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.
Hi Robert,
So where does the extra phosphorus come from? It seems to me that compost made from plants should have roughly the same elements as the plants that went into it, plus the air and water needed as well. Am I missing something?
You are mostly correct for compost made from plants. Compost made from other sources may naturally have a higher P value because of the material used.
There is one other aspect to this. Phosphorus is fairly stable (does not react with air) and is not very soluble. Nitrogen on the other hand is easily converted to ammonium and nitrogen gas, both of which leave a compost pile as gas. So the nitrogen level, even from plant material tends to lose nitrogen while the phosphorus remains in place. So composting of any material reduces nitrogen relative to phosphorous.