The symptoms of overfertilization in plants are not always obvious. Check for discoloration and try to remember how frequently you fertilized the plant. This article will provide you with detailed descriptions and photos of the most common signs of overfertilization, as well as proven solutions to such problems. Let’s get started!
The leaves are the most common indication of an overfertilized plant. Yellow or brown edges, drying, white crust, and falling leaves are all obvious signs of overfertilization in plants. So, yes, it is possible to over-fertilize your plants. What can you do to help your plants survive? Can these symptoms be confused with those of other issues?
Too Much Fertilizer: Salt and Nutrient Unbalance
If you are growing herbs, primarily perennials (here is a guide to annual and perennial herbs), they will require mineral support at some point. Indeed, given the relatively closed environment in which they live, even the best potting soil will run out of those nutrients that your herbs require after 6/12 months.
Fertilizers are great because they contain macronutrients (Nitrogen, Potassium, and Phosphorus, or N-P-K) and micronutrients (more on whether you should use fertilizer on your plants in this guide).
However, more is not necessarily better. Overuse of fertilizers can result in two major issues:
- Excessive salinity level in the soil;
- Nutrient unbalance
Let’s look at why these are issues and what they can lead to.
Too Much Fertilizer Increases the Salinity
Excessive fertilization causes an increase in soil salinity, making it difficult for plant roots to absorb water. This will cause the plant to struggle and develop brown tips, indicating that it is being drowned.
Bulletin fertilizer (particularly nitrogen-based fertilizers) is a type of salt, according to the University of Michigan and Illinois. As a result, each fertilizer is distinguished by a salt index. This is a measurement of how salty the solution becomes after the fertilizer is added.
A quick table showing the salt index for some common fertilizers on the market is provided below. More information can be found in this source.
| Fertilizer | Salt index |
|---|---|
| Urea 46 | 74 |
| Ammonium Sulfate | 88 |
| Triple superphosphate 45 | 10 |
| 3-18-18 Fertilizer | 8.5 |
However, you may be wondering why the salt in the fertilizer can harm your plants. Water can be absorbed by salt. This means that a high level of salinity in the soil makes it increasingly difficult for your plant’s roots to absorb water. The salts cause the roots to dry out, making them far more vulnerable to disease.
When the salinity content of the soil becomes too high, the salty soil will begin to extract water from the plant itself. This is known as the burn effect.
As a result, your plant will begin to dry out. In this case, the symptoms will be:
Slow growth: Water, along with air, is required to build and circulate nutrients. The first symptom of less water is slow growth. If you catch this problem early, you have a good chance of saving your plant.
Root tips black and dry: Water extraction burns the roots, especially in the lower parts. They will become more rigid and their color will darken/blacken.
Wilting leaves: Leaves lose water in their leaf cells, which, like deflated balloons, causes the leaves to lose rigidity and become droopy.
Dry leaves: The leaves will dry and drop after wilting.
Yellow/brown leaves edge: Take care here. The yellowing will begin at the edge and then move inwards, depending on the severity of the problem. This is a common symptom of high salt levels. If you want to learn more scientific facts, search Chlorosis and look at this interesting resource from;
The white layer on the soil: This is basically salt from unused fertilizer. This can be found on the soil’s surface or in and around the pot. When you touch it, it feels crystallin and coarse (a bit like fine table salt).
Although there is no set rule for fertilizer salinity (it is determined by the amount and type of fertilizer used),
The Midwest laboratory recommends a salt index of less than 50 to limit the possibility of salt damage (depending on the type of soil and plant).
Related: How to Take Care of a Succulent Plant
How Do You Know if You Have Overfertilized? Signs
It’s interesting to note that plants can’t choose which nutrients to absorb. This means that if your plants are given a lot of nitrogen, for example, they will have little capacity to absorb other macronutrients like potassium and phosphorus. This has far-reaching consequences.
