Most home compost makers, or experienced compost operators, will judge by appearance what composition of the material will provide good compost. An experienced operator can generally do proportioning, when desirable, from visual estimates of the quantity and character of the materials. In large-scale municipal composting operations, however, there may be times when operators rely on laboratory analyses to determine how the various materials should be blended or proportioned for composting.

The C:N ratio and moisture content are the two factors to be considered in blending. There is no need for blending when the C:N ratio is between 25 and 50, although 30 to 40 is a better range. If materials containing much paper, straw, sawdust, or other substances rich in carbon are to be composted, the C:N ratio materials should be proportioned to provide a near optimum C:N ratio. Similarly, materials too dry for good composting and materials too wet to compost without odors should be blended in proper proportions. Where initial shredding is practiced, proportioning can usually be done at the shredder; otherwise, the materials are mixed and placed in stacks or pits together.

Some compost operators add soil to the organic materials with the idea of increasing the number of microorganisms and thus expediting composting. But organisms necessary for decomposition are indigenous to the organic materials, and those added in the soil will have no significant effect. Dry soil is also sometimes added to reduce the moisture content and to absorb ammonia in low C:N ratio materials. This procedure is fine if sufficient dry organic materials are not available, but the efficiency of nitrogen reclamation by the addition of soil is not great. The addition of cellulose organic matter to provide a C:N ratio above 30 is much more efficient. Soil may be added to compost if the materials have a high acidity content, to neutralize acid conditions. It may be added to improve the appearance of the finished compost, to give it a more granular texture, and to increase the ease of handling by giving the compost more body.

Adding soil to the compost pile might make the mass heavier to work with. Also there is a danger of adding more weight to the materials, and the weight could add to less air penetration.