Test The Soil of Your Indoor Herbs: Full Beginner Guide


Testing the moisture and pH level of the soil of my basil plant (on a reused milk container box as discussed in one of my previous articles)

Chances are that sometimes you saw your little green friend(s) shade away without any clear explanation on the reason as you provided the right amount of water, sunlight and no pests were around. Well, in this case, the soil might be the culprit. Hence, even if you are a beginner gardener, you need to test the health of your soil, something that can be done inexpensively and can teach you a lot along the way.

Hence, how to test the soil of your indoor herbs? For pH level, moisture and sunlight (and sometimes temperature) small electronic devices are available in the market. For pH also strips are available. For nutrients tests, more expensive and complex kits that perform chemical analysis are generally required.

These are the possibility you have to test the main (not all, there are many more) health parameters of your soil. However, it is fundamental to know how to properly perform such tests to avoid misleading results. More on the above points in the following sections. Let’s dive in!

Soil Test For Your Herbs: 3-in-1 Tester

Testing your soil is extremely important, and once you realize that, you are a step ahead of many beginner gardeners that focus on watering and sunlight forgetting that herbs do not live only with air and light, they also need nutrients in the soil and specific soil conditions to thrive.

By far the easiest and more accessible tests you can perform are pH level, moisture, and sunlight. This because the market is flooded with small electronic devices called 3-in-1 soil tester that given their inexpensive price (a dozen of dollars the bottom category) are accessible to even the most amateur gardener.

This device, like the one shown in the photo below (with my funny face as well), is easily recognizable in the gardener shops as it presents a small (often green, original design choice!) plastic head with an analog indicator (a moving arrow essentially), a switch below and two metal rods coming out from it. 

This is the 3-in-1 tester that I normally use for herbs, they look more or less all the same

It is worth highlighting that exists also a 4-in-1 device that, on top of pH, moisture and sunlight it measures also temperature. However, I do not find it very useful because if not in a very special location (close to a heater, a room without any heating/cooling system or in general in any very cold/hot place that can potentially damage your plant), you potted herb should not have any temperature issue as a human-comfortable house is OK for potted herbs as well.

Although its design might (slightly) vary from vendor to vendor its features and way to work do not. So the information reported here is applicable to any 3-to-1 tester that can be found in the market. 

The 3-in-1 device is the simplest tool to use and does not even require any battery. What you need to do is just to stick the two metal probes in the soil and read the measurements as shown in the picture below.

The 3-in-1 soil meter stuck in the soil pot. Dry soil (and it was correct as I checked with the finger test)

The moving arrow on the screen will indicate the value corresponding to the indicator selected by the switch below. Changing the black switch below the screen will allow you to switch from pH to moisture measure and vice-versa. Simple as that. 

5 Tips On How To Use a 3-in-1 Tester

Although apparently simple, there are several tips you need to know if you want reliable indications of your soil moisture and pH from your 3-in-1 soil meter that many people forget or ignore the existence.

Tip 1: Distilled Water

This is one of the most common mistakes. For an accurate measure of your soil pH the soil should be watered with distilled water because it has a neutral pH (7) that, let’s say in simple words, does not affect the pH of the soil. 

Tap water is not the best choice for pH measurements

This might not be the case of tap water. In the UK for instance, the water company clearly states that the pH is allowed to vary (greatly) from 6.5 to 9.5. Very similar happens in the USA.

The water quality regulations specify that the pH of tap water should be between 6.5 and 9.5. 

United Utilities (UK main water regulator)


The U.S. Environmental Protection Agency (EPA) does not regulate the pH level in drinking water. […] The EPA recommends that public water systems maintain pH levels of between 6.5 and 8.5, a good guide for individual well owners. –

Source

And this can be a problem, especially if your tap water is far from a pH of 7. To understand why this is the case I will quickly tell you how an electrode soil meter in the 3-in-1 meter works. 

As you can notice the two electrodes are of different colours due to the different coating.

Without getting into technical details you just need to know that the two metal probes are in reality electrodes through which (a bit of) electricity flows (no worries, no risk of electroshock as this device does not even have a battery). The two electrodes are different. One can be brown in color while the other silver or both silver in with one lighter than the other.

One electrode is called glass electrode (due to a special glass-based coating material) and it is designed to attract some specific particle in the soil (called hydrogen ions, but no worries if this does not sound familiar).

