Can You Add TOO MUCH Compost? Surprising Answer

This is a new research report regarding too much of a good thing. Always people preach to me to add more compost. Some people I know boast they have in their container beds ONLY organic homemade compost and nothing more. It seems that Portlanders in cities who do this likely (even though they may be Master Gardeners) are hurting their plants and environment.

As the old saying goes: “everything in moderation.”

Read more here: Urban gardens contain too much organic matter, OSU study finds -


My soil mix

Washed Sharp sand : 3 parts
Triple Ground Redwood Bark or Peat : 2 parts
Dried Cured Compost : 1 part
Nematodes : 0


Ther is definitely a “too much” level. I always assumed it was practically impossible to reach that level.

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The article is sensationalist and commingles unrelated issues.
Too much phosphorus supposedly caused the burning they saw. But that happened due to the source of organic matter. But that is rarely the problem if one uses bark which the authors recommend.
Last time I checked, bark was organic.

Too much organic matter may indeed be a problem. But the authors are not making a convincing argument.


I read a scientific article where they tried different “concentration” of compost when cultivating parsley in Tunisia, and the perfect rate was 10%, above which the production starting dropping. I could find it again if someone is interested.


I think it’s important to make a distinction between organic matter and compost, and also how these interact with roots. Compost is the product of the breakdown and bacterial/fungal digestion of organic matter. Organic matter is anything that was once alive: Wood chips, leaves, “mulch”, kitchen scraps, etc. Therefore, compost is a concentrate or essence of organic matter. And yes, probably too much of it, especially deep in the soil around roots could be harmful. Organic matter on the other hand, if kept on the surface as mulch, without direct contact with roots, is not harmful. It becomes harmful if it’s mixed with the soil because then it putrefies in anaerobic conditions and becomes noxious to roots. Bottom line, if compost is to be used, it needs to be in reasonable amounts, and on the surface where it slowly percolates in the soil. One can be more liberal with organic matter as long as it is kept on the surface where it will slowly break down into compost and then percolates in the soil.


I am far from convinced that there is a cross the board ‘too much’ level. I have used 100% compost in large pots to grow various things in the past when I could make a lot more compost. No soil or sand, just compost from my tumbler. Things grew amazingly well in them and I never had tip burn or any issues.

I think what the compost is comprised of probably makes a bigger difference than how much is used. Planting into 100% low nutrient compost seems fine as I have done it numerous times, whereas smaller percentages of more concentrated compost probably cause the issues people are seeing.


I have seen several other articles, it’s not just them. They are stating current practice. Good advice about compost and other organic material on top. Pine and peat are not very nutritious so are very safe to add. As Richard mentioned some sand is not a bad idea. Make sure it is sharp.

No, actually it was not a sensationalist story. It just is not what you expected. They stated facts. They also stated who were the scientists studying this.

A lot of times people assume things are good. For example ever since they began donating and using blood, they assumed stored blood is good for people losing blood. It was not until me, when doctors could not understand why in the ICU my skin died when they gave me transfusions of stored blood that maybe it is not always so good that they began to be willing to question beliefs. Once the ONE doctor began asking “IS there anything BAD about stored blood” did they find out in storage something in blood goes away and then blood actually cannot transmit oxygen, and it effectively prevents the patient’s body from getting oxygen to cells where it needs it most. NOW because of being willing to look at accepted practice and question IF there is anything bad going on, they can save more limbs, and lives, by just adding back the missing ingredient…so blood CAN transmit oxygen.

So too scientists assumed “if a little is good, a lot is better” and never questioned “could some plants perhaps be harmed if there is a percentage too much?” Now that science is looking into it, they are finding that yes, sometimes there can be too much of a “good” thing, and I expect in future they will have a percentage of things listed that is good for one plant and not for another, and suggested maximums for organic matter of different sorts in composts.

Things only seem extreme or wacky when you do not know, or resist.

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The article didn’t actually supply any data, just sweeping statements that are as inaccurate as the idea that more organic matter is always beneficial, from my reading. Organic matter doesn’t necessarily bleed either N. or P even in beds of pure compost, and no results of research studying stable compost was provided in the article to demonstrate that it does.

