You just bought some compost, and you are now wondering if this can burn your plants as you might have heard around. Your suspect is well-grounded as compost can actually “burn” your plants in three specific cases.
Can compost burn plants? Despite its beneficial effects on both potted and outdoor plants, compost can negatively affect plant growth, triggering burn-like symptoms that underline nutrient unbalance issues. These cases are:
- Using too much compost in the growing medium
- Using compost that has not finalised its decomposition
- Using manure and plant-based in the same way
Hence, what is the problem of having too much compost? How ends up burning your plants? What are the most “dangerous” types of compost with which you have to be extra careful?
Compost is a great growing medium with indisputable qualities. You must use it to have the best from your potted (and not) herbs and plants. However, there is a limit.
Indeed, plants have specific requirements in terms of the nutrients they require. This is defined through the famous N-P-K ration. As discussed in the article below, this ratio indicates how much nitrogen (N), phosphorus (P), and potassium (K) your plants need.
Here where the problem with compost starts
Plants have a considerable variability on what they require, however, the majority of them have a preference for a slightly higher concentration of nitrogen compared to phosphorus.
However, compost does not provide more nitrogen than the phosporus. It does not much perfectly what plants want.
For instance, this compost on Amazon, quite known for its good quality among gardeners (both outdoor and indoors), has an N-P-K ratio of 0.5-0.4-0.5. This means that it provides an almost equal amount of nitrogen, phosphorus, and potassium. This also applies to all other brands, as also discussed by iBiblio. But remember, plants want just a little bit of phosporus and way more nitrogen!
Indeed, in the long term, your plant will exhaust the nitrogen leaving in the soil (a lot of) phosphorus. This is because phosphorus, different from nitrogen, is not easily washed away. It “sticks” into the ground for a long time and is not soluble. Hence, if the plant does not absorb it (because it does not need it), it will slowly build up.
Here when the problem arises.
Indeed, a high level of phosphorus affects the ability of your potted herbs and plants to absorbs other minerals (also called micronutrients) fundamental for their healthy growth. The micronutrients that will become “locked into the soil” and so unavailable to your plant are mainly iron and zinc.
How do you notice if the excess compost is causing a problem? This excess of phosphorus will produce, among other symptoms, yellowing between leaf veins.
Here a tip: the most recent leaves (at the top of your plant) are those that will show this symptom first! Why? Zinc and iron are not mobile micronutrients. This means that the old leaves (that grow a while ago when your oil was ok) will remain green (as these nutrients are in their tissue), while new growth (in soil without zinc and iron) will suffer from it.
How much compost should you use? When creating my potting mix, I do usually use no more than 30% compost. This is a number that is also confirmed by other colleagues on the field. You can have the advantage of the compost without the hassle of burning your plants.
As discussed by a few gardeners, compost that is not totally broken-down is a problem. Indeed, the decomposition process (yes, compost is organic decomposed material) requires nitrogen.
Then, if you place a not totally decomposed compost in your potting mix, this will absorb nitrogen (to continue the decomposition) from the rest of the growing medium, leaving your plant without it. This will trigger a nitrogen deficiency symptoms such as yellowing and browning of the leaves (dead of it).
Tip: in this case, the older leaves will be affected, as the nitrogen is mobile. It starts from the tips of the leaves getting yellow and brown/dry (dead)
Unfinished produces heat that can burn plants.
Indeed, unfinished compost, if used in the garden, will keep decomposing and producing heat. Thi higher temperature might burn the plant that is not suitable for it. Tips: this problem can be mitigated by digging the compost a few inches below the plant roots)
Is Your Compost Ready?
If the compost has been bought at the shop or (even better) from a nursery, chances are that the compost is ok, totally decomposed, and will not compete for nitrogen with your plant.
The real problem is if you prepare the compost by yourself. There is not a defined amount of time after which it is ready. The only way, as suggested by the University of Florida, is to check if:
- The pile has reduced from a third to a half of its original volume;
- The material you used is no longer distinguishable. No large chunks, just a soil looking pile;
- The compost pile should be cold.
There are quite a few types of compost out there. However, the most common are those produced starting from animal feces (manure) and those obtained from decomposition of vegetable material only.
For instance, this from Amazon is a good quality manure-based compost. Animal manure gives to the compost a higher concentration of phosphorus. As detailed by the manufacturer, this has more phosphorus (6%) than nitrogen (4%). This is not bad! The compost is quite good, actually. But you have to remember that you cannot use only compost as there is more phosphorus (0.8) than nitrogen (1.1). Indeed, as mentioned before, the plant needs, in general, more nitrogen than phosphorus. If this is unused, it will end up in the soil.
The compost you prepare at home, where very likely you use a brown and green vegetable material, has a way lower N-P-K and, more importantly, more nitrogen than phosphorus. As detailed in this article, this type of compost has N=0.5% and P=0.27%. This means that:
- The compost has a lower concentration of nutrients compared to a manure-based. Around 3 to 4 times less
- Vegetable-based compost is more close to your plant requirement is
Hence, adding 30% of the volume of compost in your potting medium is a good idea for plant-based compost, although for manure-based, I do recommend 10-15% of the total volume. This is in general enough to limit the amount of phosphorus.
Indeed, during the lifetime of your plant, very likely, you will add fertilizer. This, very often (if multipurpose), contains also phosphorus increasing the overall amount on your soil.
Mushroom compost can burn your plant. This type of compost, as discussed by Oregon State University, is the “leftover” of a medium that contains a large variety of material (such as peat moss, wheat, horse bedding straw, manure, etc…) in which, in specific temperature and humidity conditions mushrooms are grown.
The medium that remains (mushroom compost) has a higher level of salt that can easily kill seedling or burn your plants (dehydration) if you use it high quantity (above the commonly suggested 30%).
“The soluble salts in undiluted mushroom compost are too concentrated for germinating seeds, young plants, and other salt-sensitive plants –Oregon State University
Can Compost Burn Roots? The use of unfinished compost can burn the roots of your plant due to the heat released.
Can compost burn your lawn? Yes, compost, if applied in a higher percentage (over 30%), can also burn your lawn. This is because of the reason already discussed in this article.
Compost is an important component of any great potting soil. However, how to create one and which material to use is extremely important. Indeed, at the end of the day, compost is only a third of the total. What is the remaining 66%? Curios? Check the article below with my best recipe for a cheap and effective potting mix.
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