You would rarely hear someone say that there can be too much of a good thing, especially with compost for plants. It is harder to step into a plant’s shoes and understand its feelings, and one can naively assume that there is no such thing as too much compost.
Can too much compost hurt plants? The answer is yes. Just because compost adds nutrients to the soil and has other benefits for plants too, does not mean it can be added excessively, or even that it is all a plant needs.
Before we jump into why too much compost is bad, you need to understand what exactly is the compost that you are adding to your garden, and also what it is not.
What is compost?
Compost is decomposed organic matter. A lot of our waste, including food scraps, yard waste (leaves, grass and tree clippings), and animal products, are organic and decomposable (approximately 30 percent of it). Throwing them away and then investing heavily in only commercial fertilizers for gardening or farming, is a waste of money and resources.
You can be recycling your organic waste and using it as nutrient-rich food for your plants, reducing the amount of fertilizer and other synthetic supplements you buy.
This is not to say that compost can replace fertilizer, or that it is fertilizer (a common misconception)!
Think of it this way: fertilizer is readily-available food for fast-growing plants, but compost is food for the soil. It improves your soil’s fertility by improving nutrient levels, the ability to absorb nutrients, improving soil structure, and adding beneficial microbes which improve soil health and disease resistance.
What is good compost?
Good composting requires a blend of green and brown organic waste and you must have enough to make a pile at least 2-3 feet deep. You also need to sprinkle water over it regularly so that it stays damp and keep turning the pile over to aerate it and distribute nutrients and microbes well.
The process requires a good number of days if you are doing it naturally in the open – about a couple of months or more. In controlled conditions, the stages of composting can be monitored and brought around faster.
A properly decomposed pile of compost should not have remaining scraps but should instead look like homogeneous rich soil in texture. It should not be too wet or smelly either – that indicates bad composting.
How much compost should be added to a plant bed?
It is a very bad idea to fill any part of your garden or plant bed completely with compost. In fact, it is recommended to keep the organic component of your soil at about 5% only! The remaining is loose soil.
While preparing a new bed, it is ideal to add 1 – 2 inches of compost per foot of loose soil. Later on, you can sprinkle very thin layers (about 1/4 to 1/3 of an inch only) every season or after every few months.
The proper application measure is 1 cubic yard of compost per 1000 sq feet of field area. And this translates to one fourth to one third of an inch in depth, as mentioned.
But remember, there is something called too much compost, which can inhibit plant growth rather than aiding it.
Is there such a thing as “too much” compost?
Yes, it certainly is possible to have “too much” of compost in your soil, but that is unlikely to happen. This is because it is rare that you will be producing so much compost that you will have more than needed.
Most farmers would be struggling to have enough for their farms, and the same is the case with home gardening. Usually, people would like to have enough organic food for their soil – having too much seems too good to be true!
And so, you will mostly ever have enough to “sprinkle” it around, especially if you have a bigger garden or field.
Compost piles take enough time to mature naturally which also ensures that there isn’t organic food going in faster than it can be healthily utilized by plants.
How does too much compost hurt plants?
You would be wondering how too much compost can hurt, when all it contains are beneficial nutrients and microbes.
The first reason is quite intuitively understandable. If you add a thick layer of compost to the soil, you can smother grass and small growth which is closer to the ground and prevent proper sunlight and air reaching them. Thus, adding it sparsely is better and does not snuff out small plants.
You should ensure that it is well raked and spread out, as even occasional clumps of mulch and compost can cause the grass underneath them to suffocate and die, leaving dead patches on your garden.
High phosphorus levels
Compost contains certain nutrients in higher quantity, which can damage plants in higher concentrations and prevent the absorption of other essential nutrients. Phosphorus levels are high in compost, especially if it is not fully cured, and can be poisonous to plants.
High phosphorus can make it harder for plants to absorb manganese and iron. These high levels are also toxic to mycorrhizal fungi, which are beneficial microbes that live in plant soil and aid in water and nutrient absorption.