You may have heard the adage, “You can’t have too much compost.” But is it true?
A recurring theme we see in nature is balance. Too much or too little of anything typically causes problems.
The Perfect Soil
The ideal soil is made up of four major components:
- 45% minerals.
- 25% atmosphere (largely carbon dioxide).
- 25% moisture.
- 5% organic matter, such as compost.
Some fluctuation in the soil composition is perfectly normal, and most plants can handle these typical changes. Problems occur when the soil becomes seriously unbalanced, containing a significantly different proportion of these components.
What Happens If Your Soil Contains Too Much Compost?
Applying too much compost tips the balance of the soil too far in the organic matter direction.
Symptoms of excessive organic matter include:
- Poor seed germination.
- Low soil pH.
- Soil nutrient imbalances.
- Dramatic soil settling.
If your compost tends to contain a great deal of manure, you may start seeing symptoms of phosphorus toxicity in your plants. Likewise, other nutrient imbalances that can occur incidentally in homemade compost will lead to a variety of toxicity problems. Such an occurrence is rare because properly finished and cured compost tends to contain only small amounts of most nutrients. However, it can become an issue over time as those small amounts of nutrients accumulate in the soil.
Improperly cured compost poses an even greater threat. Uncured compost, particularly if it contains manure, may contain excess levels of nitrogen, which may burn plants.
Finally, if you spread too much compost on a lawn, you may inadvertently bury the plants and end up with mud.
How to Correct a Compost Excess
In the event that you happen to apply too much compost, there are steps that you can take to correct the situation.
If the problem is that you applied too thick of a layer of compost to a lawn and are concerned about smothering the grass, your best bet is to rake the excess off the lawn.
If you suspect a nutrient imbalance from long-term overuse of compost, you probably want to do an NPK test on your soil to gauge the severity of the problem. Many nutrient excesses can be corrected over time by watering generously and growing nutrient-hungry plants that can absorb the excess with impunity. Please see our Minerals in Plants guide for information on dealing with specific toxicity problems.
How Much Compost to Apply
The good news is that most backyard gardeners are not likely to make enough compost to create an excess. Problems are more likely to occur if you purchase compost from an outside source, which allows you to apply large quantities at one time.
The following are some rough guidelines for how much compost to apply per year:
- New beds. Up to two inches.
- Established beds. Up to one inch.
- Lawns. Up to one quarter of an inch.
In his Square Foot Gardening books, Mel Bartholomew advocated applying one trowelful of compost per square foot after harvest. This method is easy to remember and makes soil care simple.
Composting Quick Start
Everything you need to know to get started composting, including books, reader-favorite posts, and a step-by-step guide.