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Too Much Compost – Is It Poisoning Your Garden?

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

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.

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.

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; http://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

 

Robert Pavlis
Editor of GardenMyths.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. 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!

I hope you find Garden Myths an educational site that helps you understand your garden better.

29 Responses to 'Too Much Compost – Is It Poisoning Your Garden?'

  1. Robert do you have any peer reviewed data to indicate detrimental depths to place organic matter? I’m arguing with a soils lab that is recommending the placement of amended topsoil to a depth of 3-4 foot. The topsoil has 3-5% organic matter by weight. I’m telling them to limit the amended topsoil to a max depth of 8-12 inches and below that place unamended topsoil. Typical issues with organics placed too deeply in the soil profile include many problems such as shrinkage and anaerobic conditions.

  2. jartomy27 says:

    Hi, I read somewhere in your website that manure are good, but in this post you say we better use plant based compost because they are lower in P. Which I should use for my corn plant? Should I skip adding P because you say that most soil contain too much P? I added NPK and other syntethic fertilizer a lot last time.

    Thank you.

    • Here is the problem. You do not know which fertilizer to add because you do not know what you have in the soil. Until you know that, you will never know what to add. Either add nothing and see how the plants grow – which is what I do, or get a soil test so you know the facts.

      I just did a soil test, mostly so I could write about it and it turns out my soil P levels are not high, but they are sufficient for growing plants.

  3. Ron Hawley says:

    I think your views are most informative and interesting , however this information is going to concern many organic vegetable gardeners, myself included . Would the addition of leaf mould in large amounts plus adding lime in autumn have an effect on phosphate levels . I have used home made compost, horse & chicken manure mixed with straw/ sawdust well rotted for the last 8yrs my veg grow well but I have trouble with club root my soil being a dark peat sandy type .I apply organic fertilizer in the form of F/B/&bone .I would appreciate your reply.

    • I don’t think lime will affect the P levels, but it might as the pH changes. Make sure you really need lime – a lot of people add lime that they don’t need.

      Leaves and any other organic material will add more P to the soil.

  4. Andy says:

    Interesting blog post. While I will in no way doubt your science, and I always look to science for answers, I like Lee above do not see any problem yet. Your science is obviously going to be correct but in reality I think there might be a lot more at play. I think it needs to be put into context.

    I have very heavy clay soil. When it rains heavily my garden floods as the rain sits on top of the clay. It’s very hard to dig when wet. When it is dry it cracks up and becomes like concrete. Digging dry clay isn’t fun.

    I’ve spent the last 3 years+ digging beds and mixing in horse manure, some fresh, some partly rotted / composted. I am surrounded by wild grassland full of nettles, thistles, ragwort, buttercups and alsorts of weeds. If I leave bare soil it is over run with grass and weeds in no time.

    To combat all of this I have piled manure 2 feet deep onto a virgin bed. I grow directly into it, no great but things do grow in the main. By the next season this has broken down from 2 feet thick to 4 or 6 inches deep. Then I dig it into the clay. I grow again. Then I have piled another 2 feet of manure on and repeated. My clay soil is quickly becoming easy to work and plants are growing fine (so far).

    My manure is from horses, and sometimes has straw mixed in but normally has wood pellets that the stables use as it create less waste that straw. I take a trailer load of muck off of the stables every 2 weeks for over 2.5 years now . I use a huge amount of manure. Lots of people have told me that I’m using too much but without it my soil is too heavy. I am yet to see any obvious signs of problems.

    If this “toxicity” is a case of just reduced crop sizes, then fair enough, technically it may be reducing my yields and I wouldn’t know because I can’t compare but the yields are good compared to what I could grow without conditioning my soil with manure.

    So far, the benefits of using a lot of manure definitely outweigh not using a lot (weed suppressing, helping retain water, easy to dig, and raising the soil level to avoid flooding problems).

