How To Lower Soil pH Or Raise Soil pH

Have you ever measured the pH of the soil in your garden? If not, you should. Testing is the only way to know for certain what the soil pH is, impacting amendments and what plants you can grow.

Gardeners are often advised to put wood ash and other neutralizing compounds in the soil regardless of testing. But here, you will learn more about why pH matters, how to lower soil pH, and if needed, how to raise soil pH.

What is pH?

In basic terms, pH describes the acidity or alkalinity of a substance. The pH scale ranges from 0 to 14, where the lower number means the substance is more acidic and the higher numbers mean it is more alkaline. A pH of 7.0 is considered neutral.

PH takes a measurement of the free hydrogen ions in the solution, and that is how the number is determined. When more hydrogen ions are present, the number is lower (more acidic).

Something else you might come across while looking at pH resources is pOH. This is a somewhat similar measurement, except this measures the OH ions present and is the opposite of pH, where low numbers are alkaline and high numbers are acidic.

It seems daunting to understand pH, but it is such an important chemical property of soils that takes in a lot of different factors.

Plants vary in the tolerability of pH. In general, an optimal pH range of 6.0-7.0 is best. Crops can live outside this range, but that is the best range for the widest variety of crops. Certain crops and ornamentals prefer or tolerate more extreme conditions on either end of the range. The classic examples are blueberries and azaleas. These plants like a pH of 4.5, which is acidic soil. 

Measuring pH

A soil test is the only way to truly know what your pH is. Looking at the plants and knowing what the history of the soil is can be clues, but testing is the only way. This can be done at home by getting a good pH meter for soils, using home test kits, or by sending soil for professional testing.

Testing several areas at several depths can indicate what practices need to be done, or if action needs to be taken at all. If the pH falls somewhat closely within the desirable range of 6.0-7.0, then you do not need to worry about adjusting the pH. If you find yourself reading above 7.2 or below 5.5, then action should be taken to change the soil. 

Natural Factors Impacting pH

Environmental and climatic factors can help gardeners understand why the soils we grow in will inherently have the pH they do.

Rainfall is one of the important ones to consider when thinking about pH. Rainfall will wash away basic elements like magnesium, potassium, and calcium. This allows more acidic element ions to be present, such as hydrogen and aluminum. In climates that do not receive as much rainfall, the soils are more alkaline.

Texture (sand, silt, and clay content) plays a huge role in the chemical properties. Reducing soil pH in sandy soils versus clay soils is quite different.

Reducing pH in clay soils is more challenging compared to sandy soils. This is because of the clay particles’ ability to hold onto cation elements. The concentration of positively charged hydrogen ions is how pH is measured, so these ions are better held on negatively charged clay particle surfaces.

Sand particles do not have these types of sites on them so it is much easier to lower your pH. However, you must be careful because the pH level easily becomes too acidic. 

Why pH is Important

Although pH may seem like an afterthought in the garden, there are many ways it affects plant growth.

Nutrient Availability 

Nutrient availability is one of the more important things to consider with pH. Each nutrient has an optimal pH at which it is most available.

At both extremes of the pH scale, some nutrients are very available, but the majority are not. This sweet spot at around 6.5 is when all nutrients are best available. This is important to understand because in some cases, nutrient deficiency in plants could be due to the availability, not lack of the nutrient.

Aluminum Toxicity

Once the soil is at a pH of 4.5, it is subject to aluminum toxicity. At this pH, more metals are leached into the soil and most plants cannot grow in these conditions.

If you live in an area that naturally has a soil pH value of 4.5-5.0, plant acid-loving plants like blueberries or azaleas. The other option is to lime the soil to increase the pH above 4.5. 

Changing Soil pH

Once you’ve tested your soil to find what your pH is, decisions can be made about how to change the pH. Raising or lowering the pH is a big undertaking and a lot of factors need to be considered.

How to Lower Soil pH

There are several amendments used to increase soil acidity.

Sulfur

Compounds containing the element sulfur are used to intentionally reduce soil pH. Elemental sulfur, iron sulfate, and aluminum sulfate are all options that reduce the pH of the soil.

