Compost process require the emphasizes of nitrogen ratio, the C:N ratio and calcium. So what makes calcium to be important at composting process? One product of the breakdown of organic matter by bacteria is organic acids.
In a well aerated compost pile, these organic acids are themselves broken down in the later stages of the composting process, but if there is any shortage of oxygen, they can begin to accumulate, making the compost acidic. This is a problem because bacteria do not like acidic conditions at all.
The pH scale measures acidity or alkalinity, and goes from 1 (very acidic) to 14 (highly alkaline). Pure water has a pH of 7 and is described as neutral. Soil pH depends on geology and climate: acidic rocks, like granite or sandstone, and high rainfall result in acidic soil; calcium-rich limestone and low rainfall lead to alkaline soil. A soil pH of 6 to 7 is ideal for most plants, but there are some specialized species that grow best on acidic or alkaline soils. Few plants thrive on soils with a pH of less than 4 or more than 8.
A roughly neutral pH is perfect for composting bacteria, but they are happy in alkaline conditions, up to pH 8.5. Which is why materials containing calcium are great for the pile. Acidic soil can easily be corrected by adding garden lime (ground limestone), but there is no simple way of making an alkaline soil more acidic. If your compost pile is acidic because of poor aeration, it’s far better to treat the cause rather than attempt to treat the symptoms by adding lime. However, although most green waste contains plenty of calcium, some compost materials are naturally low in calcium.