The Perfect Compost

Compost is plant and animal-based organic matter decayed organically by bio-degradation. The decaying process involves biological transformation, and also physical breaking up of matter into microscopic components. In other words, the compost is a bio-remediated (repaired and renourished biologically) organic matter derived by decaying organic waste. It contains constituent nutrients that have undergone transformation by removal of structural bonds and simplified into plant absorbable nutrients.

A perfect compost should have a C:N (Carbon:Nitrogen) ratio less than 15:1. It should also have small quantities of NPK – about 1.5% Nitrogen, 1% Phosphorus, 1.5% Potassium – and other macronutrients such as Sulphur, Calcium, Magnesium, and trace quantities of micronutrients such as Boron, Chlorine, Cobalt, Copper, Iron, Manganese, Molybdenum, Nickel, Silicon, Sodium and Zinc.

The Carbon to Nitrogen ratio (C:N) is an important parameter for compost, which can be thought of like the pH scale. A C:N ratio of 12:1 is “neutral”; this is the C:N ratio of soil organic matter (humus) and is relatively stable. Poultry manure has an average C:N ratio of 6:1, which means that it provides Nitrogen to the soil, and actually breaks down soil humus in the process. Most plant matter has a C:N ratio higher than 12:1, so composting micro-organisms take away Nitrogen from the soil, while they are breaking down organic matter.

Most dead plant and animal matter have Carbon and Nitrogen in different proportions. C:N ratios of some common organic materials is as follows:

6:1 – Poultry manure
12:1 – Cow dung, vegetable waste, garden weeds, garden soil
25:1 – Summer grass, fruit waste
50:1 – Dry leaves
100:1 – Tree bark
175:1 – Paper
500:1 – Sawdust

Compost is made by piling compostable organic matter and soil in multiple layers. The layers can be of any length and breadth, but the organic matter (greens and browns) and the soil should be spread alternatively in layers of few inches in height. Compost piles should contain a mix of Nitrogen-rich materials (greens) and Carbon-rich materials (browns). Greens include items such as vegetable waste, fruit scraps, eggshells, meat, dairy, weeds and grass clippings. Browns are items like dry leaves, straw, mulch, wood products, shredded paper, newspaper and sawdust. A healthy compost pile generally should have 50:50 greens to browns by volume. Soil is required as the base medium for the microbial activity. The soil contains active micro-organisms and helps trigger the composting process. Other ingredients required for composting are water and air.

Composting is most efficient when the net C:N ratio of the compost pile is about 25:1. Greens to browns ratio should be adjusted to average this ratio. Rapid composting happens when C:N is less than 30. Above 30, the compost pile gets Nitrogen starved. Below 15, Nitrogen can get lost as there is a likelihood to outgas a portion of Nitrogen as Ammonia.

The composting process is carried out by a diverse population of predominantly aerobic micro-organisms that decompose organic matter to produce and reproduce. Decomposer organisms break down the organic matter when provided with the right mixture of Nitrogen, Carbon, water and oxygen. The activity of these micro-organisms is encouraged through the management of C:N ratio, moisture content, oxygen supply, temperature and pH of the compost pile.

Properly managed composting process increases the rate of decomposition and generates sufficient heat to destroy weeds, pathogens and fly larvae. Compost piles usually get hot, anywhere from 110–180°F (43–82°C). Most of these organisms cannot survive above 160°F (71°C). Unpleasant odors get eliminated. Flies, a common problem around organic wastes, are much less of a problem around compost.

Decomposer organisms fall into two broad categories: (1) physical decomposers that process the waste into smaller pieces through methods such as grinding, tearing, chewing, and digesting, and (2) chemical decomposers that perform chemical processes on the organic waste. Physical decomposers include ants, earthworms, flies, millipedes, snails and slugs etc. Chemical decomposers are bacteria – the most abundant and important of all micro-organisms in the compost. Bacteria process organic matter and break them down into its constituent nutrients such as Nitrogen, Phosphorus and Potassium and other macro and micronutrients.

During composting, carbon dioxide and water are given out to atmosphere and volume of the piles reduce by 30 to 60%. It takes 6 weeks to 3 months for the piles to decompose into compost. The end product of the process is compost that is rich in beneficial micro-organisms.

Although compost is considered to be a fertilizer, it is not; it is actually a soil amendment. Compost does contain low levels of plant nutrients, however its primary role is soil conditioning as amendment, and not supply of nutrients. Soil amendment is any material added to the soil to improve its physical properties, such as structure, aeration, water infiltration, permeability, water retention and drainage. The soil amendment provides better environment for plant roots. Fertilizers add nutrients to soil; amendments improve the soil, so that plants do better absorption of the nutrients. Another example of soil amendment is lime, added to raise pH value of soils.