Too Much Compost – Is It Poisoning Your Garden?

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Robert Pavlis

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
Too Much Compost – Is It Poisoning Your Garden, source: Oregon State University

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 3-0.5-1.5 (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 6.6-1-6.6 (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.

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.

Growing Great Tomaotes, by Robert Pavlis

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.

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.

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.

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.

The above image is the result from a commercial soil lab for a raised bed. This is clay soil that has had compost added for a number of years, including recently (80 lbs on a 4 x 8 bed). They also occasionally add “occasionally used fish liquid (3-4-3) and kelp liquid (2-3-1) fertilizer”. Note that these two fertilizers have high levels of phosphate. Phosphate in this soil is 432% of normal level, according to the analysis. This soil as 12% organic matter. These types of results are becoming common, especially for raised beds that are using organic matter instead of real soil.

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.

Soil Science for Gardeners book by Robert Pavlis

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
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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

79 thoughts on “Too Much Compost – Is It Poisoning Your Garden?”

  1. Robert, you write that high soil P can adversely affect iron and manganese levels. I assume that is by forming precipitates. But that is only at high pH.
    I’ve applied a 1″ mulch of compost to vegetable beds for decades. Organic matter n my soil tested at 15%. As suspected, the soil tests “excessive” and very high for both phosphorus and potassium. With that said, the plants grow well with no symptom of manganese or iron deficiency.
    I am curious if how much of the phosphorus is leaching out of the soil as compared with similar soils lower in organic matter levels.

    Reply
    • I know your soil has a very high OM level, and I have been on the lookout for some data on how such soil behaves, but have to find any. As you know – some believe that your OM level is too high and not good for plant growth. In your case it is obviously not a problem.

      Reply
  2. I had some mushroom compost delivered (a mix of manure and plant material–the growing medium for mushrooms), and the pile is still very hot. I’m trying to research if it’s safe to spread now (mid fall – no new veg plantings, but I do have some newly planted shrubs and perennials), if it can age on the beds until spring, or if I should leave it in the pile until it’s “done” and cooled down. I think Md University gardening office info about manure conflicts with your analysis: Plant-based composts have an N-P-K analysis of approximately 1.0-0.5-1.0. Only 5%-10% of the N (nitrogen) is mineralized (plant-available) in the year of application. Most of the K (potassium) and a small percentage of the P (phosphorus) are available in the first year.
    Manure-based composts are higher in nutrients and more of the N and P is in an inorganic, plant-available form. What do you think?

    Reply
  3. Very interesting post and comments. We generally make a large compost pile right in our garden in part to deal with our lawn clippings. They don’t break down well in our lawn, it’s arid and windy, alkaline Eastern Montana. I’m not interested in tumbling or spreading. The pile gets covered over with plants, collapses over time, forgotten and we pick a new spot. This winter we were given about ten huge bags of leaves which sat over the winter. Come spring I layered them with grass and made a ginormous pile about 25×8 across the back of the garden. I planted all my squash pumpkins and cantaloupes and cukes in pockets of soil on top and the edge. I had read years ago about planting directly on a new pile, in a book my mother had, “Lasagne Gardening” but hadn’t tried it. And planted the rest of the garden. Watered the heck out of all of it and even during these excessive temps it has grown to look like Jurassic Park. My husband threw down redwood sawdust, before the plants got going, I was sure the plants would object. Nope, they didn’t miss a beat. Plenty of grasshoppers but I can’t see any damage on the cucurbits bed. Last year we started carrying our kitchen waste out there in using paper egg cartons as a container, and setting them down all over the garden, all winter. By spring not much is recognizable. We put some gypsum down last fall, some trace mineral Azomite the year before that. That’s it. I’m more than a bit shocked how everything looks this year.

    Reply
  4. I’ve some decomposing cassava peels which I mixed with the top soil to raise Oil palm seedlings, I hope I’m on the right track

    Reply
  5. just discovered you today. OK ,,,, you are now my first tempter to convert me into a garden-oholic.

    Raised beds got mushroom soil and 10-10-10 ( I am new at this. brand new)

    was told i could add lime. that did not sound safe.

    Help!!!! very little is growing and it has been one month

    Reply
    • No idea what mushroom soil is, but if you mean mushroom compost – that can contain high levels of nutrients – you probably poisoned the plants, even without the added fertilizer.

      Reply
  6. I filled a raised bed with equal quantity of top soil and plant based compost in addition to one bag of animal mbased manure. Plants such as tomoatoes, peppers and cucumbers leaves are yellowing. Is there anything I can do to fix it now?

    Reply
  7. So informative. Thank you. My husband and I are arguing about whether to spread compost on garden beds ( flowers and shrubs) end of May. Temps today getting to 90’s. Upstate South Carolina. We continually make lots of compost with mostly grass cutting and kitchen peelings etc.
    What to do with excess??

    Reply

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