POTENTIAL OF ADAPTED MUSHROOM COMPOST AS A GROWING MEDIUM IN HORTICULTURE

Abstract

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 m 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 m 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 Proc. IS on Soilless Cult. and Hydroponics Ed: M. Urrestarazu Gavilan Acta Hort. 697 ISHS 2005 172 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. INTRODUCTION The use of renewable raw materials rather then non-renewable materials is a global concern. In Horticulture for growing media 3.4 million m of peat is used in the Netherlands (Wever et al., 2002). The worldwide use is about 35 million m. As the society is becoming more aware of the environmental effects of peat excavation consumers in e.g. the United Kingdom and Switzerland are demanding peat free growing media (Holmes et al., 2000). For this purpose in the Netherlands a project is set up in which the possibilities of peat replacement are examined (Vegter, 2004). It appears there are good horticultural alternatives for peat but the major concerns are the availability of large quantities and economical aspects. Possible alternatives are organic fibers like grass and woodfiber, urea formaldehyde and some compost types (Wever et al., 2002). SMS (Spent Mushroom Substrate) could be an alternative as this waste is available for a low price in large quantities. In the Netherlands about 2.1 million m is available annually. Experiments have been performed to test the suitability of spent mushroom compost for horticultural purposes (Lemaire et al., 1985, Li et al., 1998, Chong and Rinker, 1994, Chong et al., 1994). The main conclusion of all this research is that the SMS can only be used in minor additions as the salt content is far too high. There are possibilities of lowering the salt content by lowering the salt content of the input materials. Mushrooms need fertilization but the input materials contain an overload of the minerals needed for growing. In this experiment compost recipes with less nutrients are made to see whether in this way the waste material SMS could be used for growing plants in horticulture. The aim of this study was to lower the salt content by changing the recipe of the mushroom compost. MATERIALS AND METHODS Standard SMS is a finished mixture of compost and casing soil. Compost is made of horse manure, straw, chicken manure and gypsum. After composting and inoculating with Agaricus bisporus, the material is covered with casing soil. Casing soil is a mixture of black peat and lime. Following to the mushroom growing the mixture of compost and casing soil is steamed and used as e.g. soil improver. In the experiment manure was replaced by: urea (3.8 kg/ton), ammonium nitrate (19.9 kg/ton)or ammonium sulphate (8.7 kg/ton). aiming at an N contents of 1.4 % (w/w, dry) well below a traditional value of 2.2 % N. Traditionally gypsum is applied at (25 kg/ton), now it was applied at 5 kg/ton or was replaced by lime (3 kg/ton). To compensate for the lower nitrogen the mushroom substrate was supplemented with a product based on soy beans (Miilichamp®6000). The recipes can be found in Table 1. From the different spent mushroom substrates the most promising were selected for the growing trial with kohlrabi. The selection was based on the yield and the expected salt content. The champost SMS materials D, F, H, J and N were tested without the casing soil. The casing soil from treatment o and p were tested separately. As reference materials also Peat, Composted straw, Woodfibre (2 brands), Composted greenwaste and a commercial champost (mixture champost with casing soil) were tested. The materials with a high salt (Table 2) content were thoroughly grinded by hand and washed (rinsed) with rain water at a ratio of a minimum of 3/1 (water volume/substrate volume). If the salt content was acceptable the materials were mixed with peat in the ratio 1:2 and 2:1. It was tried to fertilize to an optimum by linear programming for the major nutrients. Based on the nutrient content NH4H2PO4, Ca(NO3)2, KH2PO4, NH4NO3, K2S04,