    I’ve now entered my 4th year and am wondering what ill effects I should be looking for considering that I’ve added about 48 inches of manure….

    Your comments are most welcome 🙂

    My soil is now good enough (ie, no longer heavy clay) for me to stop taking the waste from the stables. I was going to continue because I have only raised the soil level in the beds by inches, maybe 6 inches, and wanted to end up with 1ft tall raised beds.

    I luckily have plenty of room to start again and dig new beds without so much manure if needed 🙂

    • The question “what ill effects should I be looking for considering that I’ve added about 48 inches of manure” is a good one.

      The symptoms you see will depend on which nutrient is in excess, and that depends on both the organic matter that has been added, and the natural nutrient levels in the starting soil.

      High nitrogen and phosphorus lead to excessive plant growth. High K or P can result in iron and manganese deficiencies which show up as interveinal chlorosis. High K can look like salt damage – stunted yellow foliage, premature fall coloration, leaf scorch and twig dieback are common. High calcium and magnesium will increase the pH levels which shows up as deficiencies of other nutrients since they are less available in alkaline soil.

      Any symptom that seems to be affecting more than one type of plant is suspect. If all the plants are growing well – it is not a problem.

      • Andy says:

        Thank-you very much for your reply, and the time you spend in replying.

        I think in reality, in the field, there is a lot more going on, and each situation is different and the many possible different variables are coming into play.

        I can think of one possible reason why I am not seeing problems and that may be because I grow healthy plants in the green house, then plant them out into a layer with a lot of manure, which probably hasn’t got to the stage where the Phosphorus has been made available to plants in a big way, as the phosphor is made available over time the roots (many of them) may have gone deep enough to now be in the part of the soil where there is a lot less manure.

        My courgette plants, and runner beans, for example, do tend to go a much lighter green for a few weeks (perhaps you would argue yellowing) after planting out and suffer a bit but I put that down to the fact I don’t harden them off and they did have a few cold days and nights to contend with before the weather gets better. After that they all green up and start growing well. Perhaps older plants aren’t quite so susceptible to phosphor toxicity than younger ones plus as time goes on there is less rain moving toward thew summer so that the plant has to take more nutrients from deeper down where the soil is more moist and thereby avoiding the high phosphor higher up the growing medium.

        anyway, I dare say there are lots of reasons why I’m not having a problem and hopefully by the time the compost has broken down sufficiently enough to cause a phosphor issue I will have dug all the manure into the soil deep enough (I’m looking for 2 foot deep) so that the end result is an organic level that is closer to 10% spread over 2 feet deep rather than 80% at the surface.

        I’ll have to keep an eye open for issues.

        Thank-you once again for your reply and may I take the time to say that I think you’re giving out a fantastic service and excellent thoughts on all these subjects.

  5. Maryann Caudill says:

    I read some folk don’t believe in manure compost, and miracle gro complaints on web. Older days you just plow, or loosen soil, cover the seed, when it comes up after rains, once, throw handful of 10-10-10, and my husband’s Mother raised boys, was it 6, like this, raised her kids alone, this is how she did it, i watched her, i don’t think she even used compost; this was a one-time scene i saw, and i suppose way it was. the gardning course thru the mail was not perfect; seems me times you don’t need nothing; i rooted an inch forsythia, just put it in ground, it rooted, now it is over a foot – big, i mean it is o.k., ain’t it, if you’re poor, and can’t even afford compost, or starting out? and really mite not can make or hardly get any compost; don’t good Lord make it grow, anyway?

  6. Izham Alias says:

    Hi Robert.

    I try to find your email to ask a question about gardening but I don’t see any.

    Sorry to ask it here.

    It’s about MSG or monosodium glutamate use in agriculture.

    In my garden forum there is a hot debate about the use of MSG in organic gardening. I try to search about it but I don’t see any scientific articles commenting about it.

    How about your opinion in this issue? I really love to read it.

    Thanks!

