Growing plants requires a good understanding of the necessary growing conditions, especially when it comes to the soil. One of the key factors is soil pH, which determines the acidity or alkalinity of the soil. It’s essential to understand and test the pH of your soil to ensure your plants have the best chance of thriving.
As an environmental science student, I used to survey the hinterlands in Queensland, Australia to test soil pH. I always swore that when I finally had land of my own, I’d perfect the soil pH before planting my garden.
In the meantime, I use this knowledge on my indoor plants 😄
In this article, we’ll go over the basics of soil pH, its importance, what affects it, and how to test it:
Even if you’re not a science student like me, I’ll break it down in a way that’s easy to understand.
What is soil pH?
PH stands for “potential of hydrogen,” referring to the amount of hydrogen present that will affect the soil’s acidity, or lack thereof. The pH scale ranges from 0 to 14, with numbers below 7 being acidic, and numbers above 7 being alkaline; 7 itself is a neutral pH. No matter what type of soil you’re using to grow your plants, soil pH can vary significantly based on a number of factors and affect the health of your plants.
Why you should test the soil pH
The right soil pH is all about nutrient availability to your plants. When soil is too acidic, nutrients like calcium and magnesium become less available, whereas soil that’s too alkaline keeps iron, copper, and zinc from being absorbed by plants. Similarly, in low pH (acidic)soil, aluminum and manganese become too easily absorbed and become toxic to the plant. When too much of any nutrient is available, it becomes toxic. When plants are prevented from absorbing nutrients, it slowly starves the plant. Read our guide to plant nutrients for a deeper understanding of how plants absorb and use different nutrients.
The pH scale is used to measure the acidity or alkalinity of a substance, with the following breakdown:
- 0 to 2 is extremely acidic.
- 3 to 5 is slightly acidic.
- 6 to 8 is neutral.
- 9 to 11 is slightly alkaline.
- 12 to 14 is strongly alkaline.
Nutrient availability varies because extra, or fewer, hydrogen molecules change the bonding characteristics of the substances. When they are locked up in the soil and unable to be leached by the water, those nutrients can’t be absorbed by the plant roots. The pH range with most nutrient availability is between 6 to 7.5. A pH of 6.5 tends to be best for soil life, including bacteria and other decomposers that break down soil and make those same nutrients more available.
Different types of plants like different levels of acidity. That being said, most plants like slightly acidic soil (between 6 and 7), but there are those that prefer neutral or slightly alkaline. This has to do with their natural habitats: in areas with lots of rainfall, soils tend to become more acidic due to high concentrations of carbon and other particles in rainwater, absorbed from the atmosphere. On the other hand, areas with less rainfall tend to provide more alkaline growing environments. To make sure the soil is appropriate for each of your individual houseplants, knowing the species and monitoring the soil over the year is the best strategy for good pH management.
How soil pH changes
Indoor plants need amendments to make sure the soil stays in the comfortable balance for your plant. In nature, a consistent pH is maintained by the average amount of rainfall over the year, as well as the ground material and the nutrients available. For indoor pots, plant parents have to amend the soil from time to time, but the same factors affect the pH level: water pH, soil materials, and nutrients (in our case, fertilizer).
In nature, normal rainfall is slightly acidic because it absorbs carbon, sulfur, and nitrogen in the form of acidic gasses in the air. Tap water, however, is usually treated for purity, potentially with additives like fluoride, and is kept alkaline so that it won’t leach chemicals from the pipes used to deliver it (and won’t damage them, either).
The materials that the soil is made of are the next important factor since this is what holds the nutrients, affecting both availability and presence. You can purchase potting soil that is balanced for acidity or alkalinity, or you can make your own soil using materials that are best for the plant in question.
As soil breaks down, the quantity diminishes and nutrients are leached by watering and absorbed by the plants. In nature, rain and topsoil life decomposes the surrounding organic material for consistent pH maintenance and nutrient provision. When you add fertilizers to your potted plants, you affect the pH level by the type of material that’s added and how quickly or slowly it breaks down. Some organic materials, like fresh compost or worm castings, are higher in acid than lime-based fertilizers or a solid, slow-release synthetic fertilizer.
Why you should use a pH meter
There are several ways to test the pH of your soil, ranging in accuracy and cost. The most accurate option is to have it done by a professional soil testing laboratory, which typically costs $7-10 per sample. Another option is to use pH strips, which give you a general idea of whether your soil is acidic, neutral, or alkaline. However, the results of this method depend on how well you can read the litmus paper and only provide a range of values, not a specific number. A pH meter is the most dependable and accurate method, but it has a higher upfront cost.
