Why is the Carbon to Nitrogen Ratio Important in Compost?

When using or making compost the carbon to nitrogen ratio, oftentimes designated as C:N, is important when determining how much nitrogen should be applied when growing vegetables. The carbon to nitrogen ratio is a good indicator whether plants will benefit from an application of nitrogen or not.

Carbon to nitrogen ratios of 20:1 are frequently set as the “gold standard” for sustaining good growth and development. Carbon to nitrogen ratios lower than this (e.g. 15:1) may result in nitrogen losses to the open air.

Carbon to nitrogen ratios above 40:1 usually mean that nitrogen should be applied to sustain good plant growth. This sometimes accounts for the smell of ammonia coming from very rich compost. Other times it is lost to simply as nitrogen gas which is odorless.

A rich nitrogen compost can be a good thing. Nitrogen is contained in many fertilizers. Fertilizers cost money. A low carbon nitrogen ratio in a compost means you will not have to apply nitrogen for at least 6 to 8 weeks.

At Viragrow the carbon to nitrogen ratio of our compost varies from 20:1 to 15:1. When we make our soil mixes this compost adds plenty of nitrogen to the soil mix and it is quickly available to plants. There is no need to fertilize vegetables, annual flowers, herbs and lawns for eight weeks after their installation. For trees and shrubs, fruit trees and other perennials this amount of fertilizer is enough for one complete growing season.

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