The other electrode is called a reference electrode and has (let’s say for simplicity) a specific proportion of those particles that makes it pH neutral (7). If the glass electrode attracts the same amount of particles (coming from the soil) as the reference electrode than the soil is perfectly pH neutral (pH=7). 

On the other hand, if there is a higher concentration of these particles (hydrogen ions) in the soil (and so in the glass electrode) than in the reference electrode than the soil meter will tell you that the soil is more acidic (lower than 7 pH) because of this difference.

If vice-versa the soil will be more basic (higher than 7 pH). It is the difference in the concentration of these particles between the glass electrode (so in the soil) and reference one that truly matters for the reading.

Hence, now should be easier to understand why you should avoid tap water when assessing the pH of your soil. Indeed, in case the tap water is not pH neutral (quite possible in UK/USA as well in many other countries) it will change the overall proportion of those particles in the soil and so affecting the measurements. On the other hand, if the water is pH neutral, as the reference electrode, it will be like “transparent” to the measurement as it will not create any difference.

The farther from neutral is your tap water, the more distorted will be the measurement. As the pH is not something that you measure every day you can just buy for less than 10 dollars a 1l bottle of distilled water and use it for all your potted plants.

To make the above explanation simple I just mentioned two electrodes. In reality, for those among you a bit more experienced, the two pH electrodes are combined into 1 in many models and the second electrode (if available) is used to measure the moisture in the soil. The concept, however, is the same.

Tip 2: Be Patient

Another common mistake is the lack of patience. For mine and other gardener experience, it is advised to leave the soil meter around 10 minutes stick into the soil, definitely not a few seconds. Indeed, as explained before the pH measurement is based on an accumulation of particles in the glass electrode. This is not an instantaneous phenomenon. That’s why it is recommended to wait 10 minutes.

Time is key for correct measurements

Tip 3: Electrode Position – Keep Them Parallel

When you stick the electrodes of your soil tester into the soil be sure that the distance between them does not change significantly (this might happen if your soil is very compact, and this is a problem called compaction that you might need to address).

Indeed, the soil tester is designed to make a current measurement (originated by the difference in those hydrogen ion “particles” I mentioned before) that also depends on the distance of the electrodes. Closer the electrodes, the higher the current but the calculation performed by the soil tester (a little chip does some math inside it) is still assuming the original distance among those electrodes so providing misleading results

Tip 4: Electrode Depth – Root Level

The soil tester needs to measure pH and moisture at the root level, where your plant absorbs all the nutrients for its survival. That’s why is important for the electrode to arrive where the roots are (if you can place the meter electrodes a bit on the side of the herb). This detail is more important for the moisture measurement as this can vary with depth rather than pH that should be fairly constant, especially in a pot.

How many times did you believe your plant was in need of water when, in reality, only looking at the (drier) soil surface? This is one of the reasons for overwatering among beginners. However, you need to know what are the water requirements of your herb. For instance, basil will suffer in dry soil while this is needed for herbs such as Lavender. On this more in a future article.

I placed the meter with the moist measurement active (switch is on the left) in a dry soil

As you might not be aware of the root level of your plant I usually suggest going through ¾ of the container high, or more in case of a small container with a plant living there for a while (in such a case you might consider repotting as suggested in this article).

The same measurement after half glass of water

Tip 5: Cleaning – Keep it Simple

This post is aimed at beginner gardeners and indoor herbs. Hence, although many things written here might overlap and be of interest for the more experienced gardeners among you, the same does not apply for the cleaning process (in my opinion).

Indeed, for large scale application where pH measurements are vital for commercial reasons several cleaning solutions where submerge the electrodes, are available in the market. 

However, given that their cost is several times the cost of a common soil tester, it is not something that I recommend, they make sense only for more expensive instruments. 

Usually, a clean tissue or kitchen towel does the trick.

Chemical Soil Tester Kit: pH and Nutrients

This is the second major category of soil tester. I called them chemical not because the 3-in-1 does not rely on chemical reaction (it does indeed) but because with this second type of tester you are playing like in a chemical lab where you mix a few powders and you observe a small chemical reaction happening with corresponding colour changes.

This type of test is extremely important because it can quantify the macronutrient content of your soil (as well as your pH), something missing in a 3-in-1. This is something you do need to know if you are really serious in caring about your herbs.