Indeed, if you overfertilize with an unbalanced fertilizer (such as a 20-0-0), your plant is likely to suffer from significant nutrient imbalance. Only for specific reasons are such highly unbalanced fertilizers justified (like potting soil that lacks that particular nutrient that the fertilizer is rich in).
What will happen if a nutrient imbalance occurs as a result of overfertilization?
Well, depending on the specific imbalance caused by over-fertilization, the symptoms can be numerous. Remember that toxicity (a nutrient that is more abundant than is tolerable) is frequently associated with nutrient deficiencies.
So, to keep things simple, I’ll talk about the effects of deficiencies and excesses of the three macronutrients in your soil. This will be based on extremely trustworthy sources, which I recommend you read if you want more detail/scientific information on the subject (this one and this one and this one). As it is slightly more unlikely to over-fertilize with micronutrients, these cover the most common cases of over-fertilization.
| Nutrient | Deficiency | Excess |
|---|---|---|
| Nitrogen | Yellowing is localized on older and lower leaves. In case of advanced deficiencies, such leaves start dying (brownish) from the tips margin. | Higher production of darker green leaves and more rigid stems. Leaves tips getting brown (like they were burnt). In the case of significant excess younger leaves can become yellow due to the inability to absorb other nutrients |
| Potassium | The leaves border becomes brown surrounded by a yellow corona. Also stems tend to be long and weak. In the case of advanced deficiencies, leaves will start curling and dying. | Potassium can lead to deficiencies of magnesium, iron, zinc, and manganese. These include older leaves start getting yellow margins but still with green veins (from lack of magnesium), very pale new leaves (lack of iron). Also, spot yellowing between veins in new leaves followed by brown spots over it (lack of manganese). |
| Phosphorus | Older leaves become darker or grey to then be covered by purple dots. Stems as well will develop a similar purple hue. This is due to the production of purple-colored stress-related chemicals. In case of advanced deficiencies, the leaf will yellow and die. | Phosphorus excess will reduce the absorption of copper and zinc. In case copper is deficient, leaf tissue tips and edges will be affected with the leaf getting a variety of dark colors such as blue, black or purple due to the production of colored stress-related chemicals. In case of a lack of zinc, new leaves will show yellowing and very narrow leaves. Leaves tips begin to turn dark. |
How to Fix Overfertilization: The Easiest Way
Let us begin with the most obvious suggestion: stop fertilizing. If you were fertilizing and noticed any of the above symptoms, please discontinue. Even if the problem was not the fertilizer, it is unlikely that your plant will suffer if you skip one fertilization cycle.
Avoid the mental trap of thinking, “My plants appear unhappy, so they may require more fertilizer.” This is an almost certain way to kill your plants.
If your plant exhibits one of the high-salt-related symptoms (which is very likely), you must manually remove such salt. How do you get rid of salt in potting soil? The most common methods are leaching and flushing.
Leaching: This is what I call a “plant shower.” Simply put, you water (a lot) your potted plant soil. As a result, the salt in it will dissolve in water and leak out. However, if you want to be successful, you must know how to do it correctly.
Timing is crucial. First, place your potted plant on a sink or any other location where you do not want (a lot of) water to run on your kitchen floor (it might get a bit dirty hence be prepared). You can also use a large pot (perhaps the one you use to clean the floor) to house your potted plant. This is something I usually do on my small flat balcony. Remember to place some bricks or other similar support beneath the pot to allow water to drain from the soil.
Then gather an amount of water that is roughly double the volume of your pot. There is no need to be precise here; this is simply a rule of thumb. Wait 10 minutes after pouring half of the water into the pot (not on the leaves, just on the soil). This is the amount of time it takes for salt minerals to dissolve in water. Then, pour the remaining water into the container to flush the now “mobile” salt. This is a natural process for outdoor plants as a result of rainfall, but not for potted plants.
Pro Tip: Check that the water is draining through the drainage holes in the container’s bottom. If not, use a toothpick to move the soil through the drainage holes and the top one.
