Over-Fertilization

Over-fertilization is an increasingly common problem in home gardens.

Borage leaves exhibiting over-fertilization (Kate Russell)

It happens all the time. Your plants start out doing so well. Then they lose some of their vigor. You might see chlorosis (yellowing), cupping, less fruit production, or simply a failure to thrive. What is a gardener to do?


The traditional response was to add more fertilizer, manure, or aged compost. And it would work. For a while. Then those same symptoms would return, motivating you to add more fertilizer. And more. And more. Until it reaches the point where no matter how much fertilizer you add, your plants are simply not performing well. In fact, they seem more prone to pest infestations and diseases. How can this be?


Balanced plant nutrients

Just as we must eat a balanced diet to stay healthy, plants need access to a balance of nutrients. This is true partly because those nutrients are absorbed at the molecular level, as cations and anions, according to their electrical charge. Too many of one charge or the other makes it difficult for plants to absorb what they need. Also, some minerals, such as iron, are needed to absorb and use other nutrients. If there isn’t enough of these nutrients, or if they are made unavailable due to an imbalance, your plants can starve while sitting at a banquet. Mulder’s chart provides an image of what those nutrient relationships look like.


Toxic nutrients

Too much of a good thing can be a bad thing. In the same way. too much of a nutrient can lead to toxic levels. Phosphorus, for example, is critical to plant growth and photosynthesis and it tends to bind tightly to soil particles. Phosphorus toxicity can lead to severe stunting and it blocks plants from absorbing iron and zinc. 


Potassium is critical to enzyme reactions and water and mineral movement within a plant, helps prevent diseases, and regulates the rate of photosynthesis. Potassium toxicity causes leaf distortions, chlorosis, and yellowing along leaf margins. Potassium toxicity can cause calcium, nitrogen, and magnesium deficiencies.


Similar problems occur when there is too much of any nutrient. Compounding the problem, these excess nutrients often leach into rivers, streams, and groundwater, causing algae blooms that kill fish and create ripples of pollution and threats to biodiversity.


Too much of any one nutrient can throw a monkey wrench in the works. Too much of several nutrients can take years to resolve.


Is your soil over-fertilized?

The first step it to get a soil test. You don’t know what is in your soil without a soil test from a reputable lab. Sadly, those colorful over-the-counter soil tests are not accurate enough (yet) to be really useful. Many universities offer inexpensive soil tests. These tests can save you time and money and help your plants be healthier.


Below, you can see my soil tests from 2015 and 2019. In 2015, I learned that the property we bought had been over-fertilized for a very long time. Phosphorus and magnesium levels were critically high, and there was too much of pretty much everything. Except iron.

2015 Russell soil test (Kate Russell)

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Remember what I said about iron and nutrient absorption? Yep, my plants had been sitting at a feast, unable to do more than nibble. And it showed. The plants in my landscape were prone to fungal disease, borers and other insects, and they simply were not thriving.


For four years, I thought I was doing better. I added a little iron. I avoided using any fertilizer, besides blood meal and ammonium sulfate (for nitrogen). But I continued to add aged compost to help reduce my compacted soil. My compost is mostly made up of chicken coop bedding. It ends up that chicken poop contains very high levels of nitrogen, potassium, phosphorus and calcium. While my plants needed the nitrogen, they certainly didn’t need any of the other nutrients.

2019 Russell soil test (Kate Russell)

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How to correct over-fertilization

Looking at the results of my 2019 soil test, I realized that I hadn’t done nearly enough to correct my over-fertilization problem and had wasted 4 years in the process. Now, to correct the problem, I have stopped using my nutrient-rich compost on the ground. Instead, I am saving it for raised beds and container plants until my over-fertilization problem has been corrected.


And that’’s the cure - stop adding nutrients. The other half of the cure for over-fertilization is to remove nutrients by taking plant material out of your yard completely. Instead of grasscycling, bag and remove grass clippings. Or, you can add them to the compost pile or feed them to your chickens. Avoid using the chop and drop method for a while. Plant more heavy feeders, such as asparagus, beans, beets, broccoli, Brussels sprouts, cabbage, carrots, celery, corn, cucumbers, eggplant, garlic, leeks, melons, okra, onions, parsnips, peas, peppers, potatoes, pumpkins, shallots, squash, tomatoes, and turnips. And harvest those crops to within an inch of their lives. Take everything they have to give and get it out of your yard.


Armed with the results from my more recent soil test, I am now adding far more iron, to help my plants absorb what they need, and using wood chip mulch to counteract the compacted soil, but these actions will take time to have an effect. To monitor the effectiveness of these new actions, applying more iron and removing more plant material, I will switch to annual soil tests until the over-fertilization problem has been resolved. 


I urge you to do the same.