Basil Soil pH – How Does it Affect Growth

The soil ph is an important factor in growing herbs such as basil as well as any other plants really. Among gardeners is quite often the most neglected factor as it is harder to measure. Nevertheless, an inadequate soil pH level can stunt plant growth independently from watering and lighting conditions.

The ideal soil pH for basil is 7 (neutral). However, anything between 6 and 7.5 according to experiments will still allow any basil type to thrive. The soil pH profoundly affects the growth of plants herbs by allowing/preventing the release of minerals that can cause yellowing leaves, burns, black spots.

The pH hence has a crucial role in keeping alive your plants. In this article, you will learn what is really happening to your soil in case of an off-balanced pH change. You will be able to identify the first symptoms early in time, test the pH level, and understand if your herbs are at risk. Being able to know how to save your herbs is also essential and a knowledge that every gardener, even beginners, must have. 

What is the Soil pH?

You might already know (also some of my previous articles) that a pH is a number varying from 0 to 14 indicating the concentration of particles called “hydrogen ion” in soil. The lower the pH number, the higher the concentration of hydrogen ions in the soil, and so the more acidic it is. 

Hence, a pH of 1 (acid as a car battery) implies a very high concentration of hydrogen ions. A pH of 14 (the most upper extreme, like bleach you might use for cleaning) indicates a soil extremely alkaline (lowest concentration of hydrogen ion).

Scientists chose this “particle” as very important in many biological processes (as in the soil as in our bodies!). Indeed, the pH level, among others, affects how nutrients and metal get dissolved in water. As you can guess this is vital for herbs as they absorb them only in this state (dissolved in water). In other words, your soil can be rich in nutrients, but if not soluble in water (because of the pH outbalance) these will be out of reach for your herbs. You can easily and inexpensively check the pH of your soil to check if you have any issues to address.

Nutrients, Metals, And Bacteria in Healthy Soil

Herb’s roots absorb, through water, what is present in the soil: metals and nutrients. Herbs, like every other plant, need:

  • Major nutrients: these are pretty famous among gardeners. They are Nitrogen, Phosphorus, Potassium. Any herbs need them in an adequate amount. They are not interchangeable. They need to be present all at the same time.
  • Secondary Nutrients: Calcium, Magnesium, and Sulfur. Herbs need only a minimal quantity of them. However, them as well must be in the soil.
  • Others: Iron, Manganese, Boron, Copper, Zinc, and Aluminum are all metals present in the soil and beneficial for your herbs. For instance, copper is essential for all the photosynthetic processes, while zinc also helps in enhancing the soil structure. However, this is true if their concentration is within limit. An essential change in the pH will affect the concentration of such metals, making the soil even poisonous for your herbs.

The image below, extract from scientific research on the subject, shows how the availability of such substances varies with the pH.

Level of nutrients and metals in the soil for different pH level (on top of the figure)

Each horizontal bar represents a nutrient/metal, and its thickness is proportional to its availability in the soil. The more significant, the higher the availability. The horizontal scale is a pH scale: from left to right, the pH level is increasing (from pH 4 to pH 10 as you can notice at the bottom of the figures).

  • Bacteria: However, the above graph is only a fraction of the story. Healthy soils are also characterized by their massive population of helpful bacteria. These, (in healthy condition) provide nutrients to your herbs to survive.

For instance, you know that nitrogen, one of the significant three nutrients for herbs, is everywhere in the air you and I breathe? However, herbs, as well as any plants, are not able to use nitrogen from the air. They need it dispersed in water (from rain) or fixed in the soil through a unique process performed by special bacteria called nitrogen fixator.

Another type of essential bacteria are the “decomposers”. These bacteria break down organic matter, making many nutrients available to plants. Again, extreme pH makes them challenging to thrive and so reducing the nutrients available for your herbs.

The ideal pH for the majority of herbs (and plants) is anything from 6 to 7.5. In this perfect range, all nutrients (major and secondary) are available at their fullest. Moreover, at this pH, bacteria thrive and can multiply without issues.

