Differences in Feedstock
While all animal manures are good sources of organic matter and nutrients, it’s impossible to make a precise analysis, because nutrient values vary greatly depending on the diet and age of the animals, and the nature and quantity of bedding in the mix. For example, manure with straw or sawdust will have a different nitrogen composition than pure manure. But it’s useful to know whether the manure you’re using is rich or poor in a particular nutrient such as nitrogen.
Also please don’t be misled by the N-P-K numbers that suggest manure is less powerful than chemical fertilizers. Composted manure is actually far better because it contains large amounts of organic matter, so it feeds and builds the soil while it nourishes the plants. Slow-release nutrition is best. This is one of the primary ways that organic fertilizers have an advantage over chemical ones.
The values of manure and organic fertilizers in general, are often based on the relative amount of nitrogen (N), phosphoric acid (P) and potash (K) they contain. A direct comparison of manure, composted manure, and chemical fertilizers based on these numbers is not valid, or at the least, is misleading. But, for the record, Dairy cow manure will have typical N-P-K values of 0.25 – 0.15 – 0.25, and horse manure will have typical N-P-K values of 0.70 – 0.30 – 0.60.
Commercial mushrooms grow in a specially formulated and processed compost made from wheat straw, hay, corn cobs, cotton seed hulls, gypsum and chicken manure. The 3 to 4 week long composting period is closely supervised and managed to assure that the composting temperatures exceed 160°F for a few days in addition to a steam pasteurization which occurs about one week before mushroom spawn is mixed with the compost. Finally, a layer of sphagnum peat moss mixed with ground limestone is top dressed onto the compost, and mushrooms grow on the peat. When the harvest if finished, farmers steam pasteurize everything in the growing room and dispose of the peat moss and compost that remain.
Research into using this material as a substitute for peat moss or other organic material in commercial nursery production systems have had to use a regular feeding system because the nutrient levels are too low to produce a crop. There is also a high salt level in most spent mushroom compost that has to be leached out before the crop is planted. General analysis of nutrient levels are negligible. You will not get a major nutrient benefit from this material. Regarding chemical residues, mushroom farmers have major problems with flies and fungus gnats in their growing facilities and spray regularly with such products as methoprene, cyromazine and diflubenzuron, Dimlin and Diazanon. There are also fungal infections that can wipe out a mushroom crop and require control by such chemicals as benomyl, thiabendazole and chlorothalonil. Naturally, if treated with any chemicals or having used any kind of artificial nutrient to create a composting action, mushroom compost will not qualify for use on certified organic farms.
One of the largest concerns with using yard waste as feedstock for composting is the high residuals of pesticides as well as other contaminants contained in these curbside piles of leaves, grass clippings, brush, and other garden residues collected by municipalities. At this time, little is known about the biological degradation and residual levels of pesticides this material may contain. A key factor in this degradation, however would be the composting method. For example, at best, municipalities will typically utilize a turned windrow method. At worst these piles are left as unturned piles for two years or more, and never achieve thermophilic temperatures which would break down some of these undesirable compounds.