What is Soil pH?

Soil pH (also known as soil reaction) is a 14-unit scale (0-14) which measures the relative acidity or basicity (alkalinity) of a soil.  A pH of 7 is the neutral mark.  All values below 7 are considered acidic or sour soil.  All values above 7 are considered alkaline or sweet soil.  Soil pH is one of many factors that affect the availability of certain minerals in soil for absorption by plants.

The pH scale is based on a negative base-10 logarithm of hydrogen ion concentration.  One full unit of difference in pH means the higher pH value is 10 times more alkaline than the lower pH value, and the lower pH value is 10 times more acidic than the higher pH value.  For example, a soil with a pH value of 7.2 is 10 times more alkaline than a soil with a pH of 6.2.  Similarly, a soil with a pH of 6.2 is 10 times more acidic than a soul with a pH value of 7.2. 

While most plants prefer a near neutral or slightly acidic pH level (usually a pH in the range of 6-7), some plants do require more acidic soil.  For example, Irish potatoes grow best in soil with a pH level between 5 and 5.5 to prevent potato scab disease. 

Extremely low or high pH levels can affect the availability of plant nutrients and the concentration of minerals in the soil that are toxic to plants.  At pH levels lower than 5.5, aluminum and manganese can become highly available to the point of being toxic.  At similarly low pH levels, other elements like nitrogen, calcium, phosphorous, magnesium, and potassium become less available for absorption by plants.  When the pH level of soil is above 7, elements like iron, zinc, copper, boron, phosphorous, and manganese also become less available for absorption by plants.

Soil pH scale

In the case of extreme pH levels in soil, simply adding fertilizer to the soil will not correct the problem.  Instead, the pH must be brought back into the desired range by adding ground agricultural limestone to raise the pH or by adding aluminum sulfate or elemental sulfur to lower the pH level.  Perform a soil test to determine how much must be added to the soil to bring the pH level back into range. 

What Factors Affect Soil pH?

Many factors can influence the overall acidity or alkalinity of soil. The rocks from which the soil is created release various minerals and secondary minerals which affect pH.  Rainfall in areas where the soil has good drainage can leach plant nutrients as the water passes through the soil which in turn are replaced by more acidic elements like hydrogen, aluminum, and iron.  Soils in regions with high amounts of rainfall tend to be more acidic (lower pH levels) than do soils in dry, arid regions which measure higher in alkalinity (higher pH levels).

Ammonium nitrogen-based fertilizers increase acidity, as does the decomposition of organic matter, which releases ammonium nitrogen that gets converted to nitrate nitrogen during later stages of decomposition.  When ammonium is converted to nitrate, it releases hydrogen ions as a byproduct which increase soil acidity.

Fortunately, soils can be sampled and tested to determine pH levels, and materials can be added to the soil to adjust pH to the desired levels.  How much of these materials need to be added depend on many factors like the texture of the soil, the amount of organic matter present, the type of soil, how far you want to move the pH levels, and the types of crops or plants being grown.

To raise pH levels of soil, generally apply limestone, which is made of calcium carbonate (CaCO3).  The most common type of lime used is ground agricultural limestone.  The finer the lime particles are ground, the faster it can raise acidity levels. 

To lower pH levels of soil, apply elemental sulfur or aluminum sulfate.  Most plants produced for food or ornamental purposes thrive best in acidic soil.  When these plants are grown in soil with alkaline pH levels (above 7), their leaves yellow due to iron chlorosis.  This is often confused with the soil being deficient in nitrogen because both issues lead to the yellowing of leaves.  The problem of iron chlorosis can be reversed if you lower the pH level of the soil so that it falls in the appropriate acidic pH range below 7. 

How Does Lime Lower pH Levels?

Acidic soils can have their soil pH raised by adding limestone.  This limestone “sweetens” the soil by improving nutrient retention, promoting biological activity, and improving the structure of the soil.

