Everything You Need to Know About Alkaline and Acidic Soils 

For anyone who isn’t a chemistry buff, the term “soil pH” can be intimidating. And while it’s helpful to learn as much as you possibly can about the complexities of gardening, becoming a soil science expert isn’t actually necessary. In fact, one of the easiest things any gardener can do to improve their garden’s output is to understand the foundations of soil science, which all start with soil pH. So what exactly is soil pH, and how does it affect indoor and outdoor plants? Let’s dive in. 


What is Soil pH

What is Soil pH

At its most basic, soil pH (which stands for ‘potential of hydrogen’) measures how acidic or alkaline your soil is, using a scale of 0 to 14. This scale measures how much hydrogen and how many hydroxyl ions are available, which directly correlates to how acidic (low pH) or alkaline (high pH) a soil is. 

Most soil measurements fall somewhere in the middle of that 0 to 14 range, with pH readings of 4 to 6.5 being considered acidic and those between 7.5 to 9 as alkaline. A pH reading of 7 is considered neutral—and indicates a balanced soil pH.  

You can find out the measurement of your soil by conducting a soil test. These can be done at home using vinegar and baking soda, or for more detailed results, you can gather a sample of your soil and mail it to a local USDA extension office. Many labs only charge between $7 and $10 for a basic soil test. 

Why Soil pH Matters

Like all elements of the garden, soil pH isn’t stagnant. It can be easily changed by you or even change naturally on its own depending on conditions of the geography, water chemistry, or what your home’s previous owner was putting in the soil. 

Soil pH matters because it indicates the kinds of nutrients that are available to the plants growing in your soil. This applies to both potted houseplants as well as the ones growing in your garden. And as you might have guessed, some plants grow better in certain types of soil than others. Hydrangeas, for example, even take on different colors depending on the pH of the soil. But there are other factors of soil pH to be aware of depending on the kinds of plants you want to grow in your garden. We’ll get into that next. 

How Soil pH Affects Your Garden

How Soil pH Affects Your Garden

As we mentioned earlier, soil pH determines how available certain nutrients are to your plants. This means that even if your soil has plenty of all the right nutrients, they may or may not be soluble (and available to your plants) depending on the acidity or alkalinity of the soil. This is why it’s essential to understand not only the measurement of your soil but also the specific needs of your plants. 

Most garden plants fall into one of two categories. They either prefer soil that is somewhere between slightly acidic and neutral (4.5 to 6.5), or they prefer soil that is between neutral and slightly alkaline (7.5 to 9). 

In addition to not being able to access essential minerals and vitamins, soil that’s incompatible with plants can also be mildly toxic to them. For example, acidic soils in the 4-5 range can have large quantities of aluminum, iron, and manganese, which is toxic to many plants in the legume (Fabiaceae) family. On the other hand, plants like azaleas, rhododendrons, and conifers thrive in acidic soil. 

How Soil pH Affects Your Houseplants

How Soil pH Affects Your Houseplants

Just as soil pH can affect the potential of your garden, it can also affect the health of your houseplants. Generally speaking, most houseplants thrive in neutral, slightly acidic soils (with pH measurements between 6 and 7). This pH measurement allows for the most minerals and nutrients to easily dissolve and become available to your plants. Because of this, it’s recommended to keep your houseplants at a pH between 6 and 7 whenever possible. 

Most potting soil that you buy from a store will fall within this range of slightly acidic to neutral, but if you find that yours is too acidic (or not acidic enough), there are things you can do to change that. Next up, we’ll get into the specifics of what causes both low and high soil pH, as well as how you can fix it. 


What Causes Low Soil pH

Acidic soils naturally occur in some regions of the United States more than others. For example, coastal regions with coniferous forests (like the North and Southeast as well as the Pacific Northwest) that receive more rainfall tend to have more acidic soils. Not only does the rainfall leach out alkaline minerals of the soil, but decaying pine needles and leaves also contribute to the acidity of the soil. 

In the garden

A lot of things can cause low pH (acidic) soil in your garden, including the use of high-nitrogen fertilizers. While fertilizers can change a soil composition relatively quickly, more natural causes (such as heavy rainfall that leaches nutrients from the soil or the excessive decay of organic material) usually take years to impact soil pH but can also contribute to soil becoming overly acidic. 

In houseplants

Since most houseplants grow in a balanced potting soil, low pH generally won’t be a problem right away. That being said, many potting soils add in limestone to neutralize acidity, and if too little is added, then an overly acidic soil becomes possible. 