The rate at which nutrients are released by soil or fertilizer, the amount of water held in the soil, the size of the plant, and other environmental conditions like humidity and light affect how quickly a plant absorbs nutrients from the soil. A pH meter doesn’t replace a nutrient test, but it compliments it. Too high or too low of a nutrient reading might not be from a lack of presence, but a lack of availability from too high or low acidity.
How a pH meter works
Soil pH meters give you an accurate measure of the acidity of your pot’s soil. There are many models of pH meters, and some come in combination with light and moisture meters. Some meters can be put directly into the soil, and others test a soil sample that you mix with deionized water (which is neutral and doesn’t affect the soil pH reading). You can get either an analog or digital soil pH meter. Whether you probe or take a soil sample, taking a reading of more than one spot will give you an averaged result that will be more accurate for the pot as a whole than a sample from only one part of the pot.
PH meters that measure watered-down soil samples, known as hydroponic meters, need to be calibrated with a special solution and cleaned with a neutralizing liquid. Each model will come with its own specific directions that should be followed carefully for the most accurate results. Adjusting the pH too far in one or another way can be damaging to the plant, and using a meter while amending the soil incrementally will help you to not overdo it.
Be sure to clean pH meters after use so they will give you an accurate reading next time you use it. Probing meters should be wiped down so the metal doesn’t corrode, and hydroponic meters should be cleaned with their solution before being stored.
How to test soil ph with a ph meter
To test the pH of your soil using a probe:
- Insert the probe about 2-4 inches into the soil.
- Wait for one minute for the final stable value to be displayed.
- Check the data on the dial for the moisture and pH levels.
- Remove the probe from the soil and wipe it clean after each use.
How to amend soil for a balanced pH
There are a couple of options to balance your soil’s pH: You can either use a synthetic soil conditioning product, or by add an organic soil conditioner like compost, sphagnum peat moss, wood ash, or lime. Whichever method you choose, it’s important to add it in low amounts over a period of time, depending on the need. The pH meter will help you know how much of a change each supplement has made. Record your findings, the amendments you added, and the amounts to keep track and for later reference.
Soil that’s too acidic
Soil that’s too acidic might be due to too much organic material or synthetic fertilizer having been added to your plant’s pot. Wood ash, gypsum, and lime are natural materials that can be added to raise the pH and lower the acidity.
Soil that’s too alkaline
For soil that’s too alkaline, compost, worm castings, sphagnum peat moss, coffee grounds, rainwater, and citrus are natural choices to make your soil more acidic and lower the pH number. Synthetic soil conditioners are usually added to weekly waterings using distilled water.
PH should be adjusted by topdressing the soil, or putting it right on top, and allowing the material to be incorporated into the pot with each watering. Don’t remove the plant from the pot to mix in amendments to the soil, since this can cause unnecessary stress on the roots.
How frequently should I test my soil’s pH?
The frequency you test your soil’s pH will depend on the plants in question, your water, and the growing environment. Many plant parents do a pH test when they do a nutrient test at the beginning of the growing season, and as mentioned, many moisture meters also measure soil acidity and can be used regularly throughout the year. A basic soil test kit usually measures pH as well as nutrient levels. As plants absorb nutrients and organic material is broken down over time, the soil gets depleted and must be replenished. If you have a large plant that grows quickly, chances are the soil will need to be amended more frequently than smaller or slower-growing plants. While pH affects nutrient availability, adding fertilizer affects the pH levels as well, based on the physical materials added.
What is the correct soil pH level?
While the perfect pH level for an individual plant will vary by species, most plants like a slightly acidic soil within the 6-7 range, and you can purchase pre-mixed soils that are calibrated to an acidity level. The average slight-acid includes monsteras, jades, calatheas, dracaenas, and many others that come from tropical environments. The big exception is the aloe plant, which likes slightly alkaline soil.
Can I use a meter to measure the pH of pure water?
You should avoid using a soil pH meter for pure water, since it is made to measure solid particles. There are water-specific pH measures and tests available if you need to check the water’s pH before you add it, which will be more accurate for the water results than a soil meter would be.
more about soil
- Is that mold on plant soil? Here’s what you need to know.
- how long does compost take to turn into nutritious soil? it depends
- soil draining: how to make your soil drain better
- how to loosen compacted soil in pots
- how to choose the right bonsai soil
- The best pH meter to test soil conditions
- The best soilless potting mix for happy plant roots
- soil conditioner: the what, why & how
- 4 types of soil and what they’re good for (according to science)
- 8 best soil moisture meter options for indoor plants