    • I have never heard of using MSG or monosodium glutamate in agriculture, so i did a bit of searching for information. can’t find anything on government sites about this – so it is easier not done much or is a new process. Can’t find much research on it either.

      I did find a number of sites that seem deathly afraid of eating MSG, but it is a natural chemical found in our bodies and in all plants. This link provides some reasonable discussion: Seems perfectly safe to me except in some rare cases.

      I can’t see why it would do much on a plant. Spraying on leaves will result in very little getting into the plant. Any sprayed on the ground will be quickly used or digested by microbes. Unless I see some good evidence to the contrary – this seems like nonsense to me.

  7. Brenda Steadhelm says:

    Can this be substantiated with field tests by applying compost?

    “In response, plants grow too fast and don’t produce enough natural pesticides which leads to more pests and diseases.”

    I am a new gardener/yardener and going between your information and Howard Garett’s on http://www.dirtdoctor.com is really confusing. Some of your stuff is making horse-sense, but some assertions like this one I still have doubts about.

    It seems like the philosophy behind the organic methods like top dressing with compost is that if you use more than is required it won’t screw things up too bad. You’re saying it will definitely screw things up in just as negative a way as using too much “synthetic” fert would. Wouldn’t that take so much compost you would cover the leaves and kill whatever is there anyway? These are probably going to sound like dumb statements to you so I want to assure you this is me thinking out loud and not at all trying to put you on the spot.

    • Excess nitrogen will make plants grow faster and they tend to make soft growth. I don’t have a ready reference to support the fact that they also produce less pesticides.

      I am seeing more reports from organic farmers with new plant problems. In some cases a soil test shows very high levels of nutrients.

      I have not really followed DirtDoctor. I had a look at his web site and picked out this quote “Compost: Approximate analysis is 1-1-1. This is the best all-around organic fertilizer. After all, it’s nature’s. Apply at a minimum of 50-100 pounds per 1,000 square feet or 800-4,500 pounds per acre. Use in all potting soil mixes and to prepare all new beds. Compost is far superior to any other form of organic matter for use in building the soil.”

      “that is the best all-around organic fertilizer” – not if your soil already has high P and K! Someone I know had their soil tested and the lab told them “P and k were extremely high – don’t add any more compost until those levels come down”.

      “After all, it’s nature’s” – Dumb comment. Not everything in nature is good for your garden. Garlic mustard is also nature, but this invasive is starting to take over my forest. When someone says “nature = good” – you can’t trust the rest of what they say.

      “Compost is far superior to any other form of organic matter” – why? It is good for most gardens, but so is manure, and wood chip mulch. Both of these are organic matter as well. As a mulch wood chips work better.

      I am pro organic gardening, but I have serious concerns about proponents who are certified and follow the certification requirements to the letter instead of thinking and doing what makes sense. See Organic Gardening – Are You an organic Gardener.

      Compost is slow acting. It takes at least 5 years to fully decompose. the compost you add this year is still adding nutrients 5 years from now. Next year you add more, and the year after again. In 5 years you have nutrients being added from 5 years of applications. Yes you can add too much.

      • Brenda Steadhelm says:

        Awesome, thanks for taking the time to respond, and I believe you are telling the truth with respect to reports you are getting. I am skeptical of entrepreneurs of organic gardening. It isn’t regulated, and in some cases the folks promoting the methods are on the boards or staff of some entity that seems to have no scientific papers.

        What would you say to the recommendation that putting down compost and dry molasses on a lawn that has many weeds known to grow in nitrogen deficient soils is the best organic approach? The theory that the spike in microbes eating the sugar will aggravate the common soil pests like fire ants seems intriguing. If the owner is dead set on an organic approach, is that worth their time on a lawn that has been ignored and is now just being mowed weekly?

