Taking care of your favorite herbs without success is a very frustrating situation that many of us have already experienced. However, independently from the amount of water you provide to your little green friend or sunlight it receives the results will always be the same if the soil is not in a healthy state as discussed in this article

However, how can you check if the soil is in good shape? You need to look at moisture level, pH and nutrient content. For pH and moisture, I have already discussed here the 3-in-1 tester as best compromise between quality and price. However, what to choose to assess the macronutrient content (that is the amount of Nitrogen, Potash, and Phosphate) with so many choices in the market?

The best pick for a nutrient focused soil tester, as the ideal compromise between quality and cost is, without doubt, the Rapitest soil tester (you can check its price on Amazon). A small portable chemical lab of 4 small plastic containers, colored pills, and a dropper that allows you to reliably assess the macronutrient content of your soil.

I have already discussed in this article how such a kit in case you are curious. Here I will discuss the main advantages (and negative aspect) of it  Let’s dive in!

Easy To Use

This kit comes with four colored boxes (green, orange, blue, and purple) and corresponding 10 colored pills each (for a total of 40 pills). Each pill contains a reagent powder that needs to be used in conjunction with the corresponding colored container as detailed in one of my previous articles

How to perform the test: In short, you need to mix 1 volume of soil (it can be a few teaspoons) with 5 volumes of distilled water in a bowl and let it rest for a few hours. Then, with the provided dropper you pick up a bit of the solution (taken from the top) and you place it in the small chamber of each colored plastic container. At this point, you add the content of the corresponding pill and after shaking thoroughly you need to wait for 10 minutes. At this moment, you can now compare the color of the solution (that changed in the meantime) with the closest color provided in the scale on top of the plastic container. This will provide you the nutrient value of your soil.

The test is pretty straightforward as it uses colors rather than complex tables or number making ideal for the large majority of people. The only downside is that is not suitable for those of you that unfortunately are colorblind. In such a case you might need to opt for the electronic version that provides the results in a screen.

Small Amount Of Soil For Testing

Although many might not agree this is an interesting advantage. Indeed, for indoor gardeners, whose containers are not very large (sometimes way too small) being able to perform soil test with only a few teaspoons of soil (or less) it is a great plus. Indeed, allows you to perform multiple tests overtime for the soil of one pot without depleting it.

Tip: Remember to remove any trace of stems, leaves, pebbles or any other no-dirt material that might be in the mix of soil you use so as to not affect the results.


The beauty of this kit is its ability to be used up to 10 times per test. Indeed, it comes with a total of 40 capsules (10 for each of the single tests). This is very important because, once you notice that something is wrong with your soil, and you take actions accordingly, you need to check again if those actions affected the soil as expected. That’s why is important to be able to repeat the tests over time.

The full Rapitest kit (here on Amazon) with the 40 capsules, 10 of each color (corresponding to the container)


As you can see from the price on Amazon here, at the time of speaking this soil tester is sold for around $14 and allows a total of 40 tests (10 tests for each of the three macronutrients + pH). This is the equivalent of 35 cents per test! With this amount of money, you cannot even buy a single candy.

Rapitest allows also a pH test. In this case, my soil came out slightly acidic. This is perfectly expected as the bag of soil I used was 2 years old and so, due to the age, it goes acidic (around 6) but still OK for my basil plant.

Actions To Take: You Are Covered With a Twist

After performing your soil test you might wonder what actions you can take in the unlucky case your soil is not in perfect health. Attached to the Rapitest there is a small (but pretty useful as a start) two-page guide on which fertilizer to use and in what amount. This applies to improve both nutrients and pH level

However, as an indoor gardener, the only drawback of such recommendation is the amount. Indeed as you can also see from the photo, they are referred per pound of fertilizer per 100 square feet of the garden surface. Of course, a normal pot is way smaller than that and you might struggle to follow such suggestions as if you scale down such amounts (I actually did the math!) you will end up with 1-2 grams of fertilizer per pot, very impractical.

A fraction of the Rapitest suggestion page for fertilizers after having performed the test

In such case, I do recommend to simply use liquid fertilizer (a post coming soon) whose N-P-K ratio (more information in one of my article) compensates the lack of nutrients you have. For instance, in case your soil came out with a high deficiency in nitrogen, but it is OK for potash and phosphate you might opt for a fertilizer with a high N value compared to P and K. 

If your soil is low in Nitrogen you are in luck. Indeed, this macronutrient is quite “famous” compared to the other and as a consequence, the market is quite abundant of fertilizer richer in Nitrogen than in Potash or Phosphate. In other cases, you might need to dig a bit more online to find the right fertilizer or you might decide to create your own fertilizer using only “ingredients” richer in that specific macronutrient that your herbs are missing.