How To Fix Low Soil pH 

How To Fix Low Soil pH 

In the garden

One of the fastest ways to fix low soil pH in your garden is to add some form of lime. You can use agricultural lime, dolomite lime, or even pulverized limestone. All of these contain minerals like magnesium and calcium carbonate, which can help neutralize acidic soil. 

In houseplants

To correct acidic soil in your potted plants, you would first want to measure the pH of your potting soil. You can do this by purchasing a basic soil testing kit, or sending in a sample of your soil to a nearby testing lab. Once you know just how acidic your soil really is, you can correct the imbalance by adding in agricultural lime. This should be done cautiously, since over-liming can result in creating a soil that is too alkaline for your plants. 

What Causes High Soil pH

Alkaline soils occur naturally in the Western regions of the United States (with the exception of the Pacific Northwest), where there are fewer trees to shed leaves and contribute to soil acidity and less rainfall to leach alkalinity from the soil. The rocks in these areas also contribute to the high pH of the soil since they’re more likely to be shale or limestone rather than acidic granite. 

In the garden

Besides naturally occurring alkaline soils, garden soil can also become alkaline from over-liming (usually as a gardener’s attempt to correct acidic soil), from irrigating with “hard” alkaline water, or from relying too much on composted manures. Gardens with alkaline soil are typically found in more arid, treeless climates, as mentioned above.  

In houseplants

Much like your outdoor garden, houseplants can develop high pH from over-liming or watering with hard alkaline water. The main problem with high-alkaline soils in houseplants is that it reduces the availability of nutrients like iron, zinc, copper, and manganese. This can result in things like Iron chlorosis, which is an iron deficiency that manifests as yellowing leaves with veins that remain green

How To Fix High Soil pH 

How To Fix High Soil pH

In the garden

The easiest way to fix an alkaline soil is by amending your soil with more organic material. Adding mulch, pine needles, peat moss, and even coffee grounds can help lower your garden’s soil pH over time. If these amendments aren’t available to you, you can also use acidifying fertilizers like aluminum sulfate or ammonium sulfate, although these should be used cautiously and you should wear protective gloves and eyewear to avoid coming in direct contact with them. 

In houseplants

The process for fixing houseplants with high pH is similar to what you would do for a garden. However in this case, you might consider repotting your plant altogether— since amending soil for potted plants can be a lot messier and more complicated. 

Most store-bought potting soil is slightly acidic to neutral, making it perfect for your houseplants. If you suspect your plants are suffering from alkaline soils, try repotting them with something like Miracle-Gro Moisture Control Potting Mix


Soil pH FAQs:

What is meant by soil pH?

Soil pH refers to a measurement of your soil’s acidity (low pH) or alkalinity (high pH). Most plants prefer slightly acidic to neutral soil (which corresponds to a rating of 6 to 7 on the pH scale), rather than overly alkaline soil (7.5 and higher).

What is normal pH for soil?

While there’s no such thing as ‘normal soil pH,’ most plants have an ideal soil pH that falls somewhere around a 6 to 7 pH rating, indicating slightly acidic to neutral soil. Soil that’s lower or higher on this scale tends to make gardening harder. The good news is that it’s fairly simple to change the pH of your soil.

What pH should potting soil be?

Potting soil should ideally have a pH between 6 and 7, which indicates soil that is slightly acidic to neutral. This pH rating is appropriate for many garden plants as well as a variety of indoor potted plants.

How do you adjust pH in potting soil?

You can adjust the pH of your potting soil by adding lime (to make low pH soil more acidic) or repotting your plant in new soil (to make high pH soil less alkaline). Houseplants perform best with neutral pH ratings (between 6 and 7), so achieving this middle ground should always be the goal. Remember that most store-bought potting mixes are naturally balanced and won’t need any soil amendments.


The Final Word

Grasping soil pH can be a tricky business for any gardener, but it’s a worthy time investment to make. By better understanding the pH of your garden soil (and houseplants) you can make changes to serve the growth and success of your plants. 

The best part? It usually doesn’t cost more than a soil test and some new compost or soil amendment to see improvements and reap the benefits of happier, healthier plants.

Author

Larissa is a writer, gardener, and herbalist living in the San Juan mountains of Colorado. Her writing has been widely published in lifestyle and personal finance publications all over the country, and she's also the creator of the weekly newsletter @rootedintribe.

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