After using fertilizer and compost in it’s many forms over the years, a thought occurred to me. Can I mix fertilizer in my compost pile, and would it be beneficial in any way?
Fertilizers mixed into compost during the composting process can help balance the carbon to nitrogen ratio. This can be especially helpful if the moisture content and aeration of a compost pile is adequate but nitrogen needs to be increased without adding more moisture, or impeding aeration, which can occur from a nitrogen source such as wet manure.
I think to get a better understanding of mixing compost and fertilizer, we should first define what they are so that we can hopefully clear up any confusion.
Is compost a fertilizer?
I know many of us probably have opinions on whether compost is a fertilizer or not, I know I did. I wasn’t satisfied with my own opinion though, so I went looking for and answer and found some interesting information.
Compost is a slow release organic fertilizer made from decomposed organic matter such as manure, plant debris, or animal remnants. Compost not only adds fertility to the soil it is added to, but it also improves the structure, aeration, and water holding capacity of the soil.
This is not just my opinion though, all we have to do is look at the definition of fertilizer itself. According to Merriam-Webster fertilizer is “a substance (such as manure or a chemical mixture) used to make soil more fertile”. It’s the same from Brittanica as well “Fertilizer, natural or artificial substance containing the chemical elements that improve growth and productiveness of plants. Fertilizers enhance the natural fertility of the soil or replace the chemical elements taken from the soil by previous crops.”
We can see that compost does fit inside those definitions so we could call it good, but lets keep going. So what about compost then, does its definition classify it as a fertilizer? Well, it actually does, and you can take a look at Merriam-Webster again and see that it defines it as a fertilizer and a soil conditioner. Even the Food and Agricultural Organization of the United Nations considers compost a fertilizer. That is also a really long name, so they must know what they are talking about.
With all that being said, most of us, when talking about fertilizer will be talking about highly concentrated dry or liquid fertilizers like blood meal, fish emulsion, or synthetic fertilizers. So when I am talking about fertilizer from now on, I will be using it in the more common sense of the word, and will not be referring to compost as “fertilizer”.
It should also be kept in mind that according the the University of Massachusetts it is estimated, roughly estimated, that only 10%-30% of the available nitrogen in finished compost will be available in the first growing Season. The rest of it will be available in the following seasons. So this is another reason why it probably isn’t generally thought of as a fertilizer, because it is so slow to release the nutrients, but it is very effective.
Mixing fertilizer into compost pile, why, and when?
If compost is a fertilizer then why would anyone add fertilizer to it?
During the composting process the materials being composted are broken down by microorganisms in the soil and/or composting material. For a proper hot compost, a fast composting process, a certain ratio of carbon and nitrogen is needed for the microorganisms to thrive and to achieve a finished compost in the shortest amount of time. That carbon nitrogen ratio, according to NRCS is between 20:1 and 40:1. So 20 – 40 parts carbon to 1 part nitrogen. You can read a little about it here if you would like. At the same time adequate aeration and a proper moisture level needs to be maintained.
Carbon sources are things which, are, well, rich in carbon. Examples can be wood chips, sawdust, paper, cardboard, straw, etc. Nitrogen sources can be found in things that are green and some things that are not green. Some people just say “green” in one way are another when referring to the nitrogen source. These sources can be fresh grass clippings, green leaves, kitchen scraps, coffee grounds, urine, manure, and unfortunately dead animals.
So what if you have a compost pile where the moisture level is just right, and the aeration is adequate, but the ratio of carbon to nitrogen is too high. For instance what if you have a ratio of 50:1 in your compost pile. It may be sluggish or fail to compost properly.
If you can add some form of nitrogen rich fertilizer, organic or not, and double the nitrogen in the compost, it could be brought up to a ratio of 50:2, which is actually 25:1. This could be done very easily and without altering the aeration, texture, moisture or volume of the compost in any significant way.
That 25:1 C:N (Carbon:Nitrogen) ratio would also bring the compost in the range specified by the NRCS. This would help the compost pick up some speed in the composting process and there may be no need to alter anything else. If someone was to add enough fresh grass clippings to bring the C:N ratio into the same range it would be adding moisture as well and may also bring down the aeration and hinder the composting process. It would then need something else to help bring the aeration back up.
So you can see that adding just a little bit of fertilizer to the composting process can make things much easier if no other factors such as moisture or aeration need to be changed.
Applying compost and fertilizer to the soil, supercharge your soil.
If compost is a fertilizer and fertilizer is also a fertilizer, why would you add or mix both in the soil or planting mix at the same time? The nutrients in compost are slowly released and can actually take quite a long time to fully release the nutrients that plants can use. Fertilizer, whether it is synthetic, natural, or organic will be available quicker than what is in the compost.
Applications of compost and fertilizer at the same time can provide needed nutrients to plants quickly from synthetic or natural fertilizer, and a slow and sustained release from the compost. This can provide needed nutrients that plants may need just after transplant, or during a heavy nutrient stage of growth, and then provide sustained nutrition over a long time without a need for constant feeding.
Compost will have nutrients in it that will be available quickly, I am not saying that it won’t have any nutrients released quickly. For all the nutrients in the compost to be freed up and used by the plants will take a longer period of time. The length of time will depend on many factors such as moisture in the soil and the compost that was made.
If you would like to read more about the nutrient composition in compost, its availability to plants, and the time frame that these nutrients are available please read the article on that topic here.