The Choice Of Thousands Of Other Gardeners

I am not the only one that tried this soil tester (and I would not recommend it if this was the case). Indeed such tester is also the most sold and (rightly) appreciated online with more than a thousand reviews of which 700+ are 5 stars (the highest level in Amazon, check it by yourself here). As already discussed in this article numerous are the reasons to justify such success such as:

  1. Excellent value for money: some more experienced gardener than myself used this product dozens of time, a good deal for just a few dollars;
  2. Accurate enough for most applications: as we will discuss later a few gardeners actually compared the results of the
  3. Simple to use: the intuitive color scale is seen as a plus by many gardeners as make the Rapitest easy to use
  4. You can take action: many gardeners value the opportunity to have, for a modest price, a tool that reliably spots major issues in the nutrients and pH level of their soil. This is the only needed information to take action in the right direction.


After reading all the positive reviews, the only aspects I was still a bit skeptical was only the accuracy of the test. However, these doubts have been dissipated. Indeed, among all the people that used it, there are also quite a few that actually compared the results of this test with more accurate ones (like lab test). The results were really encouraging as detailed below from the experienced I found:

  • Lab Test VS Rapitest for outdoor field: this was also an environmental scientist with years of experience in the field. He compared the results of his closest laboratory (in Texas) with the ones provided by the Rapitest with some soil of his outdoor field. The Rapitest provided the following: N and P depleted, K maximum, and a pH of 7.5.

    The lab analysis reported a few ppm for P and K (virtually zero) and above 100 ppm (ppm=part per million) for K and a pH of 8.5. Although the Rapitest cannot provide the exact numeric concentration as an expensive lab test, it was accurate enough to give the information to the gardener to take actions in the right direction.
  • The lemon juice test: this gardener went more creative. Indeed, he created a special solution of urea (very high in nitrogen) and some lemon juice (very acidic, low pH). The Rapitest (although not designed for this purpose) was able to provide a yellow pH (very acidic) and a purple N result (very rich in nitrogen).

Moreover, you can also find YouTuber gardeners with way more experience than myself that performed such tests confirming the above. I will not post any link as I do want you, in case you are still skeptical, to verify it by yourself.

What Can Be Improved

As always in my reviews, I do try to be as neutral as possible, even when what I test is quite good as in this case. So I will go through some of the margin of improvement on this test kit or difficulties that you might encounter:

  • The pills are small (around 1cm long): this might be a bit of a challenge if you have big hands/fingers or any type of movement difficulties as you need to be able to pull them apart;
A close look-up of the pills (for potash, but they are all of the same size of around 1cm)
  • The containers are small: the test chambers as well are small and you might accidentally drop the content of the pills out of it. In this case, just use a simple piece of paper in a cone shape to help you redirect the pill content in the tiny aperture of the test chamber;
The test chamber on the left is not very large, but reasonable
  • The pills might deteriorate after a year: this is probably the main drawback. Indeed, a few gardeners complained that they especially the blue capsule might melt. However, others have found beneficial to place them within a glass jar with desiccant. Nonetheless, for less than a $20 dollar investments it is a very good deal anyway;
  • After a year the results might not be accurate: the manufacturer reported that after a year, even without any sign of clear deterioration the results might not be accurate anymore. This is probably due to a deterioration of the reagent in the pills;
  • You need to clean thoroughly the containers: this is something that many people might overlook. However, as you are going to perform many tests with the same 4 chambers you need to remember to clean them between one test and another in order to avoid any residue that might affect future tests. Just water is enough, I would avoid soap as it might leave a basic trace (they alter the pH) and few other chemicals that might react with the pills;
  • It is not the most accurate in the market: of course, they do exist tests that are more accurate. These are the notorious laboratory tests that many have used as a benchmark to assess the reliability of Rapitest.

    However, as many have already highlighted, for indoor (and outdoor) applications having a precise measurement of your nutrients is not of vital importance for two reasons: a) your herbs will not suffer for a 5-10% difference in the nutrient content b) other factors might affect way more the result of the measurement such as the place or depth at which the soil sample has been taken. 

However, do not get me wrong,  laboratory tests do still make lots of sense. Indeed, they are not only more precise but more importantly provides a significantly wider variety of information regarding your soil. Indeed, here for simplicity I just discussed Nitrogen, Phosphate and Potassium.

However, a plant needs other substances as well like Ammonium, Calcium, Magnesium, Sulfur, Iron, Manganese, Copper, Zinc, and many more. These are classified as nutrients (not macronutrients). Nonetheless, indoor gardeners aiming to grow herbs inside, especially at the beginning, can ignore all of these aspects and still be successful. These kinds of tests really make sense for the more experienced gardener with a larger lot and aimed for commercial purposes.


The Rapitest (here on Amazon or here on Walmart, depending on where is cheaper) testing the soil is a must-buy for any beginner gardeners. It allows you to easily test the soil in a reliable and fun manner. Of course, it is not as accurate as a lab test, but hey, 99% of normal gardeners (myself included) do not need such precision.

Related Questions

What are the possible results of a Rapitest test? The colored scale provides 5 possible results: surplus, sufficient, adequate, deficient, depleted;

Is it possible to use tap water for the tests? Yes, it is possible although not recommended as it might affect the accuracy of the results, especially the pH one. Distilled water is recommended;

Is possible to buy the capsules separately? Yes, the manufacturer sells the pills only but given the low price of the kit and potentially offer, it is often cheaper to buy a new soil tester.

Further Readings