Mulching with materials to achieve the correct carbon nitrogen ratio is important. Photo: 123rf.com
Composting is the process by which organic materials, such as food scraps, garden waste, and residues, are decomposed by microorganisms into simpler substances that contain nutrients that are more easily absorbed by plants.
This transformation is similar to cooking stew made from various raw ingredients, such as vegetables, meat, and spices.
While raw ingredients are difficult to consume and digest in their initial form, they become nourishing and easily digestible when combined and cooked over time.
Analogously, composting transforms various organic materials into nutrient-rich substances that are more easily utilised by plants.
There are several key steps involved in composting. The first step is to select the type of material to be added to the compost heap. Unfortunately, gardeners often err in this first step.
One important metric is the carbon-to-nitrogen ratio, which is often represented as C:N (see the list below). This ratio indicates the proportion of carbon to nitrogen in the organic substances. For instance, C:N ratios of 10:1 and 20:1 indicate that there is 10 and 20 times more carbon than nitrogen, respectively.
The C:N ratio is crucial for assessing the quality of organic matter because it indicates the amount of available carbon and nitrogen for microbes to consume. Soil microbes are responsible for decomposing organic matter using carbon as their energy source and nitrogen as their food.
The ideal C:N ratio for these microbes is 8:1. To survive, microbes must acquire sufficient carbon and nitrogen to maintain this ratio.
However, some carbon is lost through respiration, indicating that microbes require an environment with a higher than 8:1 C:N ratio to thrive. A C:N ratio of 24:1 is required for microbes to effectively break down organic matter.
This means that for every part of the nitrogen used by the microbes, 16 parts of the carbon are utilised for their maintenance and energy, while eight parts are stored in the microbes.
Therefore, when applying organic matter to gardens, it is important to use materials with a C:N ratio of 24:1 to support the microbes and promote effective decomposition.
The use of an organic material with a high C:N ratio indicates an excess of carbon compared to nitrogen. This makes it difficult for the material to decompose in soil.
Although there is an abundance of fuel from carbon, microbes are limited by insufficient food from nitrogen.
Therefore, microbes consume nitrogen from the soil rather than from organic material. This can result in nitrogen shortfall for plants, as the soil nitrogen is depleted.
In competition for scarce resources such as nutrients, microbes usually win because they are found throughout the soil, whereas plant roots only occupy a small percentage of the soil volume.
Therefore, it is advisable to avoid applying organic materials with exceedingly high C:N ratios, such as those over 80:1.
Nitrogen depletion occurs only in the immediate area around the boundary between organic material and soil. Therefore, apply organic materials with a high C:N ratio on the soil surface; do not mix them with the soil.
Also avoid using these materials as surface mulch for young plant seedlings, as their roots are shallow and near the surface, and depletion of soil nitrogen could negatively impact seedling growth.
In contrast, organic materials with low C:N ratios contain relatively less carbon than nitrogen. Therefore, it is more rapidly decomposed by microbes because of the abundance of nitrogen (microbial food) and provides a quick release of nutrients. However, it has a short lifespan in soil and must be frequently reapplied.
The ideal C:N ratio for organic material applied to soil is 24:1, but ratios between 20:1 and 30:1 remain acceptable. Any organic material with a C:N ratio less than 30:1 is considered “green”, whereas those greater than 30:1 are considered “brown”.
Please note that organic materials are categorised as either green or brown based solely on their C:N ratio and not their colour.
In conclusion, when composting, a ratio of three green parts to one brown part is recommended. Thus, the final C:N ratio of the compost would lie between 20 and 30:1.
Here is a list of various organic materials and their approximate average C:N ratios.
Greens (C:N less than 30:1):Poultry manure (7:1), Cattle manure (15:1), Sheep manure (15:1), Food scraps (17:1), Fresh grass clippings (17:1), Seaweed (19:1), Grass clippings (20:1), Legume residues (20 to 30:1), Coffee grounds (25:1), Potato peels (25:1), Vegetable scraps (25:1), Fruit wastes (25 to 40:1), Garden wastes (30:1).
Browns (C:N greater than 30:1):Nutshells (35:1), Oil palm fresh fronds (40:1), Sugarcane residues (50:1), Corn stalks (60:1), Peat (Sphagnum) moss (60:1), Rice straw (60:1), Leaf litter (60 to 80:1), Pine needles (80:1), Coconut coir (90 to 110:1), Shredded newspaper (175:1), Oil palm trunk (180:1), Sawdust (500:1), Wood chips and twigs (700:1).
Assoc Prof Dr Christopher Teh heads the Department of Land Management, Faculty of Agriculture, Universiti Putra Malaysia. His field of specialty is in soil and water conservation.
