What’s the Big Deal About the C:N Ratio?

It’s important to first understand that soil microorganisms’ bodies have a C:N ratio near 8:1. Meaning that they must acquire eight parts of carbon for every one part of nitrogen from the environment to maintain this ratio. Bearing in mind that soil microorganisms burn carbon as a source of energy, not all of the carbon a soil microorganism consumes remains in its body — some is lost as carbon dioxide during respiration. Given the requirement for body maintenance and energy production, the optimal C:N ratio is near 24:1 or 16 parts of carbon for energy and eight parts of carbon for maintenance for every one part of nitrogen.

It’s essential to keep your soil covered at all times. Just like people, soil needs oxygen, food, water and shelter! A mix of cover crops can help to balance the C:N ratio. Low C:N ratio cover crops contain legumes (pea, lentil, cowpea, soybean, sunn hemp, or clovers) and/or brassicas (turnip, radish, canola, rape, or mustard). High C:N ratio cover crops include corn, rye, sorghum, sunflower, or millet.  For example, rye straw has a C:N ratio of 82:1 – this is why too much of the wrong organic residues can leave crops low on N. Hairy vetch, on the other hand has a lower C:N ratio of 11:1. It’s important to keep these options in mind, and executing a well thought out soil program will get you and keep you where you need to be.

Meanwhile, as plant materials or organic residues are added to the soil, bacteria and fungi grow rapidly on this food source, producing a large biomass of bacterial and fungal cells that contain much of the nitrogen originally in the residues. Until the microbial biomass begins to die off, the nitrogen is immobilized and not available to the plant. A healthy soil ecosystem is likely to contain certain nematodes, protozoa, and earthworms that feed on bacteria and fungi. As these animals feed, they respire most of the carbon in the microbial cells, using only a small fraction to grow on or produce eggs. Since the C:N ratio of these animals is not too different from that of their microbial food, and since most of the carbon is converted to CO2 by respiration, the animals begin to digest more nitrogen than they can use. They excrete the excess nitrogen into the soil as plant-available mineral nitrogen. The soil animals’ feeding frenzy increases the rate of nitrogen mineralization dramatically.

Understanding the carbon:nitrogen ratios of crop residues and other material applied to the soil is important to managing soil cover and crop nutrient cycling. Providing quality habitat for soil microorganisms should be the goal of producers interested in improving soil health. Soil is a biological system that functions only as well as the organisms that inhabit it. Andaman Ag offers a wide array of soil amendments like compost teas and oceanic fish hydrolysates that significantly contribute to the cycling of microbes in the soil – the key to maintaining and hastening the right C:N ratio, improving soil health and delivering healthier and less vulnerable crops.