To compost or not to compost

Organic NZ Magazine: September/October 2007
Section: Gardening
Author: Holger Kahl and Dr Charles Merfield

Some people really love their compost heaps, but are we following good organic practices when we make compost for our gardens? Two experts take differing views.


1. Dr Charles Merfield: There is increasing scientific and practical onfarm experience which indicates that composting is not always as beneficial as its nickname of Black Gold would have us believe.

Composting has been an integral part of organics since its inception, and is generally considered a highly beneficial process that should be done as much possible. However, there is increasing scientific and practical onfarm experience which indicates that composting is not always as beneficial as its nickname of Black Gold would have us believe. I am therefore making what many people would consider to be the heretical statement – that in many cases we should not be composting but applying fresh organic matter to soils. The explanation of why is going to take us through some soil biology and the process of composting. To have a healthy, biological active soil requires healthy, active soil organisms, which requires that they have goodquality food.
Food has two principal components; nutrients and energy. Life needs nutrients to make tissues, and energy to power the process of making tissues. The problem with compost is that it’s the diet version of soil food. It’s got all the nutrients, but it’s low on energy. If our soils were overweight, this would not be an issue, but I have yet to come across an obese soil, while I have seen way too many starving soils. We therefore have a bit of a problem.

What’s the answer? Simple, stop composting and put the raw compost materials directly on to the soil, just as nature does. Next time you’re off for a walk in the woods, take a spade and dig a soil inspection pit (a hole). You will see lots and lots of dead leaves and organic matter on the surface, and then lots of dark crumbly soil underneath. Same thing for grassland but less pronounced. Try this on cultivated soil and there will be little or no organic matter on the surface, or anywhere for that matter. When fresh organic matter is left on the soil surface, the soil gets not only the nutrients in the plant matter but the energy as well.

If you want to get a picture of how much energy is at stake here, think back to the bad old days when farmers could burn the straw off their fields. These fires were so hot you couldn’t stand anywhere near them, and they often generated their own clouds, there was such an upwelling of air. The composting process also demonstrates how much energy is at stake. When aerobic composting is done well, the heap can reach and maintain temperatures of 60°C for two to three weeks. It takes a lot of energy to keep something that big that hot for that long. All of that energy is stolen from the soil organisms when biological material is composted. No energy = no organisms = unhealthy soil.

So what happens when you leave straw and crop debris on the soil surface?

No-till farming does exactly that, and the results from thousands of scientific experiments and even greater practical farmer experience is that soil heath improves dramatically. Under continuous no-till cropping (i.e., no pasture phase or livestock), soil organic matter levels build to the same levels as a long-term pasture. This is unheard of in tillage farming, where soil organic matter always decreases over time.

In no-till, there is also much greater soil biological activity, especially earthworms. This is because earthworms need fresh, not composted, material to live on, and they like it left on the soil surface where they can gradually pull it down into the lower soil levels, mixing soil as they go. The same thing was found in a long-term organic experiment done by Bob Crowder, the doyen of New Zealand organics.

He compared compost with fresh green silage-cut grass as fertiliser for an apple orchard and found vast numbers of worms under the fresh grass treatments and very few under the compost.

Now, no-till has a number of problems, including utter dependence on a very small number of systemic herbicides, so I am not advocating that we drop organics and go no-till.However, no-till beats organics hands-down when it comes to a healthy soil. And because creating a wonderfully healthy soil, full of earthworms, is a fundamental aim of organics, we need to sit up and take notice of this.

But wait, it gets worse. Not only are we robbing the soil of massive amounts of energy when we compost, but we are also stealing its nutrients. Making compost reduces the volume and weight of the material composted, even taking into account the varying moisture content. How is this? All that heat is caused by the compost organisms respiring (breathing) like crazy. A key feature of respiration is that carbon is combined with oxygen to produce carbon dioxide. So the “solid” carbon in the compost heap literally floats away as gas.

Why is that a problem? Well, carbon is a key ingredient of soil organic matter, so we are losing a great big wad of super-valuable soil organic matter, literally as hot air. As a principal aim of effective nutrient management in organics is efficient recycling of nutrients on the farm, this is not good news.

Now it’s true that the same will eventually happen in the soil as the organisms there respire the carbon to gas, however, it is much slower and colder, so there is the potential for more carbon to be retained in the soil over longer periods. But wait, it gets worse still. Nitrogen is also lost during the composting process, especially if the carbon/nitrogen ratio is below 20:1, as there is too much nitrogen to be mopped up by the carbon, so it floats away as ammonia. One study found nitrogen losses of around 50 per cent when composting material with a ratio of 20 to 25. Even for higher ratios, there is still an appreciable loss of nitrogen just due to the heat of the pile vaporising volatile ammonia.

