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.
Interesting post. Of course the amount of compost/nutrients needed to sustain plants is going to be based a lot on the CEC (cation exchange) thus the soil structure. If you have a sandy/silty soil you can add a lot more nutrients and not overload your plants. Though it’s important to consider the effects of leaching nutrients on waterways. So if you live by a river and have a sandy plot, consider growing plants that tolerate low nutrients rather than adding nutrients to the waterways which can lead to toxic algae blooms.
is usinfg the waste from your pond pump bad for your garden
no – it is just organic matter.
Right now I have abouf 24” of organic matter on top of my garden. It is garden waste, grass clippings, shredded news papers and used coffee grinds and filters. I keep adding coffee grinds until it freezes solid which should not be too long from now and I’m also hoping to get a few bags of dry leaves to spread on top. It will look nicer and help with the smell I hope. The smell comes from the in ground digester which is a years worth of kitchen scraps, about 60 gallons.
Like I said, in the past I have rototilled fall and spring with good results but have since been told that rototilling is a bad idea.
My questions are:
1. How much will the level drop by spring time if I don’t rototill?
2. In the spring will I still need to fluff up the dirt with a spade and rake or will the ground have absorbed all of that organic matter?
3. Can the organic matter be overdone? Or is more always better, considering there will be nothing planted on the garden until next spring.
1. Depends on your climate. In cold climates it does not decompose in winter, but does pack down.
2. It will take a couple of years to absorb 24″ of organic matter. Push it aside, and plant.
3. This article says too much can be toxic.
Rototilling is only to bury the stink of the digester. I know it’s not ideal but with close proximity to neighbours it’s a necessary evil.
I live in town so manure can make unhappy neighbors. I put all food waste in a digester that holds a years worth and all grass clippings in a bin. After we harvest the garden I dump all grass clippings on and as many used coffee grinds as I can locate. Then spread the clippings from the bin and the digester.
After that I get the garden rototilled before winter. In spring time I do the same thing with coffee grinds and grass clippings then get it worked before planting again.
This year on a 20×20 garden we got 50 cobs of corn and other things as well.
Have been doing this for 4 years now and it’s working so far, is this a good long term plan?
Seems like a good plan, except that you should not rototill. Rototilling destroys soil structure and it adds air to the soil increasing the decomposition of organic matter. Add the organic material as a mulch and let nature dig it in for you.
I am the guy that has put too much manure out garden and bed that my pk is off the charts I know it will take a long while for it to correct itself. My ? Is if I raise my beds up or add 7.5 inches on top of my existing beds would I be better off mixing it all together with new raised bed soil or just top the beds off with 7.5 in of new soil 2×8 =7.5 in Tks
Not sure, but I would not mix it together. That way roots can use the level they prefer. Over time it will mix.
Is it possible to have made my Sycamore tree sick with too much horse manure?
Is there anything I can do to rectify this problem and help this tree?
I have been told it might have bacterial scorched leaf disease but I need to wait till August to see the leaves then… but after reading your comments I remember putting a lot of manure under this tree last year and the year before and maybe have caused this problem myself with too much phosphorus in the soil?
Manure won’t make it sick – as in having a disease. Too many nutrients might become toxic but this would only happen if you have been adding it for several years. A one time application will not harm the tree, especially a larger one.
As someone that has and still does grow with some beds exclusively of council green waste compost and other beds exclusively of horse manure I wouldn’t do anything unless you are seeing the plants suffer. I haven’t had a problem (that’s noticeable). In one of Robert’s answers he said just grow in it to reduce the excess – I think that is by far the best answer as to what to do. After all many of us grow tomatoes in pure compost, some grow potatoes in nothing but manure (me for one) without issues.
I also think that when we grow in manure we need to remember that not all manures are the same. My manure comes from stables that mix a little straw, and a lot of wood pellet as well as wood chips. This is totally different from “manure” of 20 years ago which may well have been pure horse poo. The ratio of horse poo to wood chips / pellets varies a lot but there is more wood in my manure than poo. Other peoples horse manure is mainly pure poo. Totally different. My plants seem to love my manure and I pile it on by the foot or 2 every year, it breaks down to inches.
Robert is obviously correct scientifically but in practice many composts and manures are different.
I’m skeptical. I’m stubborn. Love compost. I can only dream of being able to add a foot of compost to my garden beds each year. I would if I could. Too much compost? Rubbish I say. There are people out there planting exclusively in compost, exclusively in soil derived from leaf mold, from woodchips. Some plant exclusively in cow manure. One youtuber has video evidence showing cow manure as the perfect soil.
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.
Not really. Most good plant books will give numbers for root depth and most roots are in the top 8 inches of soil.
I have really messed up. I have put tons of manure on my garden and raised beds all my pk numbers are off the chart by my soil samples. I know there is no quick fix , but are their some way to deplete them somewhat? I am having to foliage feed to just get by. Tks 300 to 600 range
Grow things, and remove the plant material. This removes the nutrients from the soil. I don’t know of another solution.
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.
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.
A few years back Gardener’s world BBC(England) showed a guy who dug out a 10″ diameter by 6″ deep hole and filled with compost and new soil from off his plot, into these spots he planted his young cabbage plants which were full grown and able to be harvested before the cabbage club root disease which was present in his base soil had time to infect the plants and cause issues.