The basic story of compost emphasizes the carbon: nitrogen ratio, and rightly so—this has to be at least nearly right, or nothing will work very well. However, the C:N ratio is far from the whole story, and there is one element that has not been given quite the attention it deserves: calcium. Why might calcium be important in the compost pile? 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 ontaining 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.
For example, fall leaves of oak and hornbeam are relatively rich—for tree leaves—in nitrogen, yet do not make good compost, which makes sense when you discover that both are low in calcium. Woody waste such as hedge prunings, especially of conifers, is also short of calcium, so here there is a case for adding lime to help it break down.
The correlation isn’t perfect, but materials that are high in calcium also tend to be high in nitrogen, and no low-calcium materials contain more than moderate levels of nitrogen.
High in calcium Annual and perennial weeds; brassica leaves and stalks; kitchen vegetable waste; leaves of ash, cherry, elm, hawthorn, linden, maple, and rose; seaweed.
Low in calcium Bark, bracken, and ferns generally;
conifer needles; hay and straw; leaves of beech, birch, hornbeam, and oak; moss; wood.
