Compost Preparation/79
material depends on the proportions of urine and droppings present, the essential elements nitrogen, phosphorous and potassium being contained therein. The reason horse manure is favored for
making compost is the fact that fully 30-40% of the droppings are comprised of living microorganisms. These microorganisms accelerate the composting process, thereby giving horse manure a
decided advantage over other raw materials.
Horse manure used by commercial mushroom farms generally comes from race tracks. The
bedding straw is changed frequently, producing a material that is light in urine and droppings. On
the other hand, boarding stables change the bedding less, generating a heavier material. If sawdust
or shavings are used in place of straw for bedding, the material should be regarded as a supplement
and not as a basic starting ingredient.

When horse manure is used as the basic starting ingredient, the compost is considered a
"horse manure compost" whereas "synthetic compost" refers to a compost using no horse manure. Straw, sometimes mixed with hay, is the base ingredient in synthetic composts. Because straw
is low in potassium and phosphorus, these elements must be provided by supplementation and for
this reason chicken manure is the standard additive for synthetic composts. No composts are made
exclusively of hay because of its high cost and small fiber. In fact, mushroom growers have traditionally used waste products because they are both cheap and readily available.

By themselves horse manure or straw are insufficient for producing a nutritious compost. Nor
do they decompose rapidly. They must be fortified by specific materials called supplements. In
order to determine how much supplementation is necessary for a given amount of horse manure or
straw based synthetic, a special formula is used. This formula insures the correct proportion of initial
ingredients, which largely determines the course of the composting process. The formula is based
on the total nitrogen present in each ingredient as determined by the Kjeldhal method. By using this
formula and certain composting principles, the carbon:nitrogen ratio for optimum microbial decompositions is assured. In turn, maximum nutritional value will be achieved.

Supplements
Composting is a process of microbial decomposition. The microbes are already present in
large numbers in the compost ingredients and need only the addition of water to become active. To
stimulate microbial activity and enhance their growth, nutrient supplements are added to the bulk
starting materials. These supplements are designed to provide protein (nitrogen) and carbohydrates
to feed the ever increasing microbial populations. Microbes can use almost any nitrogen source as
long as sufficient carbohydrates are readily available to supply energy for the nitrogen utilization. Because of the tough nature of cellulose, the carbohydrates in straw are not initially usable and must
come from another source. A balanced supplement is therefore highly desirable. It should contain
not only nitrogen but also sufficient organic matter to supply these essential carbohydrates. For this
reason certain manures and animal feed meals are widely used for composting.
The following is a list of possible compost ingredients or supplements, grouped according to
nitrogen content. Their use by commercial growers is largely determined by availability and cost.
This list is not all inclusive and similar materials can be substituted. (See Appendix).

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