Flushing: Many people might confuse leaching with flushing. These methods are, in fact, very similar. The only difference is that when flushing, chemicals are added to the water, which binds more easily with the salts. This will increase the rate of salt removal. For example, you can find the best flushing solution from Amazon here.
However, serious growers only use this technique on large outdoor fields before harvesting. I’d never heard of indoor gardeners taking this approach.
Repotting:
This practice is recommended to address the issue of overfertilization. Indeed, changing the soil entirely sounds like a great way to eliminate any concerns about mineral accumulation in the soil. This is only partially correct. Your plants will experience transplant shock every time you repot them. If your plants are already weak from overfertilization, this could kill them (more on the subject in 7 pitfalls that cause your basil to die after repotting).
As a result, I see this as more of a preventive measure. If you repot your plants once a year (this is the ideal time, as explained here), you drastically reduce the possibility of over-fertilization.
This is the simplest approach because you don’t need to know what specific nutrient imbalance your plants were experiencing.
Related: Easy Ways to Propagate Succulents: A Step-by-Step Guide
Plant Nutrient Functions and Mobility
Function: What is the nutrient doing? Is it encouraging flowering? Is it better to promote leaf production or root development? If you understand what they are doing, you will understand what happens when they are in excess or missing.
Mobility: Is the nutrient restricted to one area of the plant or can it move around? If a nutrient is not mobile, it will accumulate on older leaves (those at the plant’s base), whereas mobile nutrients will always migrate on new leaves. As a result, if your plants are lacking in a non-mobile nutrient, you will notice it on new leaves, and vice versa for mobile nutrients (the older leaves will be more affected).
The table below summarizes these.
| Nutrient (the symbol used in fertilizer label) | Function | Mobility |
|---|---|---|
| Nitrogen (N) | Production of plant proteins, responsible for growth, photosynthesis, and respiration. It is famous to be a major chlorophyll component that, in turn, is responsible also for the green color of your plant. It is also known for root growth nutrient | Mobile |
| Potassium (K) | This nutrient is responsible for plant respiration (stomata opening/closing), energy production, water uptake, and more in general photosynthesis regulators. | Mobile |
| Phosphorus (P) | It is important for energy production (sugar), root growth, and make the plant more drought tolerant | Mobile |
Related: Why Are My Hibiscus Leaves Turning Yellow?
Can You Over-Fertilize?
There are numerous products on the market (far too many, in my opinion) that can over-fertilize your plants. The truth is that you can over-fertilize almost anything. However, some products are far more prone to errors than others. This is because they are either extremely “nutrient dense” (N, P, or K levels greater than 30) or extremely “nutrient unbalanced” (like a 30-0-0). As a result, can you over-fertilize with:
Miracle Gro (such as this one on Amazon): with an N-P-K of 28-8-16, it is possible to have an excess of nitrogen (which can cause excessive green leaves, causing your plant to die, or brow tips);
Worm tea (for example, this good one on Amazon): this is worm casting in liquid form. There are numerous methods for producing it (interesting article here for more). This solution has a very low N-P-K value, for example, 0.1-0-0.1.
As a result, the risk of over-fertilization is extremely low. Indeed, the power of such fertilizer is provided not by readily available nutrients, but by the ability to produce them on-site when required. This is made possible by the large number of bacteria present in the solution.
Bone meal (such as this one on Amazon): bone meal contains more phosphorus (like a 2-14-0). As a result, it is recommended for root growth stimulators (in the early stages) and flowering plants (those used for decorative purposes like the purple basil). Even if the phosphorus content is low (only 14 for this product), over-fertilization is still possible. Indeed, as discussed in this study from Shandong Agricultural University, phosphorus is absorbed at a slower rate than nitrogen.
Fish emulsion (such as this good one on Amazon): this is quite unbalanced because it is mostly nitrogen. Because the content of all organic fertilizers is relatively low, As a result, the risk of over-fertilization is very low. Although the label suggests once a week, I would limit myself to once or twice a month, no more. Many plants, in fact, may not require such a large amount of nitrogen.
