Some Compost Can Be Used as a Fertilizer

Q. If I make my own compost, can I use it instead of commercial fertilizers for grass, plants, trees, shrubs?

Some composts, if they are rich enough in nutrients, can be used like a fertilizer.

A. Yes you can. But please be aware that homemade compost is not consistent in fertilizer content and quality. This is because of variability of different nutrients in ingredients used to make the compost. However, compost is universally good, whether it’s commercial or homemade, when added to soils as a soil amendment.

This is the fertilizer content of a compost supplied by a company I consult with. This particular compost contains a large amount of fertilizer for each cubic yard of compost. This compost would make an excellent fertilizer for plants. Other composts may not contain as much fertilizer as this one.

            When using compost as a substitute for fertilizer, it is important to know its carbon to nitrogen ratio, in other words how much nitrogen fertilizer it contains. The nitrogen content of a compost is critical. High nitrogen content (low carbon to nitrogen ratio) makes compost “hot” and less of it should be used. If compost has a high carbon to nitrogen ratio (low nitrogen content), then more of it should be applied when substituting it for fertilizer

When applying compost as a fertilizer for plants, it is important to keep the fertilizer away from the trunk of trees and the stems of soft, succulent plants. This type of application is okay for woody plants but vegetables and annual flowers should have the compost mixed with the soil before planting because of the high salt content from the fertilizer salts.

            Commercial composts aim for a carbon to nitrogen ratio close to 20:1 or twenty times more carbon than nitrogen. As this ratio increases to 40:1, the nitrogen fertilizer content decreases. At a ratio more than 40:1, the compost is still valuable but it’s value is greater as a soil amendment rather than fertilizer.

            The carbon to nitrogen ratio in homemade compost is managed through what is added to the compost mix before composting. “Woody” additions to compost like wood chips, sawdust and shredded newspaper (sometimes referred to as the “brown” component) increase the carbon to nitrogen ratio.

            Additions of grass clippings, leaves of trees and shrubs, and vegetable scraps (referred to as the “green” component) lowers the carbon to nitrogen ratio and make it more valuable as a fertilizer.

            Animal manure (think of it as a concentrated “green” component) is high in nitrogen and added to get the carbon to nitrogen ratio low and improve fertilizer content. If lots of different components are mixed together in the right proportions, green components are balanced with brown components, homemade compost has all the nutrients needed by plants.

            The short answer is “yes”. But substituting a homemade compost for a fertilizer application varies from batch to batch depending on what was used to make the compost.