Soil pH

Soil pH is a measure of the acidity and alkalinity in soils. pH levels range from 0 to 14, with 7 being neutral, below 7 acidic and above 7 alkaline. The optimal pH range for most plants is between 5.5 and 7.0; however, many plants have adapted to thrive at pH values outside this range. Because pH levels control many chemical processes that take place in the soil – specifically, plant nutrient availability – it is vital to maintain proper levels for your plants to reach their full yield potential.

Soil Acidity

An acid is defined as a substance that tends to release hydrogen ions (H+). Conversely, a base is defined as a substance that releases hydroxyl ions (OH-). All acids contain hydrogen ions, and the strength of the acid depends upon the degrees of ionization (release of hydrogen ions) of the acid. The more hydrogen ions held by the exchange complex of a soil in relation to the basic ions (Ca, Mg, K) held, the greater the acidity of the soil.
NOTE: Aluminum (Al) also contributes to soil acidity, but for simplicity, further discussion of soil acidity will be limited to H as the cause of soil acidity.

Source: IPNI

pH Range

5.0 – 5.5	5.5 – 6.5	6.5 – 7.0
Blueberries	Barley	Alfalfa
Irish Potatoes	Bluegrass	Some Clovers
Sweet Potatoes	Corn	Sugar Beets
	Cotton
	Fescue
	Grain Sorghum
	Peanuts
	Rice
	Soybeans
	Watermelon
	Wheat

Desirable Soil pH for Optimum Crop Production pH Range

The desirable pH range for optimum plant growth varies among crops. While some crops grow best in the 6.0 to 7.0 range, others grow well under slightly acidic conditions. Soil properties that influence the need for and response to lime vary by region. A knowledge of the soil and the crop is important in managing soil pH for the best crop performance.
Soils become acidic when basic elements such as calcium, magnesium, sodium and