QUESTION: Can you fill a raised bed with just compost?
ANSWER: For the same reasons you wouldn’t grow vegetables in compost alone when they’re planted directly in the ground, you also shouldn’t fill a raised bed with just compost. The ideal percentage of organic material garden soil should contain is five percent, and your compost counts as organic material.
Using too much compost or attempting to grow plants in nothing but compost will cause problems for your plants when it comes to nutrition, soil stability, and the soil’s ability to retain water.
Just like human beings can overdose on the vitamins our bodies need to thrive, plants can also get too much of a good thing. Compost has high levels of nitrogen, which can be toxic to plants when the soil contains an excess amount of nitrogen.
Similar problems present themselves with micronutrients in the soil. For example, a small amount of boron will help plants grow healthy and strong, but too much boron will have negative effects for your plants. Compost also tends to contain much more salt than is healthy for plants, and growing plants in compost alone can cause them to be burned by the excess salts.
When it’s mixed into garden soil at the proper ratio, compost can work wonders for the soil’s ability to drain or retain water, whichever is needed. In heavy, clay soils, compost will loosen things up and aerate the soil to allow for better drainage. When compost is worked into sandy soil, it improves the soil’s water retention, keeping moisture available for plants. But on its own, compost is too loose to properly retain water, allowing it to drain right through and drying out much too quickly for growing healthy plants.
The looseness of compost can cause another problem if you attempt to grow plants in compost alone: issues with soil stability. It just doesn’t have the presence and solidity that’s needed in order for the roots of plants to find purchase and stay upright.
Compost alone will leave plants unsupported and prone to falling over. Compost used on its own will also settle dramatically, compacting more and more as time passes. For plants growing in containers, this can be especially problematic as it will seem like the soil in the container is disappearing as time progresses.
For all these reasons, it’s best to stick with mixing compost into the soil you already have. An easy rule of thumb to remember is to spread one to three inches of compost over the top of your garden soil before you plant. Then work the compost down into the top six to 12 inches of soil.