Losing some carbon could be considered careless but losing significant amounts of hard-won nitrogen in an organic system is inexcusable. On top of all of this is the extra hassle of making compost. You have to turn it, cover it, make sure no leachate gets into the soil or rivers, and store it. At home, this is not much of a problem, but on farms it means higher labour, building and machinery costs, most of which also consume fossil fuel. So composting makes for a less healthy soil, increases the losses of carbon and nitrogen from the farm nutrient cycle, increases production costs, and burns fossil fuel. How many strikes before you’re out?

Now, to clear up potential misconceptions, I’m not saying that no compost is better than compost. Adding compost to soil is always better than not adding compost, but it would be far better added as fresh rather than composted material.

There are also a lot of times where composting is required, for example, if there are lots of weed seeds present, or food scraps, especially animal products which would give rise to a health and safety issue (although it would still be better for the soil).

Sometimes it is just not practical to put material straight on the fields – it might be too wet and nitrogen could leach. But I strongly argue that for the rest of the time, composting is a bad idea and the organic matter should be put on fresh. So, next time you’re clearing the farm yard manure from the sheds in spring, don’t spend all summer composting it, stick it straight out on the fields to give them some food just when they need it. If you are mowing the lawns at home, don’t put the clippings on the compost.

Either leave them on the lawn to feed it, or sprinkle them on your vegetable plot to feed it instead. That way you will have more soil organisms, greater recycling of soil nutrients and better soil structure all for less work. As Bob Crowder so rightly pointed out, it’s time to kill the sacred organic cow of composting.

• Charles “Merf” Merfield is an agricultural scientist with a diverse background, including managing organic horticultural holdings, the philosophy and science of computing, biological control, politics and management, botany, with specialisation in physical and cultural weed management, agricultural machinery, and is an international specialist in thermal weeding.


2. Holger Kahl: Clearly there are more points to consider than feeding the soil life directly, in order to determine what constitiutes good organic management and practice.

When asking such a question it is important to work with a definition of organics, to understand what it means “to be organic”.

The point constructed in Dr Merfield’s article is that organics means creating a healthy, biologically active soil. The argument is that compost is not the best way of using fresh organic matter to achieve this. First of all we have to investigate whether the criterion “contribution to a healthy, biologically active soil” is the only and sufficient definition for “organicness”.

Plant health, balance of nutrition, a stable and buffered soil environment and improved nutrient uptake are just as important for a healthy soil as directly feeding organisms to produce a “biologically active soil”.

Clearly there are more points to consider than feeding the soil life directly, in order to determine what constitutes good organic management and practice.

However, when you choose to go down the narrow lane you will of course discover that compost and humus are not the best ways of feeding soil organisms.
The point is: they were never meant to be. Compost has never claimed to be the best quality food for soil organisms.

It is somewhat like discussing whether buses are useful for public transport after having made the statement that good, convenient public transport needs to be fast, and then consequently condemning buses for not being able to be fast and therefore rendering them useless for convenient, public transport. But of course they are not meant to be fast; they are meant to transport lots of people efficiently. And who says that convenient transport needs to be fast?

If you make compost for an organic system, it should not be produced for the purpose of creating a substitute for a chemical fertiliser or recycling nutrients. Those would have to be regarded as side effects.

Life is more than nutrients and energy; it’s about timing and balance, right amounts in the right formation in the right place at the right time etc. Compost is more than food. It provides storage, acts as a protector, activator and balancer. Good organic producers will practice extensive rotations, including pasture phases, herbal leys and green manure crops anyway. They will minimise soil disturbance and cultivation and keep the soil covered as much as possible, perhaps even using mulches. Because they are using compost they will not give up on those good housekeeping practices.

So what is the role of good compost?

When making compost, the emphasis needs to be on creating a final product that is rich in humus. Composting provides the opportunity to create decomposed organic matter with a high proportion of mature, stable humus.

To achieve maximum humus formation, composts need to be turned and aerated, but not excessively as some quickcomposting schemes recommend, which claim to produce good compost in 30 days. What can be achieved in this short time is broken-down, mineralised organic matter with very little, if any at all, mature and stable humus. Good humus formation requires the activity of worms and at least nine to 12 months.

The carbon/nitrogen ration needs to be around 30-1, and there needs to be good balance of lignin and cellulose-rich substances (such as woody and straw materials) and fresh, proteinrich substances. With careful management, additions of bentonite clay, and avoiding lime, nitrogen losses can be minimised Soil in the compost heap will actually reduce the humification process.

When composting organic materials, the compost maker is in control of the process. It is different from burying fresh organic matter in the soil, where one can exercise little influence on the process of decomposition.

Why humus is so important

The pioneers of organics sometimes stated that humus was not a substance but a process. It can be seen as the organising principle to create balance and order from and with the results of the decomposition process of organic matter.

