Something that can be agreed upon by most gardeners is that compost is generally a good thing. It’s a good source of nutrients, it builds soil structure, and it’s an eco-friendly alternative to recycling organic waste that would otherwise end up in landfills.
However, it’s also true that too much compost, especially the one produced from animal manure, can be harmful to plants.
Plants require a certain amount of nutrients to thrive and providing them with more or less than what they need might create an imbalance that may lead to the appearance of more pests and diseases.
In this article, we will examine the reason why too much compost may be harmful to plants, and we’ll also look at what you can do to prevent that from happening.
What is compost?
Compost is an umbrella term used to describe recycled organic waste or material.
It’s what you get when organic waste is decomposed and turns into a nutrient-rich matter that can be used to enrich the soil and nourish the growth of plants. It’s important to note, however, that compost is not the same thing as mulch.
The composting process can be defined by how microorganisms living in the soil break down organic waste through aerobic biodegradation.
Gardeners spread compost over a garden bed to improve the soil’s structure and provide plants with all the necessary nutrients they need to effectively grow.
Essential nutrients in compost
The nutrients present in compost vary because it depends on the specific ingredients used to create the compost.
Compost made at home is mostly made from plant materials and will have a specific NPK ratio, which is essentially the macro-nutrient concentration present in the organic waste.
For example, compost made from plants typically has an NPK number of 3-0.5-1.5, while composed produced from animal manure has an NPK number of 1-1-1.
In other words, plant-based compost has a relatively higher concentration of nitrogen (N) and potassium (K), while animal-based manure has a relatively higher concentration of phosphorus (P).
While these are the major nutrients plants need to grow, there are also other important nutrients like calcium, magnesium, and sulfur, as well as trace minerals such as iron, manganese, zinc, copper, boron and molybdenum.
The role these nutrients play in plant growth is rather complex, but it’s important to minimally understand how they work to at least understand what type of compost you should be using to grow your plants.
Plant nutrition: What is it?
Nutrition is a process by which every living organism, whether plant or animal, ingests and assimilates its food. From a plant perspective, nutrients (which involve macro and micronutrients) are essential elements for plants to grow and thrive.
The following macronutrients play a central role in plant growth.
Nitrogen (N) – Nitrogen is a key element in plant growth. It can be found in plant cells, in plant proteins and hormones, and in chlorophyll.
Phosphorus (P) – Phosphorus helps transfer energy from sunlight to plants, stimulating early root and plant growth.
Potassium (K) – Potassium increase the resistance plants have to diseases, and helps form and move starches, sugars and oils in plants, and it can also improve fruit quality.
Plants also have secondary macronutrients, which are also essential, but they are consumed in smaller amounts by plants.
Calcium (Ca) – Calcium is essential for root health, the growth of new roots and root hairs, as well as the growth of new leaves.
Magnesium (Mg) – Magnesium plays an important role in chlorophyll production and is also vital for photosynthesis.
Sulfur (S) – Sulfur is one of the constituents of amino acids in plant proteins and is involved in energy production processes in plants. It’s also responsible for flavor and odor compounds in plants, namely the aromas in onions and cabbage.
Plants also need micronutrients (or trace minerals) which are involved in various metabolic reactions.
Iron (Fe) – Iron is part of many compounds that regulate and promote plant growth.
Manganese (Mn) – Manganese also plays a role in photosynthesis.
Copper (Cu) – Copper is an important part of the enzymes present in plants.
Zinc (Zn) – Zinc is crucial for the production of the plant hormone responsible for stem elongation and leaf expansion.
Boron (B) – Boron helps with the formation of cell walls in rapidly growing tissue. A boron deficiency diminishes the uptake of calcium as well as the plant’s ability to use it.
Molybdenum (Mo) – Molybdenum helps bacteria and soil organisms convert nitrogen in the air to soluble nitrogen compounds in the soil, and it also plays an important role in the formation of plant proteins from soluble nitrogen compounds.