Hopefully for you, as indoor gardening is a rising trend in recent years, you can find relatively inexpensive testers from reliable manufacturers for less than $30 dollars, so I suggest to give it a go.

Step by Step: How To Use a Chemical Tester

I will discuss the chemical soil tester show in the photo below that I usually use for my herbs at home.

Example of Test Kit to assess the nutrient content of your soil (sorry for my dirty carpet :D)

You need to know that there is a large variety of chemical soil testers in the market, and few of them look small chemical labs. However, for indoor application, the one I am going to discuss is, by far, the most widely used (is called the Rapitest, you can check it on Amazon) especially for beginners/intermediate gardeners given its simplicity, reliability and costs. More on that in future articles. 

There are two versions, one “purely” manual and one that comes with a small electronic device that read the water/soil sample. I honestly prefer the purely manual one (the one shown in this article). Indeed, it does not have an electronic device prone to any kind of problems (it might fall and broke, you might easily forget to replace the battery as you are not going to use it daily).

Test Chamber (where the solution should be placed) and the reference chamber

Test Kit Components: These soil testers come with 4 colored plastic containers and corresponding same colored capsule as shown in the photo below:

  1. Green: pH test
  2. Purple: Nitrogen test
  3. Orange: Potash test
  4. Blue: Phosphorus test
Here a close look up at the component included in the Rapitest nutrients and ph Test: 4 containers, 40 pills, and little water extractor, I forgot it to place in the picture)

You need first: I suggest to use a half cup of soil, hence I would not perform such tests for very small pots (and also it does not make much sense as you need to transplant them soon adding new soil). You need also 2.5 cups of distilled water and a bit of time (around 3 hours or more, no worries, you are not going to work all the time)!

Remember: the number of tests you can perform is limited by the number of capsules (40) in general. However, considering that this type of test is not performed daily the is more than enough as I will discuss later.

Step 1: Get Ready

Set aside large plastic clean bowl that you are going to fill with a half cup of soil extracted to 2-3inch (5-7cm) below the surface level. 

Set aside also 2.5 cups of distilled water. You perhaps have it at home for other uses (like ironing for instance). If not you can easily find a cheap 1l distilled water in shops.

Step 2: pH Test

The pH test is slightly different from the other three (nutrient-based) tests. You need to use the green container and fill the small side chamber (called test-chamber) with a bit of soil (around a teaspoon) until you reach the white dashed line. 

Then you need to drop in the content of one green capsule by separating the two half (just pull the extremity apart).

Once done, just fill the test chamber with distilled water until the top and shake thoroughly. Then let it sit for around a minute.

By visual inspection, you will be now able to assess the pH content by comparing the color of your soil/water solution in the test chamber with the color scale on the container.

Here I am comparing the pH level color of my soil with the reference scale. It came out slightly acidic (pH 6) due to the age of the potting soil. Still OK for my basil

You pH test is done, let’s move on. As a rule of thumb, the soil should have a pH of around 7. More information on the pH requirements of our indoor herbs in a future article.

Step 3: Nutrients Test

These 3 tests need to be performed in the same way.  First, you need to mix for around 1-2 minutes in the bowl you previously prepared your half cup of soil and 2.5 cups of distilled water. Then you need to let it rest for a few hours (up to 24). If your soil is not very coarse (unlikely for indoor potted soil) 3-4 hours are sufficient.

Once done, just add take liquid part on the top of the bowl (not the soil that is now sitting at the bottom, do not move the solution) to fill the test-chamber (the small one) of each color container until the indicated dashed line. The clearer the solution (or the longer the time you left to settle) the better.  

Once done add the content of the corresponding (same color) capsule, shake and let it settle for around 10 min. Then again, you are ready to compare the color of the solution with the color scale in the container, using indirect sunlight.

You are done, all the test have been performed!

Tips On How Often Should The Tests Being Performed 

You might read around that you need to test your soil every 2 to 4 years. However, I do disagree for theoretical and practical reasons:

  1. In a pots, everything happens faster: A test every 2 years is a suggestion of gardeners working in a large area outside. There, given the significant amount of soil, any change in its nutrients/pH might take a long time to take place just because of its volume.

However, the same does not apply for indoor gardener where the volume of soil is very limited and (more often than not) people have the tendency to plant my herbs in the same pot whose soil can be depleted way quicker than if it would be outside.