How pH Affects Nutrients, Metals, And Bacteria

Nutrients: When the pH starts getting below 5.5, you can notice that the horizontal bands of the 3 major and 3 secondary nutrients get thinner in the figure, such nutrients get locked in the soil. This is because herbs need nutrients to be dissolved into the water first (soluble). However, these cannot physically happen if the pH is too low or too high. The only exception is for calcium in very alkaline soils.

Others: Manganese, aluminum, and Iron (although not shown in the picture) get more available in acidic soil. Given what I explained before, you might think this is good news. No, it is not! Their concentration tends to rise above what an herb can tolerate becoming so poisonous. In the case of alkaline soils, the opposite problem arises. Indeed, their concentration starts dropping way below what a healthy herb need to thrive.

Microbes: The above are only a few of the critical bacteria that need to be present in healthy soil. The majority of them can only thrive with a pH from 6 to 7.5. Everything about this godlike zone will experience a real struggle.

Visual Symptoms on Plant of A Wrong pH

All the most common visible symptoms of a wrong pH are a consequence of nutrient deficiencies. There is a massive variety of symptoms that will be discussed in detail in a future article. However, to have you covered, here some of the most common to watch out:

  • Yellow leaves: this is often due to a lack of nitrogen
  • Too Dark Leaves: leaves too green is, as well, a bad sign. Lack of phosphate quite often; 
  • Yellow borders: this is generally due to a lack of potassium;
  • Yellow patches: lack of manganese is the culprit
  • Yellow border but green veins in older leaves: this might be a sign of Magnesium deficiencies;
  • Yellow new (top) leaves: iron deficiencies affect new leaves;
  • Roots: if you are so brave that you want also to inspect the root system, few studies highlighted that higher than normal concentration of aluminum might prevent the roots from fully develop (case of acidic soils);
A change in the leaves color is often signal of a nutrients deficiency, potentially triggered by an inadequate pH

This quick checklist is very hand if you want to understand if there is any problem with your soil pH. However, bear in mind that a lack of nutrients is not only due to a pH out of balance. Indeed, it can be simple that your way to fertilize (or perhaps you never did it) your potted herbs is not ideal (it might happen, if you use a random commercial fertilizer, that you are exceeding perhaps in nitrogen causing deficiencies in other nutrients as discussed for the basil here).

Moreover, the symptoms described above are generally associated with a lack of nutrients, that even in the case is due to a wrong pH, can be due to an excessively high or low pH level. Indeed, from a visual inspection, especially for beginners, it is very hard to understand the real cause of the problem.

However, the important thing is to be aware that there might be a pH problem with your soil. After you understand that you are ready to test.

How To Test The pH Level?

Testing the pH level of your soil is the only way to be sure what the problem is, and, more importantly, how to solve it. Indeed, if the nutrition deficiencies are not due to the wrong pH, but you are unaware of it, you can easily make the situation worse.

There are several ways to test the pH of your soil. 

  • Laboratory analysis: One of the most reliable, although not my favorite for potted herbs is the lab tests. In Amazon, you can easily find one those kits in which you just need to fill with soil a small bag to send it via post, with the provided envelope, to the lab. Such tests also provide a detailed analysis of your soil nutrition content, an extensive health check.
  • Testing kit: for pH test, you have essentially two options: 1) probe version, discussed here 2) and the test kit version, discussed here. In this case, I recommend the 2) choice. This not because the probe is unreliable, but just because the kit gives a complete picture of your soil health, also providing the nutritional content and can be repeated many times. Suck kit can be easily found on Amazon. I do personally use the Rapitest brand (quite known among gardeners);
Here I am holding the 3 way tester I use to measure the soil pH
  • DIY test: these are the least reliable (and the cheapest) tests. They allow you to learn a lot and play a bit with chemistry. However, bear in mind that these tests will simply tell you if the pH is above or below neutral. They will not tell you how much of you are. The most common ones are those involving adding vinegar (1/4 cup) to a sample of soil (1 tablespoon).