        • Compost and molasses will both add nitrogen. all plants need nitrogen, so you will make the grass grow better and you will make the weeds grow better. Just because a weed grows in nitrogen deficient soil, does not mean it will not grow with more nitrogen – it usually grows better. This is a problem with corn gluten. It prevents weed seeds from germinating, but since it is high in nitrogen it makes existing weeds grow better.

          I don’t know about the fire ants, but i would be skeptical unless those microbes produce some chemicals the ants don’t like.

          I don’t know of a strictly organic method to get weeds out of the lawn – but that may depend on their definition of organic.

      • ari says:

        compost isn’t even ‘natures’ The forest floor is covered in a layer of forest duff, not manure/compost. Yes, an animal may poop here and there but it is not as if the entire forest floor is covered in it

  8. Dave says:

    Good article and as the saying too much of a good thing is not always good. Will point out that in order to achieve 3-5% OM by weight the volume added for a compost would have to be between 20-30% by volume. the only organics that can be added at 10% by volume to get 3-5% by weight are peats and or nitrogen stable fir/pine bark, coir or other pure wood residual.

    • Why do you say “in order to achieve 3-5% OM by weight the volume added for a compost would have to be between 20-30% by volume”? The two numbers I gave are routinely used and i never gave them much thought.

  9. Ileen Davidson says:

    I am a Master Gardener and I find your blog to contain the most useful information I have found anywhere. You repeatedly answer the questions I have that I never seem to get answered elsewhere. Thank you so much.

  10. rogerbrook says:

    My instincts tell me that you can have too much farmyard manure, especially if it is fresh
    I think it would be very difficult to apply too much homemade garden compost from plant remains
    Although I agree that our soils often have excessive phosphorus from previous fertilisation I think you overplay the dangers of excess from compost

    • You might be right Roger. There is not a lot of information available about field effects when higher amounts of compost are used. One of the problems is the the effects are slow to develop since compost decomposes so slowly. But there seems to be more reports of organic growers having problems with plants while at the same time having high organic levels.

      I also see a lot of organic followers thinking that you can’t have too much compost.

  11. Ruth Klein says:

    Helpful information, especially since our latest community garden soil test showed almost no Nitrogen and way too much Phosphorus. We are limiting our fertilizers to blood meal and fish emulsion.
    Thanks for explaining.

  12. Rick says:

    Is there a reliable diy soil test kit to test nutrient levels in your homemade compost? If my Phosphorus is too high how do I reduce it?

    • Not really. Best way to get this data is from a reliable lab.

      If P levels are too high the best way to lower them is to grow things, and then remove the plant material from the garden. Alternatively, wait. Over time P levels will also drop as some is leeched to lower levels of soil – but this is a slow process.

  13. Lee Reich says:

    The cautions all seem correct — except that they don’t play out “in the field.” I have added an inch depth of compost to my vegetable beds every year for decades. Phosphorous and potassium levels are extremely high. The plants have never shown any signs of any deficiencies, and growth and yields continue to be excellent.

    A few possible explanations . . . Phosphorous supplied by organic materials is in a different form in the soil than that supplied by minerals. Soil tests are geared to mineral soils (5% or less organic matter). My soil is 15% organic matter; perhaps a different test is needed for soils very high in organic matter.

    Also, phosphorous does move down through the soil profile, how fast is directly related to the phosphorous load into the soil. Not saying this is good or bad, just that it is. A major source of phosphorus movement is when soil particles are carries down slopes by rainfall.

    And finally, as far as the differences in major nutrient concentrations in finished composts between plant- and manure-based composts . . . I would like to see actual data showing whether these differences exist in FINISHED composts.

    • Thanks for the post. Organic matter will also absorb free phosphate. Is it possible the OM is keeping the effective levels of P low in the soil solution? Most of the P is held in soil and only a small amount enters the soil solution. OM should increase the amount that can be essentially buffered – until some tipping point is reached and the soil is saturated. Then the excess P starts causing problems.

      I had seen a paper that looked at actual home made composts, and they found a significant variation in nutrient levels. This reference might be of interest.

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