Some plants need other nutrients – more specifically cobalt, nickel, silicon, sodium, and vanadium. But these are not relevant in this particular case.
The bottom line is that all of the nutrients are important because the lack of ONE nutrient may create an imbalance that will negatively impact the plant’s growth.
What nutrients do plants need the most?
Most people don’t know that plant growth is a process in which plants extract nutrients from the soil and those nutrient requirements vary based on the plant.
Nutrient guidelines typically only account for the three major nutrients (Nitrogen (N), Phosphorus (P), Potassium (K)), as those are conventionally supplied by fertilizers.
According to California’s Department of Food and Agriculture, the NPK requirements for field crops like corn or barley should be around 3.5-0.4-3.0 while tree crops like pistachio trees should be about 2.2-0.14-1.8.
This data is quite revealing because it shows us that plants need much more nitrogen than phosphorus – often by seven times or more.
Why would compost hurt plants?
Taking the aforementioned nutrient references into account, we need to ensure that the compost we use has an adequate NPK ratio.
Homemade compost created from plant materials generally has a higher concentration of nitrogen and potassium, and a lower concentration of phosphorus. In most cases, this is exactly what your plants will require.
However, if your compost is made from animal manure, or you purchase a commercial compost where the ratio of nitrogen and phosphorus is closer to 1-1, then that may eventually turn out to be harmful for plants.
Nitrogen is considered to be the most important nutrient, and plants absorb more nitrogen than any other element. If you have low concentrations of nitrogen and plants use up that nitrogen, you’ll be left with phosphorus in the soil, which isn’t good because phosphorus does not leech in the soil.
An excessive amount of phosphorus will make it difficult for plants to take up iron and manganese, which may result in a deficiency of these nutrients. This shows up as interveinal chlorosis, which is the yellowing of the tissue between the veins due to the decline of chlorophyll production and activity.
To fix this issue, people will oftentimes treat the soil with powdered or granular chelated iron without realizing that the problem is caused by having too much phosphorus concentrated in the soil.
Unfortunately, there are no ways to actively reduce excessive phosphorus but it can be reduced over time by avoiding the use of fertilizers that contain phosphorus. That can also be achieved by planting nitrogen-fixing plants because they increase the amount of nitrogen available in the soil without the use of fertilizers.
Opting for plant-based compost
Because compost created from animal manure and commercially sold compost usually has higher than necessary concentrations of phosphorus – it is a good idea to create your own compost at home using plant materials.
But before you do any of that – getting a soil test to measure the health and fertility of the soil is probably something you should do first. Amazon sells a comprehensive test kit that allows you to measure the soil’s pH levels, nitrogen, phosphorus, and potassium.
Knowing your soil’s current state will take the guesswork out of fixing any of the nutrient issues you may encounter in the future.
With that being said, if you’re planning to use compost – it’s better to use plant-based compost rather than animal manure-based compost since the former contains relatively less phosphorus.
Here is a great video you can watch to learn how to create compost:
Just make sure that, when following the instructions, to cut back on the animal manure.
Don’t use too much compost
Compost is an excellent source of nutrients and is great for building soil structure.
However – too much compost can be a problem. Using excessive amounts of compost may create an imbalance by providing plants with nutrient levels that are too high.
Believe it or not, plants are not different from human beings when it comes to nutrition.
Many people go on about how nutrient deficiencies are dangerous, but they forget that the overconsumption of certain nutrients can also be toxic.
Here is a paragraph taken from Nature.com on nutrient excess in plants:
“While nutrient deficiencies can pose serious threats to plant productivity, nutrients can become toxic in excess, which is also problematic. When some micronutrients accumulate to very high levels in plants, they contribute to the generation of reactive oxygen species (ROS), which can cause extensive cellular damage.”Nature.com
For this reason, you should be mindful not to use too much compost.
Don’t add more than one or two inches a year on landscape plants, and don’t add more than three inches to your vegetable garden.