You always heard about the great benefits of fertilizer, and so you bought one for your green friends. However, after a few weeks, your herb starts looking unhealthy. You are likely overfertilizing. However, how can you be sure that this is the case? This article has you covered with detailed descriptions and photos of the main sign of overfertilization, and proven solutions to fix such issues. Let’s dive in!
Hence, what are the warning signs of overfertilization? The 10 most common symptoms of overfertilization (can appear singularly or as a combination) are:
- Leaves yellowing/browning at the edges
- Leaves drying out
- A white crust on the soil surface
- Leaves fall
- Roots tips black and dry
- Stunted/stopped growth
- Darker and more numerous leaves
- Dark-colored dots
Now you know all the symptoms that your herb can show in case of overfertilization. However, what do they actually look like? What can you do to save your herb? Can these symptoms be confused with other problems? Keep reading for this and more information.
If you are growing herbs, mainly perennial (here a guide on annual and perennial), they will require, at some point, your support for minerals. Indeed, given the relatively closed environment in which they live, even with the best potting soil, after 6/12 months will run out of those nutrients, your herbs need to thrive.
Fertilizers are great as packed with macronutrients (Nitrogen, Potassium, and Phosphorus, the N-P-K) and micronutrients (more on this guide on whether you should use fertilizer on your herbs) that your herbs need to thrive.
However, more is not better. Indeed, overdoing with fertilizers can cause two main problems:
- Excessive salinity level in the soil;
- Nutrient unbalance
Let’s see why these are problems and what they can cause.
As clearly stated by the University of Michigan and Illinois, bulletin fertilizer (especially the heavily nitrogen-based) is a type of salts. That is why every fertilizer is characterized by the so-called salt index. This is a measure of how much salty becomes a solution to which the fertilizer is added.
Below you can find a quick table showing you the salt index for some common fertilizers you can see in the market. You can find more in this source.
|Triple superphosphate 45||10|
However, the question you might have is: why can the salt in the fertilizer damage your herbs? Salt can absorb water. This implies that a high level of salinity in the soil makes it harder and harder for your herb roots to absorb water. The salts cause the roots to dry up, becoming way easier victims of diseases. For instance, as discussed in this scientific publication, Cercospora Leaf Spot is a disease that only plants lacking in nitrogen are affected.
At some point, when the salinity content is too high, the salty soil will start extracting water from the plant itself. This is the burn effect.
As a consequence, your plant will start drying out. The symptoms, in this case, will be:
- Slow growth: water, with air, is essential to build and circulate nutrients. So, with less water, the first symptom is slow growth. If you manage to spot this problem first there are high chances you can save your herb;
- Root tips black and dry: the roots are burned by the extraction of water, especially in the lower parts. They will become rigid, and their color will get darker/black;
- Wilting leaves: leaves lose the water in their leaf cells that like deflated balloons will make the leaves lose their rigidity making so them droopy;
- Dry leaves: after wilting, leaves will dry and drop;
- Yellow/brown leaves edge: be careful here. The yellowing will be mainly in the edge and then, depending on the gravity of the problem, moving inwards. This is a typical sign of excess salt content. If you want to know more scientific fact, just google Chlorosis and check this interesting resource from;
- The white layer on the soil: this is essentially salt leftover from unused fertilizer. You can spot this on the soil surface or and around the pot. It feels crystallin and coarse at touch (a bit like fine table salt).
Although there is no rule about the fertilizer salinity level (this depends on the amount and type of fertilizer,
the type of soil and herb) the Midwest laboratory suggests a salt index below 50 to limit the probability of salt damage.
It is interesting to know that plants are not able to select which nutrients to absorb. This implies that if your herbs are provided with a large amount of nitrogen, for instance, they will have little capacity left to absorb the other macronutrients such as Potassium and Phosphorus. This has significant implications.
Indeed, if you overfertilize with a very unbalanced fertilizer (like a 20-0-0), there are high chances that your herb will end up with a significant nutrient unbalance. Such kind of highly unbalanced fertilizer are justified only for specific reasons (like a potting soil that lacks that particular nutrient that the fertilizer is rich)
What is going to happen in the case of nutrient unbalance due to overfertilization?
Well, the symptoms here can be numerous, depending on the specific unbalanced created by over-fertilization. Remember that toxicity (like one nutrient more abundant than tolerable) is often associated with deficiencies of other nutrients.
So, for simplicity, I will discuss the effect of deficiencies and excess of the 3 macronutrients in your soil. This will be based on extremely reliable sources that I suggest you reading if you want more detail/scientific information on the matter (this one and this one and this one). These cover the most common cases in case of over-fertilization as it is slightly more unlikely to excess with micronutrients.
|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||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.
Let’s start with the most obvious of the suggestions: stop fertilizing. If you were fertilizing and you start noticing any one of the above symptoms, please stop. Indeed, even in the case, the problem was not the fertilizer, it is improbable that your herb will suffer if you skip one fertilization cycle.