Remember that pH is based on the amount of hydrogen (H+) ions or cations present in the soil.  The more hydrogen ions present, the more acidic the soil will be.  When lime (CaCO3) is added to soil, a series of chemical reactions occur:

  • When lime (CaCO3) is applied and mixes with water in the soil to form a solution, it splits into calcium (Ca2+) and carbonate (CO32-).
  • The released calcium (Ca2+) cation replaces two hydrogen (H+) cations on the surface of soil particles through cation exchange.
  • The released carbonate (CO32-) combines with water (H2O) in the soil to yield bicarbonate (HCO3-) and hydroxide (OH-).
  • The free hydrogen (H+) cations then combine with the free hydroxide (OH-) anions to create water (H2O), thus reducing the number hydrogen (H+) cations that exist in the soil which raises the pH level, lowering acidity.

When and How Should Lime be Applied?

A soil test should be performed in the fall to determine the soil’s pH level and whether an adjustment needs to be made to the pH for the next year’s garden.  If  lime needs to be added because the soil is too acidic, it can be applied in the fall or winter.  But it is typically best to apply lime 2 to 3 months before planting.

Lime must be mixed well with the soil to maximize the effectiveness of the lime-soil reaction described above. Because most lime is not easily dissolved in water, it is very important that the soil be moist as the first step in the lime-soil reaction requires the lime to be in solution.  The lime will have little effect on pH if the soil is dry. 

What Type of Lime Should You Use?

There are various types of liming materials.  The effectiveness of each liming material in neutralizing acidic soil by raising its pH is compared to that of pure calcium carbonate (CaCO3) to determine its “relative neutralizing value” (RNV) or its “effective calcium carbonate equivalent” (ECCE).

 

Liming Material

RNV/ECCE

Burned Lime

150-175

Hydrated Lime

120-135

Dolomitic Lime

95-108

Calcium Carbonate

100

Calcitic Lime

85-100

Baked Oyster Shells

80-90

Marl

50-90

Basic Slag

50-70


Relative Neutralizing Values of Liming Materials

If using a liming material other than calcium carbonate, the relative neutralizing value of the alternate material can be used to calculate how much of the alternate material would be required to yield the same results as some fixed amount of calcium carbonate.  The formula is as follows:

For example, if you know 50 pounds of calcium carbonate are required to raise the pH of a garden’s acidic soil to the appropriate pH and you can only get calcitic lime with a relative neutralizing value of 88, then the formula above (100/88)*50 would indicate that you need 56.8 pounds of calcitic lime to achieve the same pH. 

Calcitic and Dolomitic Limestone

Calcitic (CaCO3) and dolomitic (CaMg[CO3]2) limestone are the most common sources of lime in the United States. Calcitic limestone is the naturally occurring form of calcium carbonate and contains about 40% calcium.  Dolomitic limestone (or calcium magnesium carbonate) contains 21-30% calcium as well as 6-11 percent magnesium.  For this reason, dolomitic limestone is often used when gardeners need to increase the amount of magnesium in the soil as well.

Both calcitic limestone and dolomitic limestone come in a fine, ground form.  However, they can also be purchased in pellet form that dissolves when it rains so that it can be used in a centrifugal spreader.  As previously mentioned, lime should be thoroughly mixed with the soil because of its insolubility.  It requires about 500 gallons of water to dissolve a single pound of lime.  Typically, it takes about 4 to 6 months after application for calcitic an dolomitic limestone to raise the pH to appropriate levels. 

Fast-Acting Limes

If you need to raise the pH of soil faster, there are products available for that such as quicklime and hydrated lime, both of which are calcium compounds.  Commercial growers often use these products.  Quicklime is calcium oxide (CaO), and hydrated lime is calcium hydroxide (Ca(OH)2).  Quicklime reacts faster than calcium hydroxide, but they are both quite fast.  Care must be taken when applying these products as they are corrosive and can easily burn or injure your plants.