Improving the efficiency and environmental impact of mushroom composting and cropping.

This article is a summary of the Andrew O'Neil Memorial Lecture presented at the 19th North American Mushroom Conference in San Diego, February 2007. The mushroom industries in the most developed countries are under competitive pressure from countries with lower labor costs. Environmental regulations are also more stringent than in developing countries. Improving the efficiency of cropping substrates and dealing with environmental pressures are key to ensuring a viable future for the mushroom industries in Western Europe and North America.

Straw Types and Other Compost Ingredients

Different types of straw are used for preparing mushroom compost around the world, including wheat, barley, rye, rape, rice and cotton. Comparisons made at Warwick HRI have shown that wheat straw is more suitable for mushroom composting than some other types. However, up to 20 percent of wheat straw can be replaced by rape straw without adversely affecting mushroom yield. Rape straw retains its structure and can help prevent the compost from becoming anaerobic. Most mushroom compost is made from wheat straw although this can also be highly variable. A series of experiments were conducted to examine the effect of the following factors on wheat straw and its suitability for mushroom composting:

* wheat cultivars

* growing sites

* plant growth regulators application

* nitrogen fertilizer applications.

The experiments showed that the difference in mushroom yield between the "best" and "worst" straw sources was about 50 kg mushrooms/tonne compost (20 percent yield difference). Growing site and nitrogen fertilizer application had greater effects on the chemical analysis of wheat straw and its subsequent mushroom cropping performance than cultivar or applications of plant growth regulator and fungicide. However, mushroom yields from the spring wheat cultivar Axona were better than from several winter wheat cultivars. Straw produced at the Limavady site in Northern Ireland produced compost with a higher yield potential than straw from two other sites in Northern Ireland and from two sites in England. Straw produced from higher nitrogen fertilizer plots produced higher mushroom yield than straw from low nitrogen fertilizer plots. Straw that had been stored dry for one year produced a higher mushroom yield than fresh, new season straw of the same type (wheat cultivar, growing site, chemical inputs). However, two-year old straw of the same type performed worse.

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Reducing Composting Odors

Mushroom composting in many countries is under environmental pressure due to its odor emissions. Full enclosure with biofilters is a costly and not necessarily effective option in reducing odors. Research at Warwick HRI into the problem of composting odors has been targeted at the following main areas:

* developing an objective method for quantifying mushroom composting odors

* investigating the effect of forced aeration on odors

* developing alternatives to animal manures as compost nitrogen sources

* improving the quality of recycled "goody" water.

Complaints concerning offensive odors are notoriously emotive and subjective. The standard method for making quantitative measurements of compost odor is by odor panel validation or olfactometry. The method is costly, cannot be done on-site and is subject to error caused by the varying sensitivity of panelists. The individual chemical compounds associated with mushroom compost odors were identified using gas chromatography--mass spectrometry. Sulfur containing compounds were found to be the main contributor to the odor. By measuring just two of these compounds, hydrogen sulfide ([H.sub.2]S) and dimethyl sulfide (DMS), a close estimate of the odor concentration could be obtained. These sulfides can be measured cheaply and on-site using gas detector tubes. Low concentrations of sulfides on the site boundary can be measured with a pulsed fluorescence sulfur dioxide analyser, fitted with a sulfide oxidizer. The methods that have been developed for quantifying odors can now be used to determine the effectiveness of odor-reducing measures.

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Odor and sulfide measurements were taken on air samples collected during the disturbance of pre-wet and Phase I compost in traditional stacks and ricks and in aerated bunkers from 12 composting sites in England. There was no significant difference in odor measurements between sites that used horse manure in the compost mix and those that only used straw and poultry manure. However, forced aeration of the pre-wet and Phase I reduced the odor and sulfide concentrations by about 90 percent.

Experiments at Warwick HRI have investigated a wide range of alternatives to animal manures for use in mushroom compost formulations. Reducing the amount of poultry manure, the main source of sulfur in compost, resulted in a significant reduction in composting odors. Results showed that up to 20 percent of the poultry manure could be replaced with an equivalent amount of nitrogen in the form of urea without affecting mushroom yield. Organic sources such as cocoa meal were also shown to be suitable for partial substitution of poultry manure. Current work is examining the use of vegetable waste (onions, potatoes, cabbage, peelings, leaves, etc.) as a mushroom compost ingredient and results so far have been promising.

Run-off "goody" water from composting sites can be a potent source of odor, as well as a waste of leached nutrients from the composting ricks. Samples of goody water were obtained from 13 composting sites in England and Ireland. Oxidation-reduction (redox) potential was found to be a better indication of the oxygen demand of goody water samples than dissolved oxygen. Goody water odor was correlated with redox potential and dry matter content (or electrical conductivity). Aeration of goody water by submerged pipes, propeller type aerators, or continuous recirculation reduced odor and sulfides. Surface aeration of goody water pits was ineffective in reducing odor. Screening out solid matter, particularly leached poultry manure, was important in minimizing goody water odor.

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Boom spraying of goody water over straw bales is an effective way of producing an "odor aerosol" but an ineffective method for getting water into straw. Dunking of bales is a far better method of wetting straw (an invention of Dr. Murray O'Neil in Canada). It also reduces the likelihood of downwind smells from the pre-wet area. It is also important to minimize the volume of "goody water." Diluting this liquor with large volumes of better quality water makes effective aeration more difficult. It is better to have separate storage tanks for goody water and better quality rainwater run-off. The work has shown that goody water is a highly variable compost ingredient, and this may contribute to the variability in compost analysis and quality. Regular analysis of goody water samples may therefore not only reduce odors but also result in more consistent compost quality.

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The results of the research work on odor reduction have now formed the basis for environmental regulations on the production of mushroom compost in the UK and Irish Republic. They can be found on the web-sites www.defra.gov.uk/environment/airquality and www.epa.ie/wastelicences. These regulations are likely to be reviewed in the next two years in the light of progress made in odor reduction on mushroom composting sites.

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Improving Spawn-running

As well as preparing composts that produce low or no odors, it is important to maintain high cropping productivity. Research was aimed at identifying the quality of spawn-run compost. It was found that during spawn-running, the pH of the compost falls from about 7.4 to 6.3 due to the production of oxalic acid by the mushroom mycelium. Composts that had a greater drop in pH during spawn-run tended to be more productive than composts, which had a smaller drop in pH. Compost pH should therefore be measured before and after spawn-running. Spawning at a rate of 0.8 percent w/w has consistently shortened spawn-runs by about a day and resulted in better mushroom yields than spawning at 0.5 percent w/w. Conversely, surface-spawning of the compost did not produce any benefits.

Improved mushroom casing

Mushroom casing in most Western countries is now based on mixtures of peat and chalk or lime. In some countries there is now environmental pressure to find more sustainable alternatives to peat. There can also be large differences between different types of peat and in their mushroom cropping performance. In North America, milled brown peats are widely used and they were also widespread in Britain and Ireland about 20 years ago. The light structure of these peats stimulates large numbers of "pins" to form, resulting in high mushroom yields. However, quality, particularly mushroom size, density and cleanness, is inferior to that obtained with "heavier and stickier" casing materials prepared from black, deep-dug peats. There is also less risk of "panning" heavier casings than the brown milled peat casings, which therefore require more careful watering. Increased competition, the demand for high quality from supermarket retailers, and higher labor costs for watering and picking mean that most casing in Europe is now made from heavier black peats.