Nutrients in the Compost Pile

Some types of leaves rot much faster on the forest floor than others.  Analyzing why this happens reveals a great deal about how to make compost piles decompose more effectively.

Leaves from leguminous (in the same botanical family as beans and peas) trees such as acacia, carob, and alder usually become humus within a year.  So do some others like ash, cherry, and elm.  More resistant types take two years; these include oak, birch, beech, and maple.  Poplar leaves, and pine, Douglas fir, and larch needles are very slow to decompose and may take three years or longer.  Some of these differences are due to variations in lignin content which is highly resistant to decomposition, but speed of decomposition is mainly influenced by the amount of protein and mineral nutrients contained in the leaf.

Plants are composed mainly of carbohydrates like cellulose, sugar, and lignin.  The element carbon is by far the greater part of carbohydrates [carbo(n)hydr(ogen)ates] by weight.  Plants can readily manufacture carbohydrates in large quantities because carbon and hydrogen are derived from air (C02) and water (H2O), both substances being available to plants in almost unlimited quantities.

Sugar, manufactured by photosynthesis, is the simplest and most vital carbohydrate.  Sugar is “burned” in all plant cells as the primary fuel powering all living activities.  Extra sugar can be more compactly stored after being converted into starches, which are long strings of sugar molecules linked together.  Plants often have starch-filled stems, roots, or tubers; they also make enzymes capable of quickly converting this starch back into sugar upon demand.  We homebrewers and bakers make practical use of a similar enzyme process to change starches stored in grains back to sugar that yeasts can change into alcohol.

C/N of Various Tree Leaves/Needles

False acacia 14:1 Fir 48:1

Black alder 15:1 Birch 50:1

Gray alder 19:1 Beech 51:1

Ash 21:1 Maple 52:1

Birds’s eye cherry 22:1 Red oak 53:1

Hornbeam 23:1 Poplar 63:1

Elm 28:1 Pine 66:1

Lime 37:1 Douglas fir 77:1

Oak 47:1 Larch 113:1

The protein content of tree leaves is very similar to their ratio of carbon (C) compared to nitrogen (N)

Sometimes plants store food in the form of oil, the most concentrated biological energy source.  Oil is also constructed from sugar and is usually found in seeds.  Plants also build structural materials like stem, cell walls, and other woody parts from sugars converted into cellulose, a substance similar to starch.  Very strong structures are constructed with lignins, a material like cellulose but much more durable.  Cellulose and lignins are permanent.  They cannot be converted back into sugar by plant enzymes.  Nor can most animals or bacteria digest them.