There is no shortage of gardening advice websites, videos, and books touting compost as the ultimate fix-all soil amendment. In fact, compost can improve or fix so many different garden issues that it’s earned itself the nickname black gold.
However, compost does have the ability to harm plants.
Healthy compost applied at the correct rate and at the correct time of year should never cause burning. If you notice leaves that are burnt after a compost application, it may be due to the following:
- Immature/hot compost
- High salt content
- High C:N ratio
- Chemical toxicities
- Sensitive plants
While it is possible for compost to cause problems, it is far more likely that compost will improve the health of your soil, especially if it is a high-quality product applied correctly. If in doubt, it is almost always better to use compost and see what happens.
(Source: Washington State University)
If you do notice problems after applying compost to your lawn, they are usually easy to fix. But first, you need to identify what went wrong.
Purchase bagged, finished compost online (link to Amazon).
Compost Burn Caused by Immature/Hot Compost
Bacteria give off heat as they feed on plant material, which is why compost piles heat up in the center. Once the bacteria have eaten through most of the ingredients in the center of the pile, it begins to cool.
This is the signal to turn the pile in order to maintain active decomposition.
After 5-9 turns, the heating process begins to taper off. Instead of reaching temperatures of 160o after each turn, it will slowly begin to drop until it is only 100 – 200 warmer than the outside temperature.
This is the stage where compost has matured enough to add to the soil.
If compost is added to the soil before it is fully matured, the compost will heat up as it decomposes in the topsoil, causing root and stem burn.
Whether you purchase compost or make your own, watch for signs of hot compost before you mix it into your soil:
- Hot center of the pile/bag. Compost will continue to decompose even after it is matured, so some warmth is normal, but if you notice a significant difference between the outside temperature and the center of the compost pile or bag, let it sit for a few weeks.
- Recognizable chunks of plant material. Mature compost may have a few small pieces of leaves or grass clippings, but hot compost will have recognizable pieces of leaves, twigs, and veggie scraps throughout the pile.
- Ammonia odor coming from the pile. An active compost pile may or may not smell like ammonia, depending on the amount of nitrogen, but mature compost should never smell like this. The odor is caused by large amounts of nitrogen being transformed into nitrates, which only happens in immature compost piles.
Manure-based composts are the most common cause of this type of compost burn.
Although manure is an excellent soil amendment, it’s not a popular ingredient in home compost piles, for obvious reasons. Most gardeners who use manure will purchase composted or aged manure so they don’t have to deal with collecting fresh cow patties.
Manure must be aged one year before it is applied to the soil. Manure that has been aged properly will not look like, smell like, or even closely resemble the original ingredients.
Compost Burn Caused by High Salt Content
Bacteria break down plant material into its most basic chemicals, and transforms them into a plant-available nutrient. These nutrients are all different types of dissolved salts.
Compost is a well-rounded soil amendment because it incorporates a lot of bulk into the soil along with nutrition. This helps dilute the salts in the soil so they don’t do any harm to the root zone.
However, compost that is extremely concentrated, or has been applied too thick, may be too rich in nutrients and can cause salt burn.
This is most common in composted manures; especially poultry manures and worm castings.
In general, if a compost doesn’t add bulk, you should use it sparingly.
Chicken manure and worm castings are very potent soil amendments. Always follow the recommended application rates and never apply fresh.
Compost Burn Caused by Carbon/Nitrogen (C:N) Imbalance
Compost is the result of a balanced mixture of brown and green ingredients that decomposes in a controlled manner. Ideally, compost should have a C:N ratio of 30:1.
When the C:N ratio is out of balance, it attracts different types of bacteria, which results in different types of decomposition. This can cause compost to have:
- High amounts of organic acids
- High or low pH
- Extremely low quantities of available nitrogen.
If your compost pile has too much carbon and not enough nitrogen, the nitrogen may become immobile. Bacteria use nitrogen as fuel to break down plant material. If a compost pile has substantially more carbon than nitrogen, the bacteria will use the nitrogen to fuel the decomposition of carbon.
This will result in very little available nitrogen in the finished product.
Low nitrogen will not result in nitrogen burn, but it may result in nitrogen deficiency, which can cause stunted plant growth.
If compost becomes anaerobic, it can begin to ferment, and in piles with a high C:N ratio, this can result in organic acids.
Organic acids are highly toxic to plants. If you use compost that has organic acids, it may result in immediate damage.
Fermentation is easy to spot. It is more common in bagged compost than compost piles. If you purchase a bag of compost that is pungent when you open it, and has dark, slimy pockets of plant material, empty it onto the ground and wait a week.
Spray the pile with water a few times to help leach out harmful chemicals, and wait for the smell to dissipate. Once the aerobic bacteria die off, it should be safe to add this compost into your soil.
Compost Burn Caused by Chemical Toxicities
A low C:N ratio can result in compost burn for two reasons:
- Ammonia toxicity
- Nutrient toxicity
When the C:N ratio is about 30:1, the compost will have a normal amount of ammonia and nutrients. Bacteria convert the nitrogen to ammonia, and then convert the ammonia to plant-soluble nitrates.
When the C:N ratio dips below 30:1, bacteria create an abundance of ammonia gas.
This gas produces a strong odor which makes the compost smell like a trash can. If compost is applied to the soil, the gas can burn the stems and lower leaves of plants.
If your compost smells like ammonia, add brown materials like shredded paper or cardboard and mix thoroughly. Once the compost has finished, spread it out and let the gasses dissipate before incorporating into your soil.
Low C:N can also cause nutrient toxicity.
Healthy compost piles should have a balanced ratio of brown to green ingredients. The carbon-rich brown ingredients provide bulk, while the nitrogen-rich green ingredients provide nutrition. A compost pile with too much nitrogen can build up a toxic amount of trace minerals.
Nutrient toxicities can result in malformed plants, discoloration, and chlorosis. It’s difficult to leach excess nutrients out of compost, so if you know your compost was made with primarily green ingredients, use it as a mulch around established plants to slow down nutrient absorption.
Compost Burn on Seedlings
Sometimes, compost burn has nothing to do with compost.
Compost will naturally have ammonia, nutrients, bacteria, salts, pathogens, and acids. Healthy compost will have a balance of the “good” stuff, and relatively small amounts of the “bad” stuff.
If you apply healthy compost to large, established plants or mix it into garden beds at the correct ratio (3”-6”), then the “bad” stuff will be diluted by the soil and it should not affect the plants.
However, seedlings are bound to get compost burn even if the compost is perfect.
Seedlings use the nutrition within the seed for the first few weeks of life. Seedling mixes usually have very little, if any, nutritional value. They are typically sterile to prevent rot and mildew, and they are light and fluffy to promote rapid root growth.
Compost is the opposite of a seed starting mix. Seedlings grown in compost will be flooded with nutrients and may suffer from severe toxicities. Compost is also full of bacteria; most are beneficial, but some cause mold and mildew.
Compost is too rich to be the sole ingredient in any growing media; especially a seedling mix. Use a soilless mixture or make your own with a slow-release fertilizer.
Compost is an amazing soil amendment. If compost is made correctly, it is safe to use on lawns and in gardens if you follow the recommended application rate.
If compost smells funny, feels slimy, or just seems off, let it sit for a few weeks and then spread it on more established plants or your lawn, rather than using it in the vegetable garden.
For more information on compost, read Thriving Yard’s complete guide to composting.