Know carbon to nitrogen ratio when making compost

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

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

When using compost as a substitute for fertilizer, it is important to know its carbon to nitrogen ratio, in other words, how much nitrogen 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.

Commercial composts aim for a carbon to nitrogen ratio close to 20:1 or 20 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 its 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 such as wood chips, sawdust and shredded newspaper (sometimes referred to as the “brown” component) increase the carbon to nitrogen ratio.

Additions of grass clippings, leaves, shrubs and vegetable scraps (referred to as the “green” component) lower 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, and 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.

Q: Can “hot” compost damage plants after it is applied?

A: The term “hot” is sometimes used to describe compost temperature and sometimes to describe compost very rich in fertilizer.

When applying compost, it should not be hot in temperature. Warm is OK but not hot. In winter, warm, moist compost appears to steam. This is not actually steam but water vapor condensing in the cold winter air. This steam gives the appearance that a compost is hot.

If the compost is truly hot in temperature, too hot to touch, it has not yet finished composting. Commercially, compost thermometers with long probes are used to measure compost temperature and manage it during the composting cycle.

When this temperature is reached, composts are aerated, which causes them to cool. After aeration, the compost temperature climbs again to 160 degrees F when it is turned again. This is repeated until the compost no longer produces heat. When the compost no longer becomes hot but is only warm, the compost process is finished.

The best compost should rest or cure for six months after its temperature has dropped to the air temperature. When this composting cycle is finished, the result is the gardeners’ Black Gold.

Compost is sometimes referred to as hot when its fertilizer content is high. The fertilizer content in compost is in the form of fertilizer salts. These fertilizer salts are good salts, unlike table salt.

But when fertilizer salts are too rich or concentrated in compost, the compost is considered hot, and they can damage plants if too much is applied. Hot composts because of high fertilizer content should not come in direct contact with plants but used in smaller amounts.

Q: My lemon is not a Meyer lemon, but the tree is starting to flower. I didn’t want to pick the lemons this early because they are still getting sweeter. Does the “remove lemons before the tree flowers” rule apply to this variety as well?

A: Yes, it does. It applies to all fruit trees. Try thinking like a lemon tree. The reason the tree produces fruit is to reproduce. When the fruit drops to the ground, the fruit rots and releases nutrients that feed developing seedlings.

The tree knows if there is fruit attached to its branches or not. It can’t see anything but there are other types of communication that trees have perfected. When the fruit has been picked, the tree knows the fruit is no longer there.

The reason for picking fruit before the tree begins flowering is to send signals back to the tree that it no longer has fruit attached to its branches. When fruit is missing, the reproductive or flowering cycle of the tree is encouraged.

The beginning of flowering is the beginning of the reproductive cycle. The normal flowering cycle of trees is at certain times of the year. If this time for the normal flowering cycle to begin has passed or is delayed, the tree might not flower at all or flower very lightly.

You are right. Most citrus is considered non-climacteric, or, in other words, the fruit doesn’t increase in sweetness after it is picked. It is best to wait when picking lemons to improve its sweetness, but you don’t want to leave it on the tree long enough to interfere with flowering.

If citrus is left too long on the tree, fruit quality is reduced because it becomes pithy; it starts drying out. Remove all fruit from trees before they begin their next flowering cycle.

Q: I’m composting in plastic trash cans with holes. It’s taking a very long time to make compost despite adding carbon to my grass clippings and kitchen waste. I water and turn it every few days. What am I doing wrong?

A: There could be several reasons why. Perhaps the volume of compost is too small. If the amount of compost is too small, it will never produce enough heat on its own. If the air temperature is cold and the compost pile small, the composting process will be very slow.

Maybe it’s too windy. Composting is difficult in areas where there is significant wind or too much sun. Compost develops better in shady areas of the yard without wind.

The compost ingredients might not be high enough in nitrogen. Compost ingredients are divided into two categories: “brown” ingredients which are loaded with carbon and “green” ingredients which are loaded with nitrogen. Compost should have significant amounts of “green” ingredients or use animal manure.

The compost might be too dry or too wet. Compost needs to be moist but not sopping wet. Compost is like a living organism; it needs air to “breathe” and moisture.

Microorganisms are needed in the mix. I reserve a small amount of compost from the previous mix to act as a “starter” for the new mix. Or I will put in a couple of pounds of garden soil just for the microorganisms.

Bob Morris is a horticulture expert and professor emeritus of the University of Nevada, Las Vegas. Visit his blog at Send questions to

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