Moreover, outside (if in an open place, not a farm)  there is a constant cycle of nutrients coming from earthworms, dead insects, healthy bacteria that provides a stable and balanced environment for years. The same cannot be said in an indoor herb that grows in a more “sterile” soil where everything depends on the constant gardener control (through fertilizers, watering).

For the above reasons I use a soil tester every 6 months per container (just put a weekend reminder in your paper/digital calendar) or even more regularly. This for 2 important reasons: learning and expiration date.

Learning

I am an eager learner and for me, there is nothing more exciting than experimenting. Hence, why do not measure your soil nutrients content before using any fertilizer and after? What about every 2 months you used the fertilizer? This will allow you to quickly monitor what’s going on with your soil and understand when exactly your soil needs to be replenished.

Once you know the biological rhythm of your soil there is no need to repeat it again. This is a very rewarding experience that I totally encourage. Indeed, remember that is very easy to overfeed your herbs and damage them, this will totally avoid that giving you a great knowledge on the way.

Expiration date

In general every test has around 40 capsules (10 per each test) and, although there is no clear expire data in the package, a chat with the manufacturer confirmed that they have a shelf-life of 18 months “They have a shelf life of 18 months, any longer than that and the results would be inaccurate” .

Hence, if you do not use them within a 1 year and half the results provided might be inaccurate. Also, remember that the capsules need to be stored in a dry and dark environment otherwise their lifespan might be even shorter. Hence, If you perform, (in the worst-case scenario) 1 test every 6 months this implies a lifespan of 5 years (or 1 year if you have 5 different soil you want to test). This is a decent amount of testing for under $30 kit.

Tip

Some gardeners found useful to store the capsules in a glass jar with a desiccant. This because many complained that the capsule’s envelope gets broken/melt over time. Please, also prefer a cold environment (not freezing) to store them.

What About Other Soil Tester?

Online you can find a large variety of soil testers, from electronic to the manual that looks quite different from each other. 

For instance, you might find electronic soil tester that has only one electrode rather than two. Nothing to worry about, they will be as effective as the double electrode with the advantage that you do not have to worry about distance among electrodes. These single electrode soil testers still have two electrodes but they are placed within the same metal probe.

Some electronic 3-in-1 (or 4-in-1) soil testers might cost slightly more as equipped with a digital screen (against the old-style moving arrow). To be honest, for simple practical reasons I do not prefer such devices. They cost more, they need batteries (that you might forget to buy) and in case the screen gets broken (a bad fall) the meter becomes useless even if the actual metering system still works.

In the class of chemical soil testers, you can also found paper stripes ones. These multi-use testers (adopted also saliva, water etc) can also be adopted to quantify the pH of your soil. You just need to bring a few tablespoons of your soil in a bowl, add 3-4 times the amount of water, mix for 1-2 minutes until it is a liquid solution (but not too watery) and let it rest for a few hours. Then you submerge the stripe in the solution for 20-30 seconds and you compare its color with a scale.

Regarding the test to quantify the nutrients in your soil, you can also find a large variety of test format. Some manufacturers even sell small plastic suitcase containing material for 400+ tests for more serious (commercial) applications.

In such a case, experienced gardeners buy the tester with all its tools and then, when they run out of the powder activator (the equivalent of the powder in the small capsule we mentioned before) they purchase them directly from a lab.

Related Questions

Can I leave the 3-in-1 tester in the soil indefinitely? It is not suggested because of potential corrosion and/or shortening its lifespan as always in use (analog version). Moreover, if the sensor is digital it will probably turn off after a few minutes;

If the capsules finish for the nutrient tests is it necessary to buy a whole new kit? Many sellers offer the possibility to sell only the capsule avoiding the gardener to buy again the entire kit;

What is the best possible soil test? For the most accurate and complete results, local soil laboratories are recommended. However, given their cost, these are not typically an option for indoor gardeners but rather for those interested in commercial applications or without large garden areas.

Further Readings

Best soil tester for beginners

Best potting soil for your herbs

21 Tips to grow massive basil

Andrea

A young Italian guy with a passion for growing edible herbs. After moving to the UK 6 years ago in a tiny flat, it was impossible to grow herbs outside. So I start my journey in growing indoor and so I decided to share my knowledge.

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