Blood meal (such as this one on Amazon): this powder fertilizer is made from animal blood, which is a byproduct of meat production. This is by far the most nitrogen-rich organic fertilizer. 12-0-0 is a common N-P-K. Over-fertilization is still improbable, but not impossible. If not for really nitrogen-deficient potted plants, I would use this a few times a year, especially at the start of a growing season.
Overfertilize During Winter? More Likely
Plants are living organisms with their own rhythms, just like humans. This implies that at times you may be less active and thus require fewer nutrients. Consider the winter season, when long hours of darkness and cold reduce (or even completely stop) plant growth.
In this case, you will need to fertilize (or even water) your plants for an extended period of time. You are very likely to over-fertilize your plants if you do not respect such rhythms. This is quite common because many people begin fertilizing during the growing season (spring-summer) and continue at the same rate during the winter, despite the fact that they require far less fertilizer.
Related: Easy Ways to Propagate Succulents: A Step-by-Step Guide
Overfertilized? Four Easy Checks You Can Do Right Now
This, in my opinion, is the most crucial question of all. Indeed, overfertilization is easily confused with other issues. According to Ann McCauley, a soil scientist at the University of Montana:
“Potential factors causing pseudo deficiency include, but are not limited to, disease, drought, excess water, genetic abnormalities, herbicide and pesticide residues, insects, and soil compaction”
As a result, before concluding that overfertilization is to blame, you should conduct the following tests:
Water: Except for the white crust on the soil, underwatering causes the same problem as excess salinity in the soil. Overwatering, as strange as it may sound, can also cause similar symptoms. As a result, all of the preceding checks should be repeated.
- Checks: Insert your finger 1-2 cm (0.4-0.9 inch) beneath the soil’s surface. This will give you a good idea of how much water is available to your roots. Roots, in fact, are not a soil level (usually drier than below the soil). How frequently do you water your plant? Have you forgotten about it for a long time (perhaps because you were in a faction)? Are you watering as you did in the winter during the summer? Is your plant thirsty (for example, basil)?
Soil compaction: This fancy term means that your potting soil has become overly compacted. This is normal because the water you pour week after week gently pushes the soil down. As a result, the soil pores will be reduced, making it more difficult for your plant to breathe. Suffocation of this type can cause leaf yellowing or browning, among other symptoms.
- Checks: Simply move the soil on the surface and through the drainage holes with a chopstick. Is the soil difficult to move? If this is the case, compaction is a problem that you are addressing by removing the soil. Take care not to damage the roots during the process. While a few roots will not harm your plants, if you cut too many while moving soil, your plants may suffer.
Sun: This is not a problem for some people. However, if you leave in a sunny country with long and hot summer days with 10 hours of the scorching sun, your plant’s leaves may be burned.
- Checks: Is the plant’s damage limited to the side that receives direct sunlight? If this is the case, simply move your plant away from the window. If not, then the sun is not to blame.
Soil pH: This changes over time due to the natural decay of organic matter. As a result, without your knowledge, your potting soil may be highly acidic (low pH). This affects how nutrients are absorbed by your plants, as explained in this guide. Some nutrients may become less available, while others may become more available, resulting in intoxication and symptoms similar to overfertilization.
- Checks: A test is the simpler and more reliable method. As discussed in this pH tester guide, for a few dollars, you can have a simple device that gives you reliable information on the pH level simply by sticking it into the soil. If this is the case (below 5.7 or above 7), you must address it. The University of California published a comprehensive guide on the subject. You can also find some interesting videos on YouTube, such as this one.
When Should You Fertilize Your Plants?
Fertilizing your plants is essential for providing them with the minerals they require for healthy and consistent growth.
The catch is this: carefully read.
Most of the time, you will end up over-caring for your plants, providing fertilizer when it is not required. Although it is true that your plants require minerals, these requirements vary throughout the year and according to plant growth.
