When deciding what proportions of various materials to mix together in making
compost, the moisture of the resulting mixture is one of the critical factors
to consider. The following steps outline how to design your intital mix so that
it will have a suitable moisture level for optimal composting. In the
composting industry, the convention is to report moisture content on
a wet (or total weight) basis, as the formulas below indicate.
M_{n} = ((W_{w}W_{d})/W_{w})
x 100
in which:
M_{n} = moisture content (%) of material n
W_{W} = wet weight of the sample, and
W_{d} = weight of the sample after drying.
Suppose, for example, that you weigh 10 g of grass clippings (W_{w}) into a 4 g container and that after drying the container plus clippings weighs 6.3 g. Subtracting out the 4g. container weight leaves 2.3 g as the dry weight (W_{d}) of your sample. Percent moisture would be:
M_{n} = ((W_{w}W_{d})/W_{w})
x 100
= ((10  2.3) / 10) x 100
= 77% for the grass clippings
in which:
Q_{n} = mass of material n ("as is", or "wet
weight")
G = moisture goal (%)
M_{n} = moisture content (%) of material n
You can use this formula directly to calculate the moisture content of a mixture of materials, and try different combinations until you get results in a reasonable range. If you have a browser capable of handling Java script, you can try this formula out for up to 3 materials. (Calculate Percent Moisture of Compost)
Using trial and error to determine what proportions to use for a mixture will work, but there is a faster way. For two materials, the general equation can be simplified and solved for the mass of a second material (Q_{2}) required in order to balance a given mass of the first material (Q_{1}). Note that the moisture goal must be between the moisture contents of the two materials being mixed.
For example, suppose you wish to compost 10 kg grass clippings
(moisture content = 77%). In order to achieve your moisture goal
of 60% for the compost mix, you calculate the mass of leaves
needed (moisture content = 35%):
Q_{2}= ((10 kg)(60)  (10 kg)(77)) / (35  60)
= 6.8 kg leaves
Mixtures of 3 or more materials can also be solved in a similar way (although the algebra is more complicated), but for an exact solution the amounts of all but one material must be specified. To find the mass of the third material (Q_{3}) given the masses of the first two (Q_{1} and Q_{2}) plus all three moisture contents (M_{1}, M_{2}, and M_{3}) and a goal (G), solve:
With an internet browser that incorporates the JavaScript language, you can try calculating mixtures ratios based on moisture goals for up to three materials. (Calculate Required Weight of Third Ingredient for Ideal Moisture Content)
Composting 
Engineering 
in Schools 

Cornell Waste Management Institute ©1996
Cornell University
Ithaca, NY 148535601
6072551187
cwmi@cornell.edu