In addition, ammonium-containing fertilizers also lower pH. Ammonium sulfate (also containing sulfur), urea, and ammonium nitrate all lower pH. The sulfur-containing compounds are in the soil to create sulfuric acid which is effective at lowering pH.

Trying to lower the pH of your soil is a more challenging process than raising the pH. Elemental sulfur is involved in a biological reaction with soil bacteria to lower the pH. This means the process takes more time to lower the pH, making correct amendment tricky. Be careful when adding this sulfur, otherwise, the pH will be too low and your soil acidic.

Also keep in mind that in cooler climates, soil temperature affects biological activity. Soils need to be in the right conditions for bacteria to be active. 

Organic Matter

Organic matter breaking down can also release organic acids into the soil, but the amount is much smaller compared to other soil amendments.

Pine needles and used coffee grounds are sometimes found on the internet as potential acidifying amendments. However, they do not do anything to acidify the soil. Peat moss and sphagnum peat may acidify the soil slightly, but it is temporary and less effective than these other methods. 

The same goes for most ammonium fertilizers. Ammonium nitrate won’t have as big of an impact if used in lesser amounts. The ammonium phosphates and sulfates have a bigger impact on lowering pH. 

Rates for each of the amendments are different for each situation, so it is best to look at extension or government websites for your region that may give a better idea as to how much would need to be applied. Alternatively, contact your local agricultural extension for insight on treating alkaline soils in your region.

Considerations When Lowering pH

One of the biggest things to consider is the buffering capacity of the soil. This is essentially the soil’s ability to resist a change in pH. It is especially important with lowering pH.

As mentioned previously, clay content and organic matter play a big role in buffering capacity. The clay content also plays a role in the cation exchange capacity of the soil. The ability of the soil to exchange cations is called cation exchange capacity, or CEC.

CEC and soil texture are connected as are many things with soils. The higher CEC increases this buffering capacity. As you move up in texture to sandy soils, the CEC is worse and there are fewer sites for cations. Again, sand is easier to change, but also it is easy to drop soil ph too much, resulting in acidic soil.

How to Raise Soil pH

Raising the pH should happen before the soil develops aluminum toxicity at a soil pH value of 5.

Calcitic lime is the classic example that can be found at most nurseries and places that sell soil amendments. The amount required for your garden soil will vary.

Dolomitic lime is another choice and has magnesium instead of calcium. If your soil is deficient in magnesium, then adding dolomitic lime may be a better choice. The downside to dolomitic lime is that magnesium does not benefit soil structure like calcium does. If you need to lime and you want to add magnesium, alternate the use of calcitic and dolomitic lime.

Another thing to consider is finding as finely ground lime as possible, as this increases the surface area in which the soil can react to it.  Fluid lime makes spreading it easier as there is no dust that blows away. This is more expensive but might be more cost-effective for home gardeners. Typically resources online will overestimate how much lime needs to be applied. 

Hydrated and burnt lime are both extremely effective at raising soil pH. The downside is that these two are caustic, essentially meaning they can be corrosive. In most cases, using the other liming materials above like dolomitic and calcitic lime will work perfectly. 

A final amendment that could be used for changing the acidity deeper in the soil profile, while not covered as much, is gypsum. Gypsum changes the solubility of aluminum and is more effective at changing pH at depths. Gypsum can have other benefits to the soil as well.

Key Takeaways

Soils are incredibly complicated, and your soil’s pH is no exception. The most critical thing is testing your soil and buying a soil pH meter or soil test kit.

Nature plays a huge role in soil pH. The texture and climate are the two biggest factors. This means soil pH varies greatly regionally. The soil is inherently going to work to revert to its natural state.

For example, if you live in a warm and wet climate, the soils will continue acidifying even though you have applied liming materials. This means liming will need to take place more than once to change this.

Fighting nature is hard, so trying to fight it as little as possible should be your goal, whether that means planting more native plants that are adapted to the climate and soil, or choosing to plant plants that better fit the soil.

Frequently Asked Questions