Changing Soil PH

Introduction

This is an overview of soil pH, explaining its importance in agriculture and methods to adjust it for optimal plant growth. It covers strategies for raising soil pH through liming and the challenges of lowering it, especially in lime-rich soils. The guide offers practical insights for farmers and gardeners to manage soil acidity and alkalinity effectively. 

Raising Soil pH

One would want to raise soil pH if acidity is an issue for raising crops. The most common approach to neutralizing soil acidity, and thus raising soil pH, is through the addition of a liming agent. A good liming agent must perform two jobs. First, it should remove acid generating ions (typically H, Fe, and/or Al) from soil surfaces (called cation exchange sites) by replacing those ions with basic cations such as Ca or Mg. These acid generating ions are released into the soil solution. 

The second critical job of the liming agent is to neutralize the acidity added to soil solution by these ions. Several liming materials are available at garden centers and co-ops, with their effective acid neutralizing ability always based on an equivalent basis to calcium carbonate (i.e., the benchmark liming material). This is known as a liming materials’ calcium carbonate equivalency and is essentially based on the ratio of the amount of Ca (and/or Mg) to other elements present in the liming agent in relation to calcium carbonate. A list of common liming agents and their calcium carbonate equivalencies are presented in Table 1. 

Table 1. Common liming materials. 

Liming Material Chemical Formula	Common Name	Calcium Carbonate Equivalent (%)
CaCO3	Calcium carbonate; limestone	100
CaO	Calcium oxide; quicklime; burnt lime	179
Ca(OH)2	Calcium hydroxide; hydrated lime	135
CaMg(CO3)2	Dolomite; limestone	109
Aglime	Aglime (contains impurities)	60-90

The amount of lime to add varies depending on soil texture, cation exchange capacity (i.e., the amount of positively charged nutrients a soil can hold), and organic matter content. Based on these soil properties, a soil testing laboratory can determine a soil-buffer pH test to show you the change in soil pH based on lime application. The most common test used for Colorado soils is called the Shoemaker-McLean-Pratt (SMP) Buffer method. 

If you have a lab determine the soil- buffer pH, ask the laboratory to also determine the amount of limestone needed to raise your soil pH to the desired pH; laboratories routinely provide these types of recommendations. A rough rule of thumb for quickly determining the amount of limestone required to raise soil pH from a current value to pH 6.5 is presented in Table 2. 

Lowering Soil pH

Lowering the pH of alkaline soils is typically desired to reduce iron deficiency symptoms (e.g., Figure 2). However, in soils containing naturally occurring lime, it is difficult to impossible, cost prohibitive, and thus impractical to lower soil pH. To test for the presence of lime, add some vinegar to dry soil; the formation of bubbles, or fizz, indicates the presence of lime. When this occurs, lowering soil pH is impractical. To emphasize this point, if the top 6 inches of soil contains 2% lime, that equates to 20 tons of lime per acre and would require ~ 6.5 tons of elemental sulfur (S) per acre; this would only neutralize the lime in the top 6” (not deeper depths) and not additional alkaline sources on soil cation exchange sites. 

Lowering the pH of soils that do not contain lime requires the addition of elemental sulfur (S); do not add gypsum (CaSO ) as it is not an acidifying agent. In the presence of oxygen, water, and soil microorganisms, elemental S will be converted to sulfuric acid (H SO) to help lower soil pH. A general guideline is to add about 650 lbs of elemental S per acre to lower soil pH by one unit. Keep in mind that: 

• fine textured, clayey soils and soils above pH 7.3 likely will require greater elemental S rates; 
• incorporating elemental S to a soil depth that needs acidifying is necessary and will help speed up the conversion of elemental S to (H2SO4) 
• conversion of S to H2SO4 is dependent on soil moisture, temperature, and soil microbiological activity, and thus may take several months to a year to occur; and 
• monitoring soil pH several months to a year after initial application and re-applying elemental S on a yearly basis may be needed to help reduce and reach a target soil pH. 

Table 3. Definitions. 

Term or Abbreviation	Definitions
Al	Aluminum
Ca	Calcium
Cation	A positively charged ionic form of an element, e.g. Ca2+
Cation Exchange	The interchange between a cation in soil solution and another cation attached to the surface of negatively charged soil particle, such as clay or organic matter.
Cation Exchange Capacity (CEC)	The total amount of exchangeable cations that can be held by the soil. This value is highly influenced by clay type, clay content, and organic matter.
Fe	Iron
H	Hydrogen
Macronutrients	Plant nutrients found at relatively high concentrations in plants. Usually refers to N, P, K, Ca, Mg, and S.
Mg	Magnesium
Micronutrients	Plant nutrients found at relatively low concentrations in plants. Zinc and iron are the most common deficient micronutrients in Colorado soils.
S	Sulfur