Back to basics – compost

Compost – it’s an evocative word and if it isn’t it should be. Put more mundanely compost is solid particulate material that is the result of composting, that has been sanitised and stabilised, and that confers beneficial effects when added to soil and/or used in conjunction with plants. Composting is a process of controlled biological decomposition of biodegradable materials under managed conditions that are predominantly aerobic, and that allow the thermophilic temperatures as a result of biologically produced heat, in order to achieve compost that sanitary and stable. All of this is a rather dry way of saying that composting is a natural process that recycles the nutrients from broken down organic material back to the soil and the plants that grow in it.

The process of composting organic or ‘green’ waste is an ancient one. The Ancient Greeks, Romans and the Tribes of Israel were all known to compost their organic waste mixed in with animal manure and street sweepings. In some cultures, blood and crushed bones were also added to the compost mix. The Chinese are thought to be the first people to develop larger composting sites for use in farming. The knowledge and practices used in composting were passed from farmer to farmer, from country to country, down through the generations until the present day so this is real back to basics stuff. Although the process has a long and distinguished history we can still miss the point in this day and age and there needs to be a greater understanding of the multiple benefits of compost and composting.

There are groups of farmers and growers who are not only convinced of the benefits but are actively engaged in operating and developing systems that tick all the boxes of what might be called ‘proper’ composting. In general terms composting systems range from the simple heap or stack to highly controlled and organised windrow systems typified by the Controlled Microbial Composting® (CMC) system. The aim of a successful composting operation should be to combine the right blend of ingredients and supply the right conditions for the active microbial breakdown processes to take place. Apart from the solid ingredients the other two key components are water and air. There must be enough moisture in the mix for the living organisms that actually carry out the breakdown process to survive (they will become inactive or die if the mix is too dry) and there must be a sufficient supply of air for the aerobic organisms that dominate the process and generate the heat in the early stages.

In general terms systems found on UK organic farms tend to fall into 3 main categories: passive composting (stacked heap), open windrow and covered windrow. The use of in vessel composting can be very quick and effective but it is too capital intensive to be economic and the aerated static pile approach has not found favour though some trials have been carried out. The great majority of farming systems tend to rely on the passive or stacked heap approach while true composting systems are still very much in the minority.

Despite what is thought or claimed the stacking of manure and/or other ingredients is not composting and does not produce compost as defined above. It is true that a degree of breakdown and heat generation can take place and this can become significant over time. It is also true that the outer material will break down to any great degree, the temperatures cannot be guaranteed to achieve the desired level of sanitation and there will almost certainly be anaerobic zones within most heaps and stacks. The other major disadvantage with the uncovered stack is a very real danger of nutrient loss from the heap over time. This can involve nitrates in the short term and potassium over the life of the stack. The very real advantage from the producer’s point of view is that it is cheap and convenient.

Open windrow – this is the most common approach to active on-farm composting adopted in the UK. Windrows are long narrow piles formed by front bucket machines (tall windrows) or specialised turning machines (low wide windrows). The composting period takes between 12 and 20 weeks with regular turning and re-mixing. There is no particular protocol for how frequently this is done nor is there any particular prescription for feedstock materials.

Covered windrow system as represented by the CMC® system. This is a development of the open windrow system by the Luebke family in Austria. The feedstocks are carefully chosen to include a balance of well-structured materials with an overall carbon/nitrogen ratio of around 30. A small amount of clay or clay rich soil can be added to assist the buffering of pH and labile nutrients, and a starter culture used to establish a desirable population of micro-organisms. The other key area of control is the close monitoring of temperature, moisture content and carbon dioxide production. The windrows are turned every time the internal temperature exceeds 60°C and maturity gauged using temperature and CO2 emission. This is clearly a resource intensive system but it does give a relatively uniform product within a relatively short period of time (5-6 weeks).

The production of mature, consistent and stable compost can clearly involve both capital investment and significant running costs. Is it worth it? Talk to anyone using the CMC®, open windrow or other controlled system and you will find that they tend to be absolutely convinced of the benefits for the soil and crops and the potential for the reduction of diseases on the holding. Is this confidence justified? There are usually demonstrable changes that have taken place around the holding that bear out the conclusion that the introduction of true compost has brought benefits. Is this supported by the scientific literature? Yes but only up to a point.

There has been a large body of work that has looked into the benefits of the use of compost though it should be noted that much of this work has taken place in different climates, on different soils and often using materials that are not permitted in organic agriculture e.g. sewage sludge, municipal solid waste (includes food waste), paper sludge, etc. The amount of work that has been carried out in a UK context using materials that are permitted by the standards is relatively limited. Despite these limitations many beneficial effects have been demonstrated. These include measurable increases in soil organic matter, improvements in soil physical properties such as porosity and structural stability, positive increases in beneficial soil microbial populations and activity, increased cation exchange capacity, and many, many more.

It is also quite clear that compost will add significant amounts of plant nutrients to the soil but it has to be treated as a very different material to fresh or stacked manure in this respect. It will not make a major difference to the available nitrogen in the soil unless it is applied at very high rates. Mature compost contains a significant amount of nitrogen but much of it has been stabilised and tightly bound during the composting and maturation phases of the process. A relatively small amount of this will be mineralised in the season of application though there can be useful contributions in subsequent years. If compost is applied at a rate that is trying to address the nitrogen requirement of a hungry crop there is a very real danger of over-applying phosphorus. Applied at moderate rates compost can make a very useful contribution to the available phosphorus status of a soil and there is a case for calculating the application rates on the basis of the phosphorus budget rather than the nitrogen budget.

Unlike nitrogen potassium is not bound or stabilised during the composting process and it can be as available in compost as it is in fresh manure. This can be of considerable interest to fruit growers and producers of other high potassium demand crops. This availability does put it at risk during the composting and storage processes, and careful management is essential if no losses are to occur. The use of breathable covers as in the CMC® is one approach and another is to review the availability of farm buildings to see if the composting can be carried out under cover. Compost will also contribute a range of minor nutrients though little work has been carried to identify any potential benefits from such contribution. Proponents will claim great benefits for crop quality but again hardly any work has been done to underpin such claims.

Compost making is a natural process that can be enhanced and improved by careful management and planning. The compost that is produced has much to contribute to the organic production system though it is not a universal panacea – there is no excuse for ignoring the other aspects of the system. Compost is likely to be most effective when applied to light soils with the objective of increasing soil organic matter and in a rotation that has a high proportion of legumes. In terms of crop nutrition it will provide a greater benefit to long term crops with a relatively low nitrogen demand. If the primary objective of an application is to supply nutrients to a short term, nitrogen hungry crop then stacked manure will give a better result.

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