Avoid the mental trap: “my herbs look unhappy so they might need more fertilizer.” This is a sure path to kill your herbs.
If your herb is suffering from one of the high-salt related symptoms (very likely), then you need to remove such salt manually. How can you remove salt on potting soil? Leaching and flushing are the most common methods.
- Leaching: I call this a “plant shower.” Essentially you water (a lot) your potted herb soil. In this way, the salt in it will dissolve in water and leak away. However, you need to know how to do it properly if you want the highest success.
Timing is important. First, bring your potted herb on a sink or any other place as you do not wish to (lots of) water 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 might be using to clean the floor) and place your potted herb inside. I usually do this on my little flat balcony. Remember to put some bricks or any similar support below the pot so the water can leak out from the soil.
Then gather around an amount of water around double the volume of your pot. No need to precise here; this is just a rule of thumb. Pour half of the water in the pot (not the leaves, just on the soil) and wait 10 minutes. This is the time salt mineral needs to dissolve into the water. Then, pour the remaining water to flush the now “mobile” salt. This is a natural process for outdoor herbs due to rainfall but not the case for potted herbs.
Pro Tip: Make sure the water is going through the drainage holes at the bottom of the container. If not, move the soil through the drainage holes and the top one with a toothpick (be careful not to harm the roots).
If you have a passion for science, you should be able to “see all the salts” you extracted by simply boiling the water drained down from the potted herb, as shown in this interesting YouTube video.
- Flushing: you might find many people confusing leaching with flushing. These techniques are indeed very similar. The only difference is that in flushing, you add chemicals to the water that binds more easily with the salts. This will increase the salt rate of removal. For instance, here, you can find the right flushing solution from Amazon.
However, this technique is only adopted by serious growers on large outdoor fields, before harvesting. I hardly ever heard about indoor gardeners using such an approach.
- Repotting: this practice is recommended to solve the overfertilization problem. Indeed, changing the soil altogether sounds an excellent idea to remove any question of mineral accumulation in the soil. However, this is only partially true. Indeed, each time you repot, your herbs will go through a transplant shock. This might cause your herb to die if already weak due to the overfertilization effect (more on the subject in 7 pitfalls that cause your basil to die after repotting).
Hence, I do see this more as a preventive approach. If you repot your herbs every year (this is the ideal repotting time, here explained why), you are reducing the possibility of any over-fertilization issue dramatically.
This is the simplest of the approach as you do not need to know what specific nutrient unbalance your herb was suffering. By leaching or flushing, you are resetting the soil nutrient content as detailed in this study.
Hence, after leaching, my suggestion is to not fertilize for 2 to 4 weeks, depending on the gravity of the situation to then adopt a modest fertilization regime according to this fertilization guide.
At this point, you learned how your herbs look like and what to do in case they get over-fertilized. However, I did not tell you why! This is important if you want to be able to spot all symptoms with a glance of an eye. This is related to two nutrient features whose balance is messed up by overfertilization:
- Function: what the nutrient is doing? Is promoting flowering? Is promoting leaves production or root development? If you know what they are doing, you will know what is happening when they are in excess or missing.
- Mobility: does the nutrient stay in a specific area of the plant, or can it move around? If the nutrient is not mobile, it will accumulate on older leaves (the one at the bottom of the herb), while mobile nutrients will always migrate on new leaves. Hence, if your herb has a lack of a not-mobile nutrient, you will notice on new leaves, and vice versa for mobile ones (the older leaves will be more affected).
These are summarised in the table below.
|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 colour of your herb. 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 regulator.||Mobile|
|Phosphorus (P)||It is important for energy production (sugar), root growth, and make the plant more drought tolerant||Mobile|
There are countless products (too many, in my opinion) available in the market through which you can over-fertilize your herbs. The truth is that you can over-fertilize with essentially everything. However, with some products, it is way easier to make mistakes than others. This is because they are very “nutrients dense” (N or P or K higher than 30) or very “nutrient unbalanced” (like a 30-0-0).
Hence, can you over-fertilize with:
- Miracle Gro (like this one on Amazon): having an N-P-K of 28-8-16 it is possible to lead an excess of nitrogen (this can cause excessive green leaves to then let your plant die after, or brow tips);
- Worm tea (like this good one on Amazon): this, in simplistic terms, is worm casting in liquid form. There are various techniques on how to produce it (interesting article here for more). This solution has a very low N-P-K, such as 0.1-0-0.1.
Hence, the possibility of over-fertilizing is very low. Indeed, the power of such fertilizer is not given by the readily available nutrients but on the capability to produce them on-site when needed. This is possible thanks to a vast plethora of bacteria present in the solution.
