Keith has been involved in the gardening and landscaping industry for the past 21 years. From a jobbing gardener to running his own landscaping services....
As gardeners, we are constantly regaled with tales of the magic of compost and how fantastic it is for plants and your garden in general. However, can too much compost hurt plants? Can excessive amounts of black gold actually cause harm to your garden?
Clearly, there is a wide range of benefits for the plants and soil in a garden from adding compost; thousands of years of producing and using compost suggests as much. Whilst this is true, too much of a good thing can, in fact, be not only detrimental to your plants but also to the broader environment.
There is more to compost than meets the eye, a dark side to compost that you probably didn’t realize existed. There are some things that you need to consider before you start adding large quantities of organic material into your garden.
The Composition of Compost: Nitrogen, Phosphorus, and Potassium (NPK)
The first place to look when considering what problems compost may cause is in its chemical makeup.
The key benefit of compost is that it feeds the soil by adding essential nutrients to it and helping the soil’s ecosystem. The principal nutrients are nitrogen, phosphorous, and potassium with the amount contained in the compost of each varying, depending on the ingredients used.
In general, compost produced at home largely uses material from plants and kitchen scraps and will have an NPK of around 3 – 0.5 -1.5 as opposed to shop-bought compost which is closer to 1 – 1 – 1.
Different plants will absorb these nutrients at different levels at different periods of their growth. Generally, plants use much more nitrogen than phosphorous, This means that compost produced at home is often a good fit for your garden, having approximately six times more nitrogen than phosphorous.
Nitrogen and Phosphorous in The Soil’s Ecosystem
Nitrogen and phosphorous behave very differently in the soil. Both nutrients are essential to having thriving plants but their capacity to be retained by the soil to feed plants is different.
Nitrogen is transient and won’t remain in the soil for long periods. Any that is not absorbed through the roots of plants will usually either escape as it reacts with oxygen and forms a gas, or will be washed away. Phosphorus, on the other hand, stays in the soil for extended periods. Any phosphorous that is not absorbed by plants will remain as it doesn’t convert to gas like nitrogen nor wash away quite as easily.
The result could be that if you composted every year in order to supplement your soil’s nitrogen levels you could get a build-up of phosphorous and this might not be such a good thing.
Too Much Phosphorus in the Soil
In the right amounts, phosphorous is an important nutrient. It is a necessary macronutrient. Phosphorus is most often thought of in relation to the formation of strong roots in plants, but in reality, plants need it for a wide variety of crucial areas of plant development.
The Phosphate bonds in phosphorous contain a lot of energy and can be found inside of plant cells, where they serve the purpose of providing fuel for the plant’s “metabolic process” and, eventually, growth. It is often used in amendments when laying a lawn to help promote strong root growth. Without adequate phosphorus, the rate of growth in the plant’s leaves, roots, and stems is significantly slowed.
It is possible however to have too much phosphorous, the consequences of this are that instead of thriving, an accumulation of phosphorus may lead to poor plant growth and with some even dying in certain cases.
The plants fail to grow because the phosphorous content crowds out other essential nutrients that most plants need, particularly zinc and iron. The capacity of the plant to take up these other necessary micronutrients is hindered. The yellowing that may be seen in between leaf veins is a telltale sign of an iron deficiency. A whitening of the tissue is seen in the presence of zinc deficits.
Different Plants Different Phosphorous Requirements
To complicate matters, different plants absorb different amounts. This is why when seeking to establish a lawn through seeding or hydroseeding you are going to use amendments with higher levels of phosphorous. The roots of the grass will easily absorb this and once established you would feed the lawn with more normal levels. However, in plant beds where there are fewer plants, you need much less phosphorous, and standard home-produced compost is perfect for this as the phosphorous level is much lower.
In truth, unless your soil already contains high levels of phosphorous, it is unlikely that you are going to get to the point where there is too much in your soil through home-produced compost, it is more likely if you use commercial compost or fertilizer. It is important to test your soil, especially if you are concerned about plant growth as the results will give you all the answers.
How Much Is Too Much Phosphorous in Your Soil?
If your soil has anything over 100 ppm (parts per million) you may need to take remedial action. Levels above 150 to 250ppm can take between three to five years to correct, and, if you have levels above 350ppm, you are likely to require expert help, which will take even longer to correct
How to Reduce Phosphorous Levels in The Soil
Testing the soil is crucial to understanding the problem. Indeed regularly testing your soil is advised so that these problems can be prevented. On testing, even if there appear to be adequate levels of zinc and iron, the phosphorous is likely to be preventing them from being absorbed by the plants. You will need therefore to increase the level of zinc and iron to rebalance.
- This first step is to avoid adding any further phosphorous to the soil.
- The second step is to add ‘foliar’ iron and zinc appendments, you will need to do this every couple of weeks
- If nitrogen is needed use zero phosphorus fertilizers
Too Much Nitrogen in the Soil
Too much compost can also result in too much nitrogen in the soil and although this can be more easily rectified it can cause considerable damage to the grass while it lasts.
Nitrogen is an essential nutrient for growth. An accessible amount of nitrogen present in the soil is essential for the growth and productivity of plants; in its absence, plants cannot survive. If nitrogen is not made accessible to plants, through either nitrogen-fixing soil microorganisms compost or by the use of fertilizer, crops will not grow as productively.
The problem is that too much nitrogen can be detrimental to plants. It can cause leaf burn and root rot and can ultimately kill the plant.
