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.
What are your thoughts on what you might add to a deep tomato hole when transplanting besides your regular garden soil you dig up?
I saw somewhere that you were somewhat favorable of the Pro-Mix BX soils out there. I have been using it in addition to various potting soils in starting all my seeds with success but not sure of any uses for transplanting.
Nothing. The roots need to grow in your garden soil – just let them do that.
If you have a known phosphate deficiency, you can add the fertilizer at root level.
You could also add compost, but then I would prefer to dig a bigger hole and spread it around.
I recently just came across Mr. Pavlis on YouTube Tube and immediately became intrigued by his science worthy information. Being an avid vegetable gardener for over 40 some years (also having a solid Biology background) I am learning valuable information that I am excited to begin using! I don’t think I will watch any other so called “garden gurus” anymore with all of their conflicting information. I have been swearing by mushroom compost the past 10 years or so and now will definitely rethink how I can successfully use it. Would love to have a response on his theories on mushroom compost. Thanks again Ron.
Thanks for the kind words.
There is no such thing as “a” mushroom compost. Each one is different and made from locally sourced material. In general, there is nothing wrong with it, except that it can be high in nutrients and a bit fresh for gardens.
https://www.gardenmyths.com/salt-mushroom-compost-garden/
Are you sure about OM absorbing free phosphate, except as it sponges up water with free phosphate in the soil solution. Free phosphate is anionic, and OM lacks anion exchange sites (https://lawr.ucdavis.edu/classes/ssc219/biogeo/table11.htm).
Do you have another reference for comparisons of nutrient levels of plant- and manure-based composts. The link you supplied doesn’t show anything.
Thanks.
It is my understanding that OM has both positive and negative charges, but the negative ones are the majority of the charges.
“Soil Organic Matter (SOM) has some functional groups which form positive sites: e.g. R-N-H3+
Anion exchange capacity of soil materials is much lower than the CEC–on order of 20 x less.”
https://web.njit.edu/~kebbekus/Soil%20chem%20notes.htm
This contains links to some lists of NPK for composts
https://www.gardenmyths.com/compost-fertilizer-numbers/
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.
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.
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?
If you have a concern, spread it a bit thinner.
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.
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
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
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.
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?
get aa soil test done so you know what the problem is.
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??
Yes spread it. I never met anyone who had excess compost.
Grass clippings can be left on the lawn.