Have you ever measured the pH of the soil in your garden? If not, you should, especially if you have added amendments with the hope of changing pH. Testing is the only way to know for certain what the soil pH is. Gardeners have often been advised to put wood ash and other neutralizing compounds in the soil regardless of testing. Reading through this article, you will learn more about why pH matters, how to lower soil pH, and if needed how to raise soil pH.
The internet is a wonderful place for many things, but often information related to soils and gardening is not as accurate as it could be. One of the goals of this article is to alleviate some of this confusion by giving background on the science behind soil pH and why some of these suggested methods are often inadequate.
It seems daunting at first 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 of 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.
What is pH?
In basic terms, pH describes the acidity or alkalinity that a substance has. The pH scale ranges from 0 to 14, where the lower the number means the substance is more acidic and the higher numbers mean it is more alkaline. A pH of 7.0 is considered neutral. When the number is below 7.0 it is considered acidic, and when it is above 7.0 it is basic, also called alkaline. 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). It is important to note when changing between pH values that it is increasing or decreasing by a multiple of 10. The graphic below showcases this. This means the difference between a pH of 5 and 4, for example, is more significant than a difference of 1 unit.
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. This is a less dominant measurement, but it may show up while looking into acidity and alkalinity.
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 climatical factors can help us as gardeners understand why the soils we grow on will inherently have the pH they do. Rainfall is one of the more important ones to consider when thinking about pH. Rainfall will wash away more of the basic elements like magnesium, potassium, and calcium. This allows more of the 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, meaning they have a pH of above 7.
As with most things related to soils, 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 of a challenge compared to sandy soils. This is because of the clay particles’ ability to hold on to cation elements. As I have mentioned the concentration of positively charged hydrogen ions is how pH is measured, so these ions are better held on negatively charged clay particle surfaces. Increasing the organic matter in soil can help with this problem since these organic matter particles can take up some of the surface areas on the clays. Sand particles do not have these types of sites on them so it is much easier to lower your pH. You must be careful because the pH level easily becomes too acidic.
Why pH is Important
Nutrient Availability
Soil pH and its impact on plant nutrients. Source: University of California Riverside
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. The graphic above does a good job of displaying these important nutrients for plants and where they are most available. At both extremes of the pH scale, you find that 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 the nutrient deficiency being displayed on the plant could be due to the availability, not lack of the nutrient. As I will explain, when pH is too low, the soil is subject to aluminum toxicity.
Aluminum Toxicity
Once the soil is at a pH of 4.5 it is subject to aluminum toxicity. Aluminum toxicity is not good. 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 conditions with a soil pH value of 4.5-5.0 then plant acid-loving plants like blueberries or azaleas. The other option is to lime the soil so that the pH increases above 4.5.
Changing Soil pH
Once you’ve tested your soil to find where your pH is at, 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. Here I will break down lowering soil pH and raising soil pH.
Lowering Soil pH
Testing one’s soil pH is crucial before amending soil. Source: Karl Dawson
There are several amendments that can be used to amend basic or alkaline soil to increase soil acidity. Mostly 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. Sulfuric acid is sometimes directly used but would be much harder for the home gardener to use.
Sulfur compounds can be tricky, and trying to lower the pH of your soil is a more challenging process than raising pH in soil. It is less straightforward as there is a difference in how the pH is lowered. Elemental sulfur is one of a few organic soil amendments that can change pH. Elemental sulfur is involved in a biological reaction with soil bacteria to lower the pH. What does this mean? It means that this process takes more time to lower the pH. 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. Iron and aluminum are acidic cations that work to chemically reduce soil pH. This is much different than the elemental sulfur. It cannot be stressed enough that the biological reaction with microbes using elemental sulfur means applying and waiting.
Organic matter breaking down can also release organic acids into the soil, but the amount is much smaller compared to other soil amendments. Organic materials are an important thing to build in the soils but ultimately are not a good choice to change the soil pH. Pine needles and used coffee grounds are sometimes found on the internet as potential acidifying amendments. They do not do anything to acidify the soil. Peat moss and sphagnum peat are common recommendations for the soil. While they may acidify the soil slightly, it is temporary and less effective than these other methods. The same is 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. Cations are positively charged elements such as calcium and magnesium. These cations exchange in the soil solution with particles on the negatively charged clays. 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.
Raising pH
Raising the pH should happen before the soil develops aluminum toxicity at a soil pH value of 5. Many amendments can be added to raise the pH of acidic soils. Some options are calcitic limestone, dolomitic limestone, hydrated lime, burned lime, fluid lime, and wood ashes. Wood ashes are a very common recommendation to add to soils, and many gardeners do this yearly. This will raise the pH, but without actually testing the soil, this may present a problem, creating alkaline soil. Wood ash is the least effective compared to the other amendments listed.
Calcitic lime is the classic example that can be found at most nurseries and places that sell soil amendments. It is incredibly common to use this to raise soil pH, but 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.
A fun historical note about raising soil pH is that oyster shells were commonly used by indigenous people in my home state of Maryland. The reason these were used is that they are made of calcium carbonate, the same thing that is calcitic lime.
Container Growing
Growing plants in containers with soilless media presents several considerations. All of the properties are different. Many of the growing media do not conduct CEC and do not have clay content, amongst other differences from actual soil. Most common mixes contain coconut coir or sphagnum peat moss, both of which have benefits for growing, but also have environmental consequences. Growing in these media may avoid some of the aluminum toxicity found in regular soils. Most of the potting mixes when you buy them will be at the appropriate pH for growing container vegetables. If there are specific plants that thrive in different pH values, container growing mixes are sold that specialize in higher pH or lower pH levels.
Final Thoughts on pH
Home testing of soil pH can tell you if you need pro tests done for more detail. Source: Chiot’s Run
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. Reading soil tests could be done in an entirely separate article, but buying a pH meter can indicate the pH of your soil. That is the only way to truly know if you need to reduce pH or raise it. You cannot tell exactly what the pH is just by looking at it. Garden soil ph can vary greatly depending on where it came from or where you live.
Nature plays a huge role in soil pH. The texture and climate are the two biggest factors. This means soil pH varies greatly regionally. You cannot change soil texture or the climate of your region. 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. It must be kept in mind that when you add reducing or raising amendments you are changing the soil. Fighting nature is hard, so trying to fight it as least 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. As an example, one might attempt to plant blueberries in an area with a pH of 8.0 instead of one with a pH value of 6.0. The difference may seem insignificant, but changing 8.0 to a 4.5 or 5 is a difficult task, and changing a 6 to a 5 will be statistically easier. It will ultimately be a lot more work than just planting your berries in better-suited areas.
Frequently Asked Questions
Q: What fertilizer will lower pH of soil?
A: Ammonium and sulfur fertilizers will lower the soil of pH. Ammonium sulfate, ammonium nitrate, and urea are all examples of these fertilizers. Inversely, nitrate fertilizers will raise the soil pH.
Q: Can you use vinegar to lower pH in soil?
A: Vinegar is acidic, but ultimately is not effective compared to other methods outlined in this article.
Q: Why is my soil pH so high?
A: There are a few reasons to explain this. One could be simply the environment you live in. Soils receiving less water and that have higher sand content naturally have a higher pH. Other reasons could be the continuous application of wood ash and alkaline amendments.
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