    If the soil reacts with vinegar (bubble formation), than the soil is alkaline. If not, it might be alkaline. In such a case, you can add to a soil sample (1 spoon) a bit of distilled water and baking soda (¼ cup). If you notice any visible reaction (bubbles), then your soil is acidic. Others even suggest using cabbage water.
Someone also uses cabbage to measure the soil pH, there is no limit to creativity

How Can You Correct the Soil pH?

Hence, until now you learned:

  1. pH is important for your herbs wellbeing;
  2. The pH can cause nutrients problems;
  3. You have identified such problems;
  4. You tested your potting soil and found a pH problem;
  5. Now what?

Now you need to take action to solve this issue. Typically you will need chemicals that have a pH able to counterbalance the one of your soil.

In the case of acidic soil: you need to add a strongly alkaline material (very high pH). Among gardeners, limestone (pH=9.9) is widely used (you can easily find it on Amazon as well). This is a white powder that needs to be uniformly mixed with the potting soil. Hence, you need to transplant the plant first.

Tips: to avoid a plant transplant, you can add the needed limestone to water and use the solution so obtained in your plant. Remember: 1) avoid the contact with the leaves 2) Before watering you need to check that your soil is dry (to avoid overwatering) 3) I do suggest to move the soil with a toothpick (without damaging the roots), straight after, in order to better disperse the solution through the whole container 4) do not use too much water otherwise it will leak (with the required limestone) through the drainage holes.

Some gardeners as a general rule of thumb recommend half a cup of limestone for a large pot (like a 9’’). There is not an exact science for it. Another material widely used, if available for you, is wood ash. However, this should be obtained only from wood an nothing else. Indeed, you do not want dangerous chemicals to end up in your soil, your plant, and, ultimately, in your table if you are growing herbs for culinary reasons.

In the case of alkaline soil: you need to reduce your soil pH making it more acidic. A sustainable and effective solution is to add organic material like compost to your potting soil. This will not only provide nutrients but anything organic increases the acidity level of the soil. The main drawback is that such an approach, as the majority of the natural methods, it will take a long time (months) to produce effects. If your potted herbs are already struggling, this will be a too-long wait for them to withstand

Fresh coffee grounds (or unused) can be another natural strategy that you might have heard of. However, again, this as well might take time as the caffeine present need to be decomposed by the bacteria in the soil.

A way quicker solution is to use aluminum sulfate to mix with your potting mix. This produce (that can also be found on Amazon) will raise the pH immediately providing a fast help for your plants. However, as everything chemical, it must be used with caution.

Indeed, small quantities are sufficient, especially considering the little volume of soil you have in a pot. The exact amount is generally reported in the product instruction. This depends on the concentration by which the sulfate has been produced. 

Indeed, small quantities are sufficient, especially considering the little volume of soil you have in a pot. The exact amount is generally reported in the product instruction. This depends on the concentration by which the sulfate has been produced. 

Tip: In such a case, I would use around 70% of the amount recommended. That will make it very unlikely to burn your herbs, and you will definitely see results. Moreover, if still needed, you can add it more over time.

Some gardeners even suggest using sphagnum peat to be added on top of the soil of your potted herb. Also, natural acidic substances like vinegar are recommended (1 tablespoon for half a gallon of water). A similar effect can be obtained with lime juice. Their application is suggested every month until the soil does not reach the 6.5-7 pH level required for the majority of your herbs.

Also, you need to know that exists acidic fertilizer. These are used for acid-loving plants. However, you can use it for your very alkaline soil producing a boost of nutrients and solving the pH problem. A Good one is the Jobe’ one that can be found on Amazon.

Related Questions

Is pH dependent on volume? No, pH indicates a concentration of hydrogen ions, so, like concentration, is independent of the volume

Does the pH change with the location? In large fields or outdoor garden in general, this might happen. This is not the case for potting soil where the pH is essentially the same across the pot. Indeed, such a small amount of soil is exposed to the same conditions, and it comes, very often, from one single source (potting mix bag).

Further Reading

Best pH tester for your soil

How to test the soil of your herbs: full beginner guide

Interesting DIY test pH video

Article on Fertilizer for Basil

Educational (and funny!) video on pH and plants

21 tips to grow giant basil indoor

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