How Do I Know When My Compost Is Done?

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For many gardeners, the best part of the composting process is seeing the results of your work and effort. Taking your shovel into your pile and revealing the dark, spongy, and earthy material that you’ve created. You’ve transformed primary materials into the nourishment for your soils and plant. 

Depending on your materials, climate, and methods you’ll experience different biological processes that can result in vastly different time-spans for the completion of your compost. Some composts can be completed in weeks but others take months.  So the question is, how do I know when my compost is done and ready for use?

Many gardeners will often use compost before it’s fully completed its decomposition process. I’ve been part of this camp many times (because I’m impatient). In most cases, this isn’t a problem because the larger bits just continue breaking down. In some cases, however, it could have detrimental impacts on the health of your soil and plants. In most cases using compost once it’s fully completed ensures that your soil will get maximum results. 

How to Tell Your Compost Is Done

Your Pile Has Cooled Down 

When your compost is highly active with bacteria and fungi it naturally begins producing heat. When the decomposition process is complete your compost pile should have heavily reduced activity and be at ambient temperature. To check the temperature of your pile dig into the center and compare it to the ambient temperature. Internal temperature can be vastly different than that of the outside.

Your Compost is Fine and Homogenous

The texture of compost is one of the chief ways to tell when your compost is finished. Long story short, it should look like good dirt. At the end of the cycle, compost is going to be a fine and homogeneous material. You want it to be broken down into small sediment like particles that are dark and spongy. If your primary materials are still large and recognizable then you know that your compost still needs time. 

If most of your materials are broken down into fine pieces but a few things have survived the decomposition process just separate these by hand. This is also known as the screening processes and can easily be done by running your compost through a once inch mesh. Things like avocado pits, corn cobs, or wood chips that take a while to decompose can be added to a more active pile

Compost Should Have a Sweet Earthy Smell

When compost is finished it should have a slight earthy smell but no foul odors. If you used manure any unpleasant odors should be completely gone. If your compost has a strong unfavorable smell something has gone wrong. A sour smell suggests that the compost is not done and should be left to age. Unpleasant odors could be caused by uncomposted materials or if your compost is too wet. Lack of oxygen caused by excess water will promote the growth of anaerobic bacteria that release unpleasant odors. These bacterias can be harmful to your plants and can remove nutrients from your compost! Remove any excess moisture sources and dry out your pile by flipping.

Aging Your Compost

Some composting processes involve aging your compost for long periods after the majority of decomposition has taken place. This is done by letting your compost sit for up to a year before using it. This process is important when commercializing compost or using compost for potting soil mix. Seedlings, young plants, and tender annuals will be most sensitive to unaged composts.

3 ways to test your compost to see if it’s done:

If you are uncertain if on how to tell if your compost is done, there are a few different tests you can perform that will give you a better idea:

Plastic Bag Test – By placing your compost in a closed plastic bag you can test to see the activity of certain microorganisms. An acidic smell and inflated bag after 2-3 days will suggest that there is still plenty of decomposition taking place!

Germination Test – Another test that can be easily conducted at home using radish seeds. By comparing germination rates with a verified commercial compost you can see if there is still any germination inhibiting compounds.

Lab Tested – Another option is getting your compost tested in a lab. This is particularly important if you are commercializing compost and want to ensure you’re providing the best product and reducing the risk of contaminating crops with harmful pathogens.

Five Issues with Using Unfinished Compost

Using compost that is still in the process of decomposition can cause some issues with the health of your soil and plants. For this reason, it’s recommended to always use a well-decomposed compost before applying them to your plants. This is especially true when making a potting mix or a seed starting mix. In some scenarios, unfinished compost can be used in your garden as a mulch.

1. Excess Carbon Can Remove Nutrients From Your Soil

If your compost has a lot of undecomposed carbon then it may rob nitrogen from the soil and result in reduced fertility. This happens when microorganisms use available nitrogen to break down the carbon.

2. Excess Nitrogen Can Burn Your Plants

Compost with undecomposed materials rich in nitrogen can catastrophically harm your plants. The excessive levels of nitrogen in the soil will disrupt the osmotic balance and reduce your plant’s ability to absorb water. This is commonly known as “burning” your plants. Highly active hot composts can cause this effect.

3.Unfinished Compost Can Inhibit Germination

Another side effect seen with unfinished composts is that seed germination can be inhibited. This is caused by certain phytotoxins found in certain plant materials. For this reason, using well-aged compost is best for making a soil mix for seed starting.

4.Unfinished Compost Can Contaminate The Product

One of the largest risks of using unfinished compost is the risk of contamination. While composting usually kills harmful pathogenic bacteria like E. coli, unfinished compost can still be teeming with these organisms. If you’re using it as a mulch for large perennial fruit trees then you won’t likely experience as much risk as if your growing leafy greens or fruiting annuals.

5.Unfinished Compost Can Contain Weeds, Pests, and Disease

A proper composting process ensures that any harmful organisms have been removed from the product. This includes pests or diseases that were living on the plant materials used for making compost as well as their seeds. 

When It’s Okay To Use Unfinished Compost

Once your compost has reached a sufficient level of decomposition it can be okay to use it in some contexts, particularly as a top-dressing or mulch. Ideally, you would know your materials are free of any pests or diseases. Excessive undecomposed nitrogen-rich materials, like manure, could still burn your plants. As long as the carbon-rich materials aren’t mixed into the soil they will just act as a protective mulch and won’t suck up nitrogen from your soil.

Speeding Up Decomposition When Your Compost Isn’t Done

If you’re still waiting for your compost to finish decomposing there are a few things you can do to speed up the process. The simplest is just flipping your pile routinely. This will give it more oxygen and stimulate the decomposition process significantly. Breaking up your materials into smaller pieces is also extremely helpful. If your pile never got hot then it may be lacking nitrogen-rich materials like manure or vegetable scraps. Alternatively, you can also feed your pile with 1-2 cups of molasses to stimulate the growth of microbes and the rate of decomposition. 

How to Store Your Finished Compost

Once your compost is complete you will want to make sure it’s properly stored. You will want to make sure it’s out of direct sunlight and not exposed to the rain. Ideally, keep it somewhere it can continue being exposed to oxygen and avoid putting them in closed containers like plastic bags. Sacks are fine as long as they are somewhat breathable. 

In the end…

At the end of the day, knowing whether or not your compost is finished is probably not a significant sticking point. Unless you’ve been sticking toxic plants or lots of weed seeds in your compost pile, if it looks done, it’s probably fine to use. So get spreading!

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