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 that 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 not performing well. 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. Plants absorb nutrients at the molecular level as cations and anions. Those are positively and negatively charged particles, respectively. Too many of one charge makes it difficult for plants to absorb what they need. Also, some minerals, such as iron, are critical components of the absorption process of other nutrients. If there aren’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 binds tightly to soil particles. Phosphorus toxicity can lead to severe stunting. It can also block the absorption of 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. And 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 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 is 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 do you any good. Many universities offer inexpensive soil tests. These tests can save 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 long time. Phosphorus and magnesium levels were critically high, and there was too much of everything except iron.

2015 Russell soil test (Kate Russell)

Remember what I said about iron and nutrient absorption? Yep, my plants had been sitting at a feast, unable to get more than a nibble. And it showed. The plants in my landscape were prone to fungal disease, borers, and other insects, and none were thriving.


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

2019 Russell soil test (Kate Russell)

How to correct over-fertilization

Looking at my 2019 soil test results, I realized I hadn’t done enough to correct my over-fertilization problem. I had wasted four years in the process. To resolve the nutrient imbalance, I stopped using compost in my garden. Instead, I saved it for raised beds and container plants.


And that’s the cure - stop adding nutrients. The other half of that cure 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. Add the following heavy feeders to your garden to use up those excess nutrients: 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 within an inch of their lives. Take everything they have to give and get it out of your yard.


Armed with more recent soil test results, I added much more iron to help my plants absorb what they needed. And I switched to using wood chip mulch to counteract my compacted soil. 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 soil nutrient levels are balanced.


Your garden may have different issues, but only a lab-based soil test can tell you what your plants need.