- Bone meal (like this good one on Amazon): bone meal is higher in phosphorus (like a 2-14-0). Hence, it is suggested for root growth stimulator (in the early stage), and flowering herbs (those used for decorative purposes like the purple basil). Even if the phosphorus content is relatively low (only 14 for this product) it is still possible to over-fertilize. Indeed, as also discussed in this study from the Agricultural University of Shandong phosphorus is absorbed in a lower rate than nitrogen;
- Fish emulsion (like this good one on Amazon): this is quite unbalanced as it contains mainly nitrogen. As all the organic fertilizer is relatively low in content (5-0-0 in case of the one here suggested). Hence, the possibility of over-fertilizing is quite slight. Although the label suggests once a week, I would go for once or twice a month, no more to be honest. Indeed, many herbs might not require such a large amount of nitrogen.
- Blood meal (like this good one on Amazon): this powder fertilizer is obtained from animal blood produced as a byproduct of meat production. This is by far the highest organic fertilizer rich in nitrogen. A typical N-P-K is 12-0-0. Still low enough to make over-fertilization unlikely, but not impossible. I would apply this a few times a year if especially at the beginning of a growing season if not for really nitrogen-deficient potted herbs.
Herbs are living organism that, a bit like humans, they have their own rhythms. This implies that in some periods might be less active and consequently need fewer nutrients. Think about the winter season, when long hours of darkness and cold reduce (or can even stop, dormancy) the herb growth.
In this case, you do need to fertilize (or even water) your herbs for a long while. If you do not respect such rhythms, you are very likely to over-fertilize your herbs. This is quite common as many starts fertilizing during the growing seasons (spring-summer) and keep the same pace during winter, while in reality, they need way less (or nothing).
For more on when and how often fertilize to avoid over-fertilization, have a look at this fertilization guide for herbs.
Overfertilized? Four Easy Checks You Can Do Right Now
In my opinion, this is the most important question of all. Indeed, it is very easy to confuse overfertilization with other problems. As stated by the same Ann McCauley, a soil scientist from 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 compactionSource
Hence, before deciding that overfertilization is the culprit, you need to perform some other checks:
- Water: underwatering cause the same problem of an excess salinity in the soil, except for the white crust on the soil. As strange as it might sound, also overwatering can cause similar symptoms. So all the checks mentioned above should be repeated.
Checks: Stick your finger 1-2 cm (0.4-0.9 inches) below the soil surface. This will give you a clear idea of the level fo water available for your roots. Indeed, roots are not a soil level (usually drier than below the soil). How often are you watering your herb? Have you forgotten it for a long while (perhaps you were in faction)? Are you watering as you were doing in winter while in summer? Is your herb water-demanding (basil, for instance)?
- Soil compaction: this fancy term means that your potting soil got too compact. This is normal as the water you pour over time, week after week, gently push the soil down. This, in turn, will reduce the soil pores making it more difficult for your herb to breath. Such suffocation can cause leaf yellowing or browning, among other symptoms.
Checks: just move the soil thoroughly but gently with a chopstick on the surface and through the drainage holes. Is the soil hard to move around? If so, compaction is a problem, and you are actually solving it by loosing the soil. Be careful not to cut the roots during the process. A few roots will not harm your herbs, but if you cut too many while moving the soil, your herbs might suffer.
- Sun: this, fortunately (or unfortunately), is not a problem for me as I live in the north of the UK where a sun apparition is a rare event. However, if you leave in a sunny country, characterized by long and hot summer days with 10 hours of a scorching sun can burn the leaves of your herbs.
Checks: is the herb damage concentrated only the side exposed to direct sunlight? If so, then just move your herb away from the window. If not, then the sun is not the culprit;
- Soil pH: this over time tend to change, due to the normal decay of organic matter. Hence, without you even realizing, your potting soil can be very acid (low pH). As detailed in this guide, this affects the way nutrients are absorbed by your herbs. Some nutrients might become less available, while others might be released in higher amounts, causing intoxication and symptoms similar to overfertilization.
Checks: the easier and more reliable way is to perform a test. As discussed in this pH tester guide, with a few dollars, you can have a simple device that just by sticking into the soil gives you reliable information on the pH level. If this is the problem (below 5.7 or above 7), then you need to correct it. The University of California wrote an extensive guide on the matter. Also, on YouTube, you can find some interesting videos like this one.
When Should You Fertilize Your Herbs?
Fertilizing your herbs is important to provide them with the minerals they need for an healthy and steady grow.
Here is the catch: read carefully
More often than not you will end up overcaring for your herbs providing fertilizer when no needed. Indeed, althought it is true that your herbs need mineral, this requirements changes through year and accordingly to the plant growth.
Hence, what to do? Well, check the article below (click on the image) for a detailed guide on how I fertilizer my indoor herbs!
Is it possible to over-fertilize flowers? Yes, this is a common problem, especially among beginner gardeners in applying fertilizer too frequently, at the wrong time when not needed or at a concentration higher than recommended.
Are over-fertilized herbs more susceptible to diseases? Yes, the excess of fertilizer and consequent nutrient imbalance will weaken the plant’s immune system making it more susceptible to bugs and disease.
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