Slow release N can contaminate if plants don’t seize it before it goes below roots or into waterways, so the theory that this sometimes happens in gardens is sound, but I’d like to see the research. I long ago saw research that harmful leaching of organic matter N does occur after fires in large areas, but that’s a lot different than a small vegetable garden.

Once P binds with soil, it is mostly immobile except through erosion, as I understand it.

For fruit growers in the humid regions I believe the key danger is that high organic matter in the soil greatly expands the amount of available water, which can make fruit big and watery and also encourage excessive vegetative vigor- depending on rainfall levels during the ripening process from about 6 weeks before harvest on.

As far as growing vegetables in pure compost, I’ve seen gardeners doing this for years for potatoes and carrots and getting incredible production. The burned pepper plants in the article were probably because the compost had fresh animal waste in it or some form of quick release N. That’s the only thing I’ve seen burn roots. Poor drainage could also explain bad results.

And too, nutritional balance needs to be considered with all soils, regardless of OM amounts and given the high nutrition content of OM the danger could increase as OM rates rise, but I’ve rarely seen it. More often plants suffer from nutritional deficiency than excess. However, I do suspect that excess K may lead to an inability of some apple varieties prone to corking to process calcium properly. Composts do tend to be very high in K.

Last season I applied foliar manganese on apple trees prone to corking and seemed to get much better pack out of sound fruit. These trees were growing in soil with excess K and I have posted on this forum an article by a commercial orchard consultant that manganese neutralizes the consequences of excess K. .


Yeah, the article was pretty vague. It didn’t specify what type of mulch the gardeners were using, if they were also supplementing w/ added synthetic fertilizer, etc.

Many mulches have very low levels of nutrients. They are mostly carbon (hence the high levels of organic matter - i.e. carbon). Wood chips have C:N ratios of 100-500:1 That’s less than 1% N by weight. In most cases a lot less. I doubt much leaching is going on.

If the mulch is manure, that’s different. Manure is high in P and can run 1 to 3% in N. Alan is correct, P is very immobile and pretty much only contaminates waterways by soil erosion. The article mentioned that, but really how many gardeners experience erosion of their vegetable or flower beds? Probably could find an anecdotal case here or there, but I’ve never seen enough erosion of a vegetable or flower garden to wash soil into a waterway.

I’ve no doubt, if carried far enough, mulch could leach nutrients, as anything carried too far generally produces negative results, but that’s stating the obvious. The article was just designed as a headline grabber.


I tried growing tomatoes in pure compost purchased from a local supplier (Living Earth in DFW). Didn’t apply any fertilizer, thinking all organic material was fine. Well the plant grew well but did not flower or set any fruit. Lesson learned.

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Great reply

I’ve often read about tomato plants that have too much N becoming entirely vegetative and failing to set fruit, but I’ve never seen it. I’ve never tried growing them in pure compost, but I dress my plants with a lot of pure compost and they certainly push lots of root into them. I keep them juiced on my own urine until they start setting fruit, and in the decades I’ve used this routine my tomatoes have
always been some of the earliest in my area to produce ripe fruit. Never had a single plant fail to set fruit in a timely manner.

Container plants are always grown in a very high OM medium but tomatoes tend to set fruit anyway, including plants I’ve grown in a mix made up of nothing but compost and perlite.

However, all composts may be similar once they are stable but no two are alike.

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I agree that applying well rotted and balanced compost is fantastic for most any plant. Commercial compost is highly variable of course. Living Earth compost likely is deficient in mineral content and nitrogen. Now I just add Tomato-tone or similar and all is good.

What I always thought the danger with tomatoes was is too much N. Did your failed plants also fail to grow vigorously?

I successfully grow tomatoes in straight clay amended only by adding lime and sprinkle 10-10-10 around the plants when they are planted.

I planted Sungold tomatoes for this commercial compost (probably too much shredded wood component) experiment. Full sun and watered through the summer. It grew to about 4-5 ft. For reference, the same variety grown in same area with native soil (clay) + compost mix grew to 6-8 ft the year before and fruited very well. You probably know Sungold can grow 8-12 ft as it’s a hybrid indeterminant cherry.

Yes, Sungold is one of my staples, and they can even grow 20’ if you train them to height. It does amaze me that you got no fruit from a dwarfed plant, though. But then, horticulture is a never ending source of surprise.

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too much N definitely prevents
tomato set.