the fact that, as mentioned above, the breakdown of the plant materials takes place actually in or on the soil. To understand this we need to look in more detail at how this breakdown takes place.

The most important agents of this breakdown of plant material are the bacteria and fungi, which feed on the fresh plant materials. They use part of the carbon in these materials to provide themselves with energy, breathing it out as carbon dioxide like we do. The rest they leave in the form of more stable or more inert substances in the compost or in the soil, as the case may be. They will flourish as long as there is fresh plant material for them to feed on. So, in the compost heap, they multiply very rapidly as the compost heap initially warms up, but, as the decomposition of the plant material proceeds, they gradually die out again. By the time the compost has matured, it contains very little suitable food for the microorganisms to feed on.

So when compost is applied to the soil, it does very little to promote microbial growth. When mulch is applied, on the other hand, since it consists of fresh plant material, it causes a great increase in the populations of bacteria and fungi in the soil and these populations will remain high as long as we continue to apply fresh mulch.

Mighty microbes

Drawing: Christine Mackay

But why, you might ask, is it so important to have a high microbial population in the soil? After all, if we are using compost, the job of the microorganisms in breaking the plant material down has already been done. The point is that soil microorganisms, especially fungi, have other important functions to perform besides plant decay. In particular, they make a major contribution to stabilising soil and protecting it from erosion. They do this by gluing the very small soil particles together to give the soil the coarse crumb structure that is so characteristic of a good agricultural or horticultural soil.

So we can make the optimum use of plant residues by applying them to the soil as a mulch rather than as a compost. In that way we not only feed the soil but also help to conserve it for the future.

In Praise of Compost

In the above I may have appeared to be completely dismissive of compost. I did not mean to be. All I was trying to say (in a somewhat muddled fashion) was that, if you just want to improve the soil with some fresh organic material, whether it be vegetable waste or crop residues or weeds or green manure, you will impart more organic matter and nutrients to the soil by using the material as a mulch than by composting it and then putting the compost on the soil.

However, this is not to say that the process of composting does not have important uses. There are a number of specific ways in which the composting of plant waste can bring great benefits. First of all, the composting process itself can kill off plant pathogens (disease-causing organisms), as well as weed seeds and vegetative parts of weeds, such as couch grass rhizomes. This sanitising effect of composting depends mainly on the high temperature that can develop in the compost heap. Few living things can survive a prolonged period of heating above 50°C. There are some fungi, however, that will survive up to 60°C or more (for example, the fungus that causes club-root in brassicas), so, to be on the safe side, it is best to ensure that the compost heap heats to 65-70°C, which should kill off all microorganisms.

Viruses may survive even this temperature, but most viruses require a vector (carrier) to spread them to the host plants – usually a fungus or a nematode (eelworm). Since these vectors are killed in the composting process, most viruses will be deactivated. There are however, a few viruses that spread directly without a vector, for example, the tobacco mosaic virus, which attacks tomatoes. When crops have been infected with such a virus, it is better to play safe and burn the plants.

Compost benefits

Besides the beneficial effect of the composting process, the finished compost will confer important benefits on the soil or on a soil-less growing medium to which it is added. Apart from the general benefits that are due to any organic matter, compost has two particular effects – it stimulates plant growth, especially root development, and it suppresses the incidence of certain soil-borne plant diseases – typically various root and stem rots of fungal origin, such as damping-off.

The reason why compost stimulates plant growth is not yet fully understood, but it seems to involve the presence in compost of humic substances. These are substances resembling the components of the humus in the soil and they are believed to facilitate the passage of minerals and other nutrients through the cell walls of the plant roots, thus increasing the plant’s uptake of nutrients from the soil or other growing medium.

The suppression of diseases happens because compost contains microorganisms that are antagonistic to the pathogenic fungi. They may compete with the pathogenic fungi for nutrients or feed on them parasitically or inactivate them by producing antibiotics. However, they do not work to order. It is not like using a pesticide, where you spray it on and expect it to exterminate every pathogen for miles around. The action of these antagonistic microorganisms is much more uncertain than that. It depends on a lot of environmental factors. Also some plant pathogens are only attacked by specific antagonists and there is no guarantee that the right organisms will be present in the compost you are using. But in general, if you mix a good quantity (say, 20%) of well-made compost into a growing medium, you have a good chance of preventing some of the common diseases like damping-off.

The antagonistic organisms are not peculiar to compost. They are found in soil, in plant material and even in the air. So, if they, and humic substances, are also found in soil, that begs the question as to why soil does not have the same growth-promoting and disease-suppressing effect as compost. Well, some soils do have such properties, but generally the effects are much weaker in soils, largely because the agents responsible for the effects are much more dilute in soil than in compost.

Tea time!

Drawing: Christine Mackay

Another and often more convenient way of imparting the benefits of compost to a soil or growing medium is to apply a compost tea. This is a subject for a separate article, but, briefly, it can easily made by suspending a cloth bag containing compost in a clean bucket of clean cool water for a couple of days. (Rainwater is better than tap water for this purpose.) It is good to agitate the bag frequently so as to mix up the compost and aerate the water a bit. When ready, remove the bag and use the tea straight away and just as it is, without any dilution, applying it directly to the soil or growing medium.

Finally, it is important to stress that, to get the above-mentioned beneficial effects, it is essential that the compost is well made, otherwise it could do more harm than good. For example, a compost that has been made under anaerobic conditions could contain organic acids, which are toxic to plants. Material that has not been left long enough to complete the composting process may contain considerable quantities of ammonia, which is also phytotoxic. There is also a danger that, when added to the soil or other growing medium, such material may continue to compost. The consequent burst of microbial activity takes up a lot of oxygen from the soil and may produce anaerobic conditions, resulting in damage to the growing plant roots. So it is important that, when applied, the compost has reached stability, that is, that the intense microbial activity of the composting process has come to an end.

Take time!

Besides stability, the compost also needs maturity and that takes time. After the hot phase of the composting process has finished and the heap has cooled off, two important secondary processes have to take place – the populating of the heap with antagonistic microorganisms (to give the compost its disease-suppressing properties) and the formation of humic substances (to give it its growth-promoting properties). Both these are very slow processes, so compost may take several months to reach maturity.

Editor’s note: when making compost tea, do try to use rainwater as suggested by Dave; tap water usually contains chlorine, which will tend to kill off the beneficial organisms.

This article is from two articles that appeared in Growing Green International magazine Num 19 (Summer 2007), p6 and Num 20 (Winter 2007), p38.