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Variety of factors influence compost structure

Compost structure is a reoccurring discussion point in the sector, both for growers and composters. It is also a subject where many parties have a different interest. At the time of the new straw harvest it is a good idea to re-examine the factors influencing compost structure. 

In the mushroom cultivation sector the following remark is often heard during discussions about cultivation processes: ‘If the basis is good, not a lot can go wrong’. This is open to discussion as the more that is known about the basic material, the more questions are raised. The primary material meant here is compost. But to be entirely correct, what we are actually talking about is the production of the raw materials used to make compost. One of these is straw, which brings us to cereal farmers. For cereal farmers, grain is the main product and straw a by-product.


Straw production and storage

The type of cereal crop and the soil it is grown on have a major influence straw structure, but the cultivation process is also a contributing factor: is it ecological or conventional? Ecological cultivation systems allow no pesticides or fertilizers, which also means no growth inhibitors. For cereal farmers it is very important that grain kernels do not develop too high up on the stems, otherwise the first heavy storm will flatten the crop. Growth inhibitors and/or modified cereal varieties can be an option.

The weather during cultivation and harvesting are also influential to straw quality. Last summer in Europe was dry, which gave a short straw quality, i.e. the compost quality for the next season. In France the quality of dry harvested straw is good. It is too soon to speak of a good harvest as so much depends on the weather during harvesting.

High labour costs in West-Europe mean that the development of agricultural mechanization is an on-going process. With harvesting and threshing machinery, speed is becoming as important as quality, which can mean the crop is harvested more aggressively. Certain cereal varieties, where the kernel is tightly held in the ears, also have to be shaken more intensely. These measures obviously mean that the handling frequency of the straw will be higher. Some balers used to compress straw into bales have multiple blades, partly to achieve more weight per bale and to reduce transport costs. Large or small bales, the density and various suppliers all affect on compost structure.

The next factor is how the straw is stored: by the producer or the trader? Is it stored on a hard surface so that clay or sand particles are kept away from the straw, and little or no ground water can seep through? Is the stack under cover or in the open air? If straw is rained on during storage, it may later develop mould, with detrimental results during composting.

Is the straw transported after storage straight to the composting company or to a stable or riding school first?

In stables, straw is subjected to many influences, which can cause a fluctuating final quality. The structure differences in straw in horse manure is far more diverse than in straw compost. The difference in moisture content is an example.



We have now arrived at the start of phase 1. A straw bath or other pre-treatment is necessary to start decomposing the wax layer. In outdoor systems this occurs in the flat heaps belonging to the traditional methods used in our country in the past. Here too, the structure depends on the method used. When other ingredients (broiler manure, gypsum, horse manure, straw and percolate) are added in phase 1, achieving a homogenous mixture is essential. This is easier said than done.

A fundamental rule of composting is mixing but retaining structure. With outdoor systems mixing is important, but with indoor systems the shortened process time means precise mixing is even more vital. Mixing compost means the structure is changed. The question is not only the length and intensity of mixing but the type of machinery used and the type of mix. Short straw cannot be made longer. A good raw material like long straw is important to get the right compost structure.

For a good structure excellent aeration properties, in other words oxygen supplies and temperature management, are essential particularly in tunnel systems. With the development of phase 2 in tunnels, oxygen became the greatest focus of attention. An oxygen supply means fresh air in lightly filled compost with good aeration properties. Slatted or spigot floors and drag mats also play an important role.

It quickly became apparent that if the compost in a tunnel was too heterogeneous the process failed to progress optimally, with all the related consequences. This leads us back to the importance of a correct process in phase 1 so that the fresh compost supplied is of the highest quality, allowing tunnel companies to produce a selective nutrient medium in phase 2 for mushroom production.


Mixing, temperature and ammonia

In phase 1 extreme demands are placed on the product, buildings, machinery and the human element. A lot happens in a short space of time. The input materials are mixed as thoroughly as possible in a relatively short period.

However, speed should never be allowed to be at the cost of quality and structure. How often is the mixture lifted by the teeth, how often does it travel from one conveyor belt to another and how often are loaders used?

In addition to all these mechanical factors, the process itself also affects the structure. To activate the right microbiological and chemical processes the correct proportions and aeration techniques cause the temperature to rise as high as 85 degrees Celsius. An important aspect of phase 1 is softening the straw, which is achieved by this high temperature combined with a high ammonia content. The high temperature during phase 1 is beneficial for the compost; there is little decomposition of the organic matter in comparison to decomposition during the phase 2 process, namely conditioning. Oxygen use in phase 1 is less.

This high temperature destroys many micro-organisms. This in turn produces more ammonia, which remains present, as it cannot be absorbed in the lignin humus-complex. The limited fresh air cannot disperse the ammonia. The straw is in a condition where bacteria and mould can do their work effectively. Ultimately, the mushroom mycelium could also develop in the straw and not only on the surface. All these influences soften the straw, which in turn influences the structure.

When the fresh compost produced in this way is delivered to the tunnel company for phase 2 and 3 many treatments again influence the structure, such as emptying the composting tunnels, transport and pre-treatment at the phase 2 tunnel company, filling the tunnels, spawning via hauling winches, the incubation process and actual supplying.

When the spawned compost is finally delivered to the grower, expertise is needed to fill the growing room optimally. Each grower knows that structure of the compost used is a decisive factor in determining the final quality of his crop and profits.