Too Much Compost – Is It Poisoning Your Garden? – Garden Myths

Can you have too much compost? Compost is the best thing for your garden – if you believe what you read so how can you have too much? The truth is that too much compost, especially manure compost, is harmful to your soil and plants.

Too Much Compost - Is It Poisoning Your Garden? - Garden Myths

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Too Much Compost – Is It Poisoning Your Garden

Compost NPK

Compost is partially decomposed organic matter. For a more detailed discussion of this have a look at Benefits of Composting.

One of the benefits of compost is that it adds nutrients to soil. The amount of nutrients depends on how it is made, and on what the input ingredients are. Homemade compost, which is made mostly from plant material has an NPK number of +61404532026 (ref 1) and commercial manure compost has an NPK value of about 1-1-1. Compost based on manure tends to have a higher relative amount of phosphorus.

Nutrients Absorbed by Plants

What does a plant need?

The numbers will vary by plant type but values for agricultural crops are reported as +61404532026 (ref 2) for corn and 7-1-5 (ref 3) for general crops. For simplicity I’ll consider the value to be 7-1-6.

As you can see plants need much more nitrogen than phosphorus (the middle number), about seven times more.

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Too Much Compost

Assume you follow the advice of most references and you add some compost to your garden each year. If you use your own compost that is made mostly from plant materials the nitrogen level will be about 7 times that of phosphorus, which is what your plants want.

However, if your compost is made from manure, or you use commercial compost which is based on manure the ratio of nitrogen to phosphorus is closer to 1:1. In this case, once all of the nitrogen is used up by plants, most of the phosphorus is still left in the soil.

You can easily see that adding manure based compost at the right nitrogen level will lead to excess levels of phosphorus. This compost does not seem to be a good choice for fertilizing your garden, but the story is even worse than that!

Nitrogen and Phosphorus In Soil

What happens to nitrogen and phosphorus in soil?

Nitrogen moves through soil fairly quickly and can be easily washed away by rain. Nitrogen can also be converted to N2 and N2O, both gases that escape to the air. Excess nitrogen, that is not used by plants, easily leaves the growing layer in the soil.

Phosphorus on the other hand moves very slowly through soil at a rate of an inch or two a year. It does not wash away easily, nor does it get converted to gasses that escape. Excess phosphorus accumulates in the soil and for the most part, it stays put.

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Because of the different way nitrogen and phosphorus move through soil, even plant based compost will result in an accumulation of phosphorus. If this is done yearly, there is a steady build up of phosphorus levels in soil until it reaches toxic levels.

The Problem with High Phosphorus Levels

What happens if phosphorus levels get too high?

High phosphorus levels make it more difficult for plants to take up manganese and iron resulting in deficiencies of these nutrients in the plant. This shows up as interveinal chlorosis of the leaves. Some people try to solve this problem by adding more iron to the soil, but if the problem is caused by too much phosphorus in the soil, the last thing the soil needs is more iron.

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High phosphorus levels are also toxic to mycorrhizal fungi which are very important to landscape plants. They provide phosphorus and water, as well as other nutrients to the plant. Without mycorrhizal fungi, plants need to expend more energy making larger root systems. Less energy is then available for growing, flowering and fruiting.

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You Can have Too Much Compost

Compost is a good source of nutrients, and it builds soil structure – both are good for plants. But too much compost can be a problem. This is true for plant based and manure based compost, but it is worse for manure based compost.

Native top soil contains about 5% organic matter by weight (10% by volume). More than this will start causing problems for plants by providing nutrient levels that are too high. In response, plants grow too fast and don’t produce enough natural pesticides which leads to more pests and diseases.

If you are going to use compost, it is better to use plant based compost than manure based compost since the former contains relatively less phosphorus.

Keep using compost, but don’t add more than an inch or two a year on your landscape plants. Because you harvest from a vegetable garden and remove nutrients in the form of food, you can use up to three inches there.

References:

  1. Compost – Is it an organic Fertilizer; https://www.gardenmyths.com/compost-organic-fertilizer/
  2. Roots, Growth and Nutrient Uptake; https://www.agry.purdue.edu/ext/pubs/AGRY-95-08.pdf
  3. Nutrient Uptake and Metabolism in Crops; http://prairiesoilsandcrops.ca/volume6.php
  4. Photo Source; Oregon State University

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