Excessive nitrogen often results in a chemical imbalance in the soil which can affect the way water is used by the plant. The osmosis process is changed and instead of the plant drawing water from the soil, the water is drawn to the nitrogen, leading the plant to suffer from dehydration.
Spotting excess nitrogen as being responsible for plant damage is harder. The yellowing of the leaves and stunted roots could be the result of various problems so the most obvious way to spot that nitrogen is the cause is through soil tests.
How Much Is Too Much Nitrogen in Your Soil?
There should be around 40ppm (parts per million) of nitrogen in healthy garden soil. The advantage of nitrogen over phosphorous is that nitrogen can disperse quickly combining with the oxygen in the air or can be easily washed away. Thus, unless you have much bigger levels you should be fine
If you start to get levels in excess of over 150 to 200 ppm then you could start to see problems. It is likely that these will be in concentrated areas. A good example would be where a dog constantly urinated on the same grass spot, you are very likely to see the patch turn yellow-brown, which is a sign of leaf burn.
How to Reduce Nitrogen Levels in The Soil
In that nitrogen dissipates fairly easily, unlike phosphorous, it shouldn’t be a long-term problem. What doesn’t turn to gas or wash away can be dealt with fairly easily.
Organic Solution to Reducing Nitrogen Levels in Soil
There are several ways to reduce excessive nitrogen levels in soil, perhaps the easiest being to put down mulch.
Covering the soil with a carbon-rich mulch will draw nitrogen out of the soil. The nest mulch would be wood chips as it won’t decompose as rapidly as other types of mulch. You would put down between one and two inches of mulch which you would leave in situ for between two and three months.
At this point test the soil to make sure it has returned to under 100ppm of nitrogen. You can then till the soil and add nitrogen-hungry plants such as cabbage, corn, squash, tomatoes, and sunflowers to name but a few.
Perennials can also be used. These plants do not need to be replanted for at least two or more growing seasons. Because they lengthen the growing season over which water and nitrate are drawn from the soil, perennial crops are the least leaky type of plant. This allows them to considerably lower nitrate burdens. The vast majority of perennial crops are able to be cultivated almost anywhere so are a good solution if you don’t want to put down mulch.
Chemical Solution to Reducing Nitrogen Levels in Soil
There are chemical solutions that can be used to reduce the nitrogen levels and rebalance the soil, but you are unlikely to need to use these and if you have to consider them you will need to take expert advice.
Too Much Compost Can Cause Salt Toxicity
In the same way, too much phosphorous and nitrogen can be harmful, an excess of other soluble salts can also cause problems.
A large amount of excess soluble salts can be caused by too much compost could induce salt poisoning is another problem that might arise in soils that get an excessive amount of compost. This is especially the case where there is little opportunity for leaching which can allow soluble salts to build up to dangerously high levels.
This can particularly become a problem where the compost is made from manure, as this generally has a greater concentration of salts than compost made from leaves and garden waste materials.
Dealing with Salt Toxicity
Testing the soil is the first step in correcting the problem caused by applying an excessive amount of compost. A good soil test should provide readings for soluble salts, calcium, magnesium, sodium, and ammonium in addition to the basic series, which consists of pH, organic matter, phosphorus, and potassium.
Environmental Repercussions for Using too Much Compost
The real ‘dark side’ of compost comes from the effect that overuse can have on the environment. It is not just plants that can be hurt. If you use too much or the wrong type of compost, there is a considerable environmental knock-on effect. In particular, both phosphorous and nitrogen can have considerable detrimental effects on wider plant and animal life, especially by seeping into groundwater and through runoff, into rivers.
The movement of excessive amounts of phosphorus, potassium, nitrogen, and other salts in runoff water is harmful to the environment because it poses a danger to the quality of the water supply. The pollution of water sources such as groundwater as a result of these minerals contained in compost and fertilizers damages the natural equilibrium in a variety of different ways.
Nitrogen is a very difficult element. It disperses easily through the air, through the ground, and through runoff, which can lead to gardeners overcompensating to maintain soil nitrogen levels. The double effect is that nitrogen is a greenhouse gas and also poisons the water supply, killing both plant and animal life.
In terms of phosphorus, phosphate is a limiting factor for aquatic plants, too much phosphate present in the water will result in the growth of eutrophication (algae). The subsequent rise in bacterial activity that occurs as a consequence of eutrophication leads to decreased amounts of oxygen in the water resulting in the death of fish and other water-dependent animals.
It is true that these problems are much more serious in terms of farming but if you took as a collective all the residential users of compost and fertilizers and even if only a fraction of these used too much compost you still have a huge environmental knock-on effect.
Summary: Can Too Much Compost Hurt Plants
There is no doubt that overuse of compost, particularly commercial compost can cause harm to your plants, and lawn and contribute to damaging the wider environment as a whole. The addition of amendments, such as compost, is of course very important to the health of your plants and for maintaining the soil structure and ecosystem.
In truth it is probably going to be difficult to cause too much damage by composting, it is unlikely you would put down that much. The problems generally lie in the existing mineral content of the soil. If minerals such as phosphorous exist at elevated amounts before you compost, it is here that you are likely to find trouble.
Thus, prevention is by far the best way of avoiding over-composting. A gardener should always test their soil at least once a year, or if you find problems, several times a year. By testing your soil regularly, a whole plethora of growing problems, not just problems caused by over-composting, can be avoided.
 Missouri State Coordinator, Steve Callis: Another Short History of Composting
 Purdue Agronomy-Turf Science, Cale A. Bigelow, William T. Tudor, and Jared R. Nemitz: Facts About Phosphorus and Lawns