POTENTIAL OF ADAPTED MUSHROOM COMPOST AS A GROWING MEDIUM IN HORTICULTURE

Durable use of materials is promoted in horticulture. Especially in countries like the United Kingdom and Switzerland the use of peat is discouraged. In the Netherlands approximately 3.4 million m³ of peat is used per year. An alternative should have a good quality and should be available in large quantities. Spent mushroom compost is a potential alternative for peat. It is available in large quantities, approximately 2.1 million m³ per year in the Netherlands only. Spent mushroom compost is a steamed mixture of compost and casing soil, could be used as an alternative for peat, but has a high EC (salt content). The aim of this study was to lower the salt content by changing the recipe of the mushroom compost. Mushroom compost is made from horse manure, straw, chicken manure and gypsum. Both the organic manure and the gypsum increase the salt content. In an experiment the manure was replaced by urea, ammonium nitrate or ammonium sulphate and the gypsum was replaced by lime. To compensate for a lower nitrogen content the compost was supplemented with a product based on soy beans later on in the mushroom cultivation stage. After growing white button mushrooms, Agaricus bisporus, mixtures of spent compost were made with peat and compared with the growth on peat substrates with different EC-levels and mixtures of peat with wood fiber, composted straw, casing soil or compost. Also the compost products were washed to lower the salt content. As a test crop Kohlrabi, Brassica oleracea var. gongylodes was used. The mushroom yield was low if urea was used as a nitrogen source. The combinations of ammonium nitrate or sulphate with additional supplementation seem to have a prospective although the physical circumstances and additional supplementation will have to be optimized. The addition of lime instead of gypsum seems to lower the mushroom yield. The adapted mushroom composts had a salt content of 25 % of the commercial spent mushroom compost. However the level is still too high for high dosages in e.g. peat based growing media. The high level is caused by potassium from the straw and additions of calcium nitrate or sulphate. It is therefore impossible to balance the nutrient levels in the mixtures for growing plants, especially if a high percentage is added to the peat. Based on the nutrient content it is however tried to fertilize the mixtures to an optimum nutrient level using NH₄H₂PO₄, Ca(NO₃)₂, KH₂PO₄, NH₄NO₃, K₂S0₄, KNO₃, MgSO₄, Mg(NO₃)₂ and micronutrients at different levels. Compared to the other tested materials all spent mushroom composts showed growth reduction, especially at higher concentrations of the spent mushroom compost in the growing medium. This was mainly caused by a high salt content (EC). Washing the spent mushroom compost reduced most of the problems concerning nutrient content and growth reduction. The spent casing soil apart from the spent compost seems however to be suitable. This spent casing soil represents however only 30 % of the dry weight of the spent mushroom compost. Next to the salt content, nitrogen fixation by microorganisms was one of the factors in the growth reduction. Only if NH₄NO₃ was used as a nitrogen source, the level of available nitrogen in the spent mushroom remained at a sufficient level. All the other spent mushroom types showed very low available nitrogen at the end of the trial. This was also found for the composted straw and to a lesser extent for the tested wood fibers. If spent mushroom compost is to be used in the future as a growing medium on a large scale not only the recipe of the compost has to be changed but also washing the material before use in horticulture seems necessary. Research concerning the impact of washing techniques on environmental impact is important and should be compared with the profits of enlarging the life cycle of the spent mushroom compost.