Remember your high school physics-chemistry classes: the more H+ ions in soil, the more acidic it is. A pH (potential of hydrogen) of 7 is described as neutral; between 7 and 14, it is basic, or alkaline; between 1 and 7, it is acidic. You’ve no doubt heard that soil pH is an important concept, but how do you measure it?
Determine the pH of your garden soil
Because of pollution, soils tend to acidify, even though efforts have been made in Europe to curb acid rain in particular. Not all plants will appreciate this. Some will tolerate it, but others will prefer an alkaline soil, while most plants will thrive best with a soil that is fairly neutral, or slightly acidic (pH 6.0–7.5).
Is your soil acidic or alkaline?
Most soils have a pH ranging from 4 to 9. Beyond precise measurements, a few clues can guide you. In general, a calcareous soil will be alkaline. A soil that is very rich in humus and/or sandy is likely to be acidic.
Note: a pH between 6.0 and 7.5 suits the majority of vegetable and ornamental plants.
Also observe the plants growing naturally on the land: they are excellent “bioindicators” of your soil’s composition, especially its pH. If you see blueberries, horsetail, heather, rhododendrons, maidenhair ferns, or foxgloves, there is a very high chance your soil is acidic.
If, on the contrary, you notice primroses, geraniums or mallows, your land is probably calcareous/alkaline.
Take a garden sample
When you’re setting up a new garden, it can be worthwhile to take more precise measurements. The first step is to determine whether a single sample will suffice or if you should take two samples.
To do this, observe your terrain: are there sandier areas? Can you distinguish clay-rich zones? If no area stands out, take a single sample, but from several spots combined into one composite sample.
First, gather glass jars – and here you can recycle a jam jar or any similar container without issue. Also grab a small hand trowel.
It’s not about sampling surface soil, but rather the plow layer, the soil in which most plants draw their nutrients. You’ll take a layer of soil located between 8 and 12 inches (20–30 cm) deep, in several spots around the area in small scoops to obtain a representative sample by mixing.
Measure the soil pH — how to do it?
1. DIY: analyze soil pH with red cabbage
This method is used by many gardeners. It’s a red cabbage season, of course… We start by getting into the kitchen:
- Thinly slice the red cabbage;
- Boil pure distilled water;
- Then place the red cabbage in the water and let the pieces soak for about ten minutes before removing them (you can use the juice as you like).
- Set aside the purple juice, which, logically, has a neutral pH (7).
If you wish, you can first check the colors by testing the juice with baking soda and lemon juice: the alkaline – the baking soda – turns the solution blue-green, the acid (lemon juice) turns pink. This gives you a reference point for comparison.
Place a bit of the juice in a clean container, and add two tablespoons of soil. Let this mixture rest for 30 minutes to an hour before checking the color:
- Purple: neutral pH.
- Pink: acidic pH; the deeper the pink, the more acidic the soil.
- Blue/green: alkaline pH; the brighter the color, the more basic the soil.
This test isn’t highly precise, but it provides a first analysis that can already help you pick plants that will thrive in your soil.
2. Do a test with everyday products
You likely have in your house some baking soda and white vinegar, especially if you’re already making your own natural cleaners.
Divide your soil sample into two containers. In the first, pour white vinegar. If the vinegar fizzes, your soil is alkaline.
Add water to the second container to form a paste. Then add baking soda, which should fizz, indicating the soil is acidic.
If neither test reacts, the soil is probably neutral (pH 7). This test also provides a preliminary analysis.
3. Analyze the soil with a kit
To go further: you can buy this kit at a garden center, or sometimes even at an organic store, for about $7–$9. This type of analysis is not complete: it lets you know the soil pH quickly, but it won’t give you the precise composition of your soil.
The setup closely resembles the lab exercises you may have done when younger (or not): you use a test tube, distilled water, and paper test strips. You fill the test tube with the soil mixture and top it up with water before sealing it, then shake the contents vigorously.
The soil will then naturally form a sediment, and it will be time to dip a reactive strip into the tube to determine the soil pH based on the color the paper assumes. The alternative is a probe tester that you insert directly into the soil.
4. Know the soil composition
Another test is possible, albeit more expensive. It is done in a laboratory. You’ll need to submit a much larger sample (up to about a kilogram) and bring it to either a garden center that coordinates with a lab, or directly to a specialized laboratory.
