Compost can burn or damage plants if it contains manure or vegetable matter that has not completely broken down. Fresh manure can burn the roots of plants because of its high ammonia content. If compost is added to soil before it is fully broken down then it can reduce the nitrogen available to plants and stunt their growth.
It is important to add compost in the right amount and in the right break down stage to support plant growth. Compost is considered ready to use when all of the ingredients have been broken down to a point that they cannot be identifiable individually.
This article will explore when compost can burn plants and what you can do to avoid this happening.
When compost will burn and damage plants
Here are the top ways that compost can burn or damage your plants and soil. Check out each one and make sure that you are not doing this to your garden at home.
1. Adding compost that contains plant material that is not properly broken down
Compost that contains plant material including food scraps, hay, grass clippings or weeds that are not fully broken down can cause problems in your soil.
When this partly broken down compost is mixed into your garden soil the bacteria and worms will go to work to continue to break it down. The bacteria will draw nitrogen from the soil around them.
This temporarily reduces the nitrogen available to the surrounding plants. The nitrogen will eventually be returned to the soil but by this time your plants can suffer from slow growth or even plant death.
It is important to only add compost to your soil that is finished. That means that it looks like light crumbly soil, it will not have large pieces of organic matter still visible, and all of the ingredients will be thoroughly mixed together. Remember if you can still see food scraps it needs longer to break down.
2. Adding compost that contains fresh animal manure
Fresh animal manure can burn plant roots if it is added with your compost into the soil. Animal manures such as horse, cow, sheep or goat that are fresh will need to break down in your compost for around 6 months to break down the nitrogen in the form of ammonia.
Soil bacteria and worms will help to break down this substance so that the finished compost will not burn or damage your plants. This will also help to break down any harmful bacteria that might be present in the manure. After 6 months in your compost it will be ready to add to your vegetable garden or flower beds to improve the soil.
Do not use compost in your vegetable garden that contains dog or cat poo. For home compost it is best to keep dog and cat poo out of your compost and use a specialized system in another area of your yard. This compost will naturally work its way into the soil or can be used for top dressing for non-edible trees.
4. If too much compost is added to the soil
Too much compost can cause problems in your soil. It can cause water to drain out of the soil quickly and it can cause the soil to lack the structure that it needs to support healthy root growth for perennial plants.
Compost will generally only contain organic material and will not have the sand, clay or silt component of soil. This is important to provide valuable minerals to the plant and help it to grow strong and stable in the soil.
Growing in pure compost can work for seasonal vegetables including pumpkins which can naturally spring up in your compost pile.
Perennial plants or plants that grow longer than a season can grow short term in compost but over time they will need more. Make a no-dig garden with compost and aged manure and allow the plants to naturally reach down into the ground soil over 3-6 months.
When adding compost to the soil it is important to add only up to 1/3 to your soil mix as a maximum. If you are adding this much to your soil, allow it to break down for 2 weeks before planting if you can. When preparing vegetable beds or new flower beds, mix through a shovel full for every square foot and this will be plenty if you want to plant straight away.
5. If compost it is too acidic or too alkaline
Compost that is not balanced or is too acidic or alkaline for the plant that you are growing can cause problems. Some compost mixes will be slightly acidic depending on what you add to your mix. Coffee grounds for example can have mixed effects on compost depending on where the coffee beans come from and how they are processed.
The only way to truly know if your compost is acidic or alkaline is to test it with a pH kit. Another great way is to add a huge variety of ingredients so that they balance each other out. I like to add food scraps in small amounts, add citrus fruit in small amounts and add a handful of garden lime to increase the pH and make it less acidic.
How to add compost without burning plants – Top Tips
Here are my top tips for adding compost to your garden without burning plants
- Make sure the compost has completely broken down. For most mixes this will take 6-9 months and the finished compost should look like soft, crumbly soil. It will be dark in color and there will be no large pieces left.
- Do not add dog or cat poo to your compost. This can cause plants to burn and introduce nasty pathogens to your soil.
- Wait for fresh manure to break down for at least 6 months in your compost before adding to your soil.
- Mix the compost thoroughly to make sure that everything is incorporated evenly.
- Balance the ingredients and add a hand full of garden lime if you add lots of acidic fruit.
Will Compost Burn Plants? | Summary
Compost can burn plants if it contains material that is not fully broken down including fresh manures, too many acidic food scraps and plant material. Allow all of the ingredients to break down fully for at least 6 months finding a good balance of green and brown material. The ratio of ¼ green material to ¾ brown material works best for most compost and add a handful of garden lime to sweeten the mix.