Can you use too much compost? If you believe what you read, compost is the best thing for your garden. If that’s true, how can you have too much? The truth is that too much compost, especially manure and commercial compost, is harmful to your soil and plants.
The NPK of 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 the input ingredients. Homemade compost, which is made mostly from plant material has an NPK value of around 3-0.5-1.5 while commercial composted manure has an NPK value of about 1-1-1. Compost based on manure tends to have a higher relative amount of phosphorus.
Plant Needs for N and P
The numbers will vary by plant type but values for agricultural crops are reported as 6.6-1-6.6 for corn and 3-1-2 for general crops and garden plants. As you can see plants need much more nitrogen than phosphorus (the middle number), about three times more.
Since plants need more nitrogen than phosphorus, it is impossible to supply the right amount of both nutrients with commercial compost. If you use enough compost to supply the needed phosphorus you won’t provide enough nitrogen. If you provide the right nitrogen level, you will apply too much phosphorus.
The best solution is to use a combination of compost and synthetic nitrogen. Add enough compost to provide the needed phosphorus level, and top it up with fertilizer to provide the required nitrogen.
It is also important to take into account the fact that compost takes about 5 years to completely decompose and release its nutrients. If you add compost every year, it accumulates, increasing the available nutrient level each year.
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 into the air. Excess nitrogen, which 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 less than an inch 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. Once in the soil, phosphorus is difficult to remove.
Because of the different ways nitrogen and phosphorus move through the 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 the soil until it reaches toxic levels.
The Problem with High Phosphorus Levels
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, adding iron won’t solve the problem.
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.
Gardens Use Too Much Compost?
The above image is the test 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 ” fish liquid (3-4-3) and kelp liquid (2-3-1) fertilizer”. Note that these two fertilizers have high relative levels of phosphate. Phosphate in this soil is 432% of the normal level and has 12% organic matter. This type of result is becoming common, especially for raised beds that use organic matter instead of real soil.
A study done by Oregon State University found that “the 27 gardens averaged 13% organic material; the recommended minimum is 3% to 5% by weight, according to numerous studies, including by Cornell University Extension Service”. One of the test beds had 30% organic matter and the owner found that pepper plants burned and died by the end of the season.
A similar study in Minnesota found very high levels of phosphorus in gardens and urban farms. “Garden soils had a median Bray P value of 80 ppm, showing a buildup of plant-available P far exceeding recommended levels.” The cause was an over-application of compost which led to phosphate pollution in local waterways.
The Right Way to Use Compost
Good quality native topsoil contains about 5% organic matter by weight (10% by volume) with lower amounts in sandy soil. More than this will start causing problems for plants by providing nutrient levels that are too high.
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. If you do use manure-based compost or commercial compost with an NPK ratio of 1-1-1, consider using less compost plus synthetic nitrogen fertilizer. This will allow you to meet the target ratio of 3-1-2.
Keep using compost, but don’t add more than an inch or two a year.
Thank you for your response. I appreciate it.
Really appreciate the information. I am a new gardener and do not know how I fouled up my soil so bad. My plants are dying. They look like they are being burned. I mostly have 4×4 bins. I bought compost from a local landscape company as well as top soil. The problem is, most of the bin is compost. At most, the top 4” are top soil. Is there any way to fix the problem without ripping everything out? I am sick! I spent so much time and effort and research. I do not know how I missed this most important detail.
Sorry – you either rip it apart and start again, or wait until the compost decomposes some more.
It is not all your fault – many places on line tell you to use lots of compost.
Help! I just planted all my ground plants and flowers in manure compost that I bought at a garden center. Do I need to pull everything out and replant with topsoil mix? I threw a couple of inches of compost at the bottom of the holes. Put in my perennials and put compost around them and on top. Now I’m worried they have way too much compost. What should I do?
Same here. I did almost the same thing. Planting my fig tree and grapefruit tree directly on top of a few inches of composted chicken manure.
I have composted Chicken manure from the deep bedding in the coop. Am so glad to find this post. I have sprinkled a lot in the garden which is newly tilled. I have the problem of my husband not wanting to do anything different from what his parents did. Personally, I would love to do the Ruth Stout method. I’m 63, so the easier the work the better.
Hi Robert,
I used composted chicken manure with the ratio if 1:4 (1 being tge chicken manure) with potting soil, sand and pumice. It burned/killed my cilantro overnight. It’s my first time ever using manure and I’ve never planted cilantro. I’m experimenting (you can probably see that). My question is, how do I neutralize the soil? The ph is 7.5. Thank you,
Dawn
You are not trying to neutralize the soil, and pH 7.5 should be fine.
The manure probably added too much nitrogen. That will reduce with time, and can drop fairly quickly. Watering a lot will wash out much of the nitrogen. Then try again. Other nutrients can also be toxic and they take much longer to leave the soil.
Since this sounds like a mix for pots – dilute it by mixing in potting soil and sand.
Oh no! I think I’ve done it…I used too much..all of my soil is water logged and doesn’t drain well..I used one bag to 2 bags potting soil..I think pretty much just waisted all the money I spent creating a garden this year..I’m an urban grower so everything is in containers, is there any help for me here?? 😩🚨
Take the soil out and mix in more potting soil. Then use it. But I am not convinced your watering problems is from too much compost.
Hi, So I have added compost to my raised beds for awhile. I also added manure one time a year ago. I suspect the beds have too much compost and perhaps the phosporus level is too high.
What to do now to correct this? Sorry if I missed this. Thanks!
All you can do with high levels of phosphate is grow things, and remove organic matter. Slowly phosphate will be reduced.
This year I shoveled out my fire pit and got approx 1/2 a wheel barrow of wood ashes. Instead of spreading on the garden I layered them in the Earth Machine compost bin with approx 50/50 used coffee grinds and ashes in layers about 6-8” thick. Depending on how much it settles after it melts in the spring will determine what I do with it. If it settles enough that I will have room for a summers worth of grass clippings I’ll wait till next fall to spread it. If it hardly settles at all I will have to spread it to make room.
Do you think it will settle to about 1/4” of the current level which is how much it needs to for there to be enough room for grass clippings? That would be my hope because it would give it another full summer to compost and for worms to help it along.
This year I did things different than I have other years. I have in the past rototilled every spring and fall. This year I finally tried not tilling. Garden was good but in the fall when I spread compost it took a lot longer for smell to go away thAn when I rototill.
I used a broadfork over the whole garden which helped with the smell then continued to get coffee grinds until the snow started. I also started a pile in the middle of the garden that is mostly the last of the garden waste and more coffee grinds just to hold them in place. That pile is probably 5’x3’.
Garden will stay like this until spring at which time I plan on spreading out the pile and going over the whole thing with the broadfork.
I’m curious to see how different the pile and the coffee grinds will look by spring time. Definitely a neat experiment.
Also is it beneficial to do a year of summer fallow in a garden every few years? I know farmers who swear by doing it.
If I do summer fallow should I add mulch to it the whole summer and just knock down any weeds that pop up?
Having a fallow garden periodically is good for the soil, but even better is to plant a cover crop, and incorporate that into the soil, preferably by just letting it die down on its own.
If you pile on the mulch to a depth of 24” how much will it settle by the spring time. Not decompose, settle. That is if the mulch is lawn clippings, coffee grinds, and compost.
Depends on many things. A guess is 60% of original.