Managing Spent Mushroom Compost

By Gerard J Walsh, Teagasc

The management of Spent Mushroom Compost (SMC) in an economic and environmentally friendly manner is one of the foremost challenges facing the Irish Mushroom Industry. Mushroom compost is made from wheat straw, horse manure and poultry manure. Up to 242,000 tonnes of SMC are produced in the Republic of Ireland annually.

Spent Mushroom Compost is a useful source of nutrients and organic matter but, like all sources of nutrients, if not used in a safe and responsible manner has the potential to impair the environment.

Properties of Spent Mushroom Compost:

The main advantages of SMC are:

High organic matter content.

Improves soil structure and water holding.

Low content of heavy metals.

A significant source of plant nutrients especially P. and K.

Provides trace elements.

No unpleasant smell.

Easy to handle.

No weed seeds.

No plant pathogens.

Principal Disadvantages:

High salt concentration in fresh SMC due to high Potash level. The Potash (K) is water-soluble.

N and P issue.

Nitrogen (N)

Although the total N is significant, this is slowly released and the amount of available N in Nitrate or Ammonia form for immediate use by plants is low.

Phosphorus (P)

The total P content is high as a result of poultry manure used in the production of compost. Poultry are fed supplementary P in their diet and this results in the P in SMC being high.

Up to recently, the P in SMC was 3.9kg/tonne but the use of the enzyme, Phytase, in poultry feeds has reduced the P in SMC to 2.5 kg/tonne. The lowering of the P level has improved the possibilities of using SMC for a range of agriculture applications.

Options For Use

Replacement of chemical fertilizer by organic manures including SMC will generate savings to the national economy in terms of reduced fossil fuels and reduced green house gas emissions helping towards Ireland's other environmental goals.

The most obvious use of SMC is on tillage crops. The problems of location, distance from tillage lands, dry matter content, lack of tradition and storage must be overcome. The concept of using organic manures for tillage will have to be promoted to tillage farmers supported by research findings. The organic matter in SMC also will help replace losses in continuous tillage, it is estimated that 2% of organic matter of soils is lost per year in continuous tillage.

Trials

Vegetable and potato crops have a high nutrient requirement, and in Teagasc/Bord Glas trials using SMC in potato crops, the replacement of chemical N, P and K. by SMC was successful when the required extra nitrogen was required.

Grassland

Grassland has a lower requirement for nutrients. If land is in permanent pasture there will be fewer options to spread SMC because of N and P recycling. The organic matter in SMC is beneficial for water retention, particularly on light soils. There is need to accommodate organic manures being applied to sites where, in the professional judgment of a trained assessor who has carried out a site vulnerability assessment there is minimal risk of P loss to water. When the Nitrate Regulations are finalised, the detailed nutrient recommendations for SMC on grass and tillage crops will be worked out.

Other Uses of SMC

The Environmental Protection Agency has suggested that SMC is attractive to landfill operators for daily and intermediate cover and for use in the restoration of mine wastes (it is worth noting that in one Co Council dump, 27,000 tonnes for daily cover was used in 2005.

Spent Mushroom Compost

Total Nitrogen	8 kg/tonne
Total Phosphate	2.5 kg/tonne