You know how much of an investment it is to build good soil. Amending native soil with compost and amendments can be expensive and time-consuming. If you put time and money into your soil last season, only to find that your soil reverted back to its original composition, you might be frustrated.
Unfortunately, it’s a fact of nature that compost (and other soil additives) disappear over time. Amending your garden soil is not a one-time task. Read on to learn exactly why compost disappears and how you can get the maximum life out of your compost pile.
Reasons compost disappears
If you’re noticing that the compost you put down on your garden beds disappears from season to season, that’s probably a sign that it’s being used for the purpose it was designed for. But there are a few specific reasons why your top-dressed compost might feel like it’s just vanishing into thin air.
The elements might be the number one reason you see compost disappear so quickly. Compost can be washed away by water, including rain and snow. Compost tends to be a little bit lighter than native soils like clay and sand, and when dry, can be blown away by the wind. If you’re worried about weather washing away your compost, don’t just top-dress your beds–use a broadfork to mix the compost thoroughly into the garden beds.
Insects and animals
If your compost seems to be vanishing, small organisms might be making a meal of it. This isn’t a bad thing–rich compost will draw worms, beetles, and other beneficial insects to your garden. Microorganisms consume the compost and excrete it, making humus–another organic material rich in nutrients and easily digestible by plants.
Your compost might be disappearing because plants are growing in it, which is the entire point! As plants drink water and nutrients, the structure of the compost will change and decompose. Plants’ roots might grow in and around the compost, further breaking it down.
Compost “cooks” or decomposes
If you’ve maintained your own compost pile before, you know that compost shrinks as it ages. This is part of the process of composting–bacteria and microorganisms eat the larger pieces of waste and digest it into something smaller and more easily digestible by plants.
Active compost is in the process of composting, while stable compost is what remains after the cessation of the composting process. Stable compost is essentially humus, an airy soil rich in organic matter. If you apply active compost to your beds, it will continue to break down, and likely more quickly than it was before.
Solutions to elevate shrinking raised beds
If your raised beds seem to be shrinking year after year, there are a few preventative measures you can take.
Add mulch and soil amendments
Before planting, do a soil test and determine if your native soil has any deficiencies. Add the needed soil amendments, along with compost, perlite, and mulch to improve the texture and increase drainage.
Don’t compact the soil
Take care when watering your raised beds to use a gentle setting on the watering wand. If you blast your plants with a jet stream for their morning drink, you’ll stress your plants and compact the soil.
If you can help it, don’t ever walk in your raised beds. Weed with a hand tool or with your hands, gently. The more pressure you put on the soil, the more compacted your beds will be – and both plants and microorganisms like loose, fluffy soil.
Mound extra soil
Plan for shrinking garden beds by adding more soil than you think you’ll need at the beginning of the season. You can usually add soil after planting as well, but some plants don’t like to be buried under too much soil. Understand your plants’ needs before you make any drastic changes.
Reasons the compost pile is shrinking
So far we’ve discussed why compost disappears from garden beds. But if your compost pile is shrinking, that’s perfectly normal! Compost will naturally reduce as it ages, but if your compost is disappearing more quickly than you anticipated, there might be other factors at work.
Pile size and shape
Smaller compost piles decompose more quickly than larger compost piles. Why? Because of the ratio of inside volume to outside surface area. Planet Natural Resource Center troubleshoots the issue of disappearing compost very well, and I will summarize their findings here.¹
Smaller particles (or piles) decompose more quickly than larger particles (or piles) because there is more surface area for reactions to take place. Think about it: the majority of a small compost pile is in its surface area, but big compost piles have much more volume inside the pile than outside of it. This is also true for wide piles versus tall piles.
Tall, big piles have so much more inside volume than outside surface area that microclimates form inside the pile. The inner environment is protected from the outer environment. Physics also explains why piles that decompose faster are also hotter – small particles break down into smaller particles, which break down into even smaller particles–creating a snowball effect.
Hot temperatures encourage faster decomposition, and colder temperatures slow the process down. Of course, there are other factors that affect the speed at which compost decomposes– like moisture and oxygen levels, as well as turning your compost more often – but of all these variables, temperature and volume affect compost decomposition the most.
How to store compost for maximum use
While compost is at its peak ripeness months after finishing, we often need to store compost for years. Compost doesn’t ever go bad, but there are ways to store compost to maximize its “shelf life.”
If you have a small amount of compost, it can easily be stored inside a bin. Keep the pile in a garage, basement, or outbuilding – somewhere not too hot nor too cold, as the temperature has a big impact on the decomposition process. If you notice that the compost is looking a little dry, add water.
The most common way to store compost is outside in a pile. Protect your compost pile from the weather by tarping it. For maximum longevity, try and shape your compost pile to be tall rather than short and wide.
Compost holds so many benefits for your garden, like incorporating organic matter back into worn-out dirt and improving the texture and drainage of native soils. Don’t be put off by the fact that compost disappears over time – it’s actually a good sign that your compost is being put to use! Use the above tips to prolong the life of your compost, and your garden will thank you with its beauty and productivity this season.
¹ “Physics of Composting,” Planet Natural Resource Center, https://www.planetnatural.com/composting-101/science/physics/