Making compost provides us with the opportunity to optimise conditions for humus production. Humus is usually a mixture of various substances which share the characteristics of being complex, organic molecules based on a lignin structure with various other compounds – often nitrogen-containing phenols. It usually a dark colour and contains a range of various humic substances, some of which are humic acids. Humus, although in parts extremely stable, is constantly changing. It is therefore difficult to pin it down and identify it as a particular substance; it is more the functions it fulfils that characterise it.

Humus is a special type of organic matter. And, like organic matter, it creates a range of improvements to a soil for growing crops, including nutrient supply and storage, improved structure, increased water-holding ability, improved disease suppression and plant health, buffering of soil pH and contaminants, increased root growth of plants, and improved plant uptake of potassium and nitrogen.

Colloids and CEC

Humus is a soil colloid. There are also other colloids in the soil that are the clay minerals.

Colloids (from the Greek kola, meaning glue) are usually heterogeneous materials of a size less than one micrometre, but not as small as simple molecules. Solid colloids in a liquid go into a dispersion, not a solution. Soil colloids have large surface areas which are mainly negatively charged. This makes them the most chemically and biologically active parts of a soil.

Humus can also bond with clay particles and form clayhumus complexes, the gold of soil fertility.

These colloids are able to hold onto and store nutrients in the soil, mainly cations. The ability of humus to store nutrients is significantly higher than that of clay minerals. The CEC (cation exchange capacity) of humic acids can be up to 1000 me.% (CEC is usually measured in units known as milliequivalents –
me.%.) in comparison to clay minerals which are ranging between 20 and 200.


Humus is able to hold essential micronutrients in forms that are readily available to plants, while protecting them from influences that could otherwise make them unavailable to plants. This is mainly done through a process called chelation (from the Greek word chele, or claw). The long, complex humus molecules “wrap” themselves around micronutrient ions, holding them in a claw-like fashion. Plants grown in humus-rich soil show higher concentrations of essential micronutrients.

Humification – Mineralisation

Dead organic matter in the soil is undergoing changes which are influenced by the climatic and environmental conditions and the biological activity, and are more or less rapid. Most of the organic matter will be decomposed completely into minerals or nutrients which can then be taken up by plants again (the law of return). However, some of it is rearranged and put together in a special way to create humic substances.

These resist further decomposition, are usually darkcoloured, and contain a number of different complex organic compounds. Earthworms are greatly involved in mixing the humic substances with the clay minerals of the soil into long-lasting humus in the form of clay-humus complexes.

Mineralisation without humification would lead to a great supply of plant nutrients in the soil but would be prone to leaching because there wouldn’t be much around to hold and store the nutrients in the soil.

Humification without mineralisation would lead to a great potential for storing nutrients, but very little nutrients to store and make plants grow. There needs to be a balance between these two processes.

Any soil that is intensively managed for production will be in need of humus formation. Good composting is a great way to ensure a good amount of humus.
In applying compost materials directly to the soil (and risk losing organic certification), you have to be aware that nature puts back amounts in balance with the capacity of the soil to process and digest. And the natural soil is undisturbed. With larger amounts and cultivated soil, things are different. Needless to say, uncultivated, undisturbed soil has more organic matter and macro and mesofauna than cultivated soil. We should all be striving to disturb the soil as little as possible.

Little organic matter is not the unavoidable consequence of cropping and horticulture. With balanced mineralisation/humification and good residue management, including generous green manuring, this needn’t be the case.

Fresh organic matter in the soil can be processed well, assuming that the soil is already healthy enough to aerobically digest the organic matter. But this is often not the case; we get “pickled”, anaerobic, acid and toxic organic matter. Fresh organic matter in the soil takes time to break down. Composting can save that time and provide valuable growing time. Hot composting will speed up the decomposition and ensure weed and pest and disease- free organic matter.

The application of fresh organic plant matter to the soil doesn’t only lead to the decomposition of those parts added, but also results in an increased breakdown of more stable humus fractions in the soil.

Green manures, additions of fresh organic materials (possibly with the support of beneficial micro organisms such as EM) and low-tillage farming all have their place in organic production.

But so has compost. It is only when we fail to see beyond nutrients and energy that we cannot appreciate the full potential and value of compost and humus.

Dr Merfield has raised a number of valid points, and surely compost is not the only answer to the challenges of good organic management.
But don’t make it either-or. Use fresh organic matter for the soil as well as compost: they are both part of good organic management.

• Holger Kahl is a biologist who studied organics in Germany and the United States, has taught organic horticulture in New Zealand, managed organic properties, is the southern co-ordinator for the Organic Advisory Programme of OANZ, and is on the national council of the Soil and Health Association