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Abstract: All commercial mushroom growing depends on a composted material. Although the time and nature of the composting process has been modified, no one has demonstrated that it may be omitted from the preparation of a suitable medium for the mushroom bed on a practical scale (FLEGG, 1961; LAMBERT, 1941; LAMBERT and SAN ANTONIO, 1964; SINDEN and HÄUSER, 1950, 1953). Research has centered on the addition of nutrients to the compost piles at the beginning or during the composting to improve the nutrient value, and, hence to increase mushroom yields. Compost ingredients harbor a great number of microorganisms. With the addition of water to the dry ingredients and with the building of a pile, these microbes grow and reproduce (FERGUS, 1964; HAYES, 1969). Their growth requires: (a) suitable temperature, (b) adequate moisture, (c) sufficient oxygen, and (d) available food. The available food is supplied by the bulk compost ingredients and the compost supplements. The addition of nutrients at this time is directed therefore toward the feeding of a microbial population in the compost rather than direct nutrient addition for the mushrooms. In addition to carbohydrates, which are supplied primarily by the bulk compost ingredients, microorganisms also need nitrogen if they are to grow and reproduce (SCHISLER and SINDEN, 1962; SINDEN and HÄUSER, 1950; SINDEN and SCHISLER, 1962). The nitrogen content of the compost pile should be 1.5 to 1.7% before the composting process begins to assure that the microbes have a sufficient supply of nitrogen to efficiently complete an active fermentation. In horse manure compost, the straw contains both droppings and liquid waste from horses giving it an average nitrogen content of 1.2%. In the case of synthetic compost (SINDEN, 1946; YODER and SINDEN, 1953), the protein in the hay is the nitrogen source and although good quality hay may have a nitrogen content of 2%, when mixed with corn cobs the total nitrogen content of the bulk ingredients is just slightly greater than 1%. To raise the nitrogen content of the compost to the desired level (1.5 to 1.7%), supplements containing primarily organic nitrogen such as poultry manure, brewers grains, cottonseed meal, malt sprouts, sewage sludge, and dried blood are added (EDWARDS, 1961; FLEGG, 1961; SINDEN, 1946; YODER and SINDEN, 1953). Gypsum is added to both manure and synthetic composts to improve their physical characteristics. Fish solubles is a by-product of processing fish meal. It contains much of the soluble gelatinous, blood and interstitial protein of the fish, plus minerals, part of the oil, and some particulate matter that pass through the pressing screens (SCARES et al, 1970). The overall nitrogen content on a fresh weight basis (approx. 50% solids) is approximately 5% which compares favorably with the commonly used mushroom compost supplements. The cost of fish solubles fluctuates rather sharply depending on supply, however, the general market price prior to last year was $50/ton which also compares favorably with other supplements. The following experiments were designed to determine whether fish solubles could be substituted for one or both of the organic nitrogen supplements used in the standard compost formula (SCHISLER, 1967) at the Mushroom Research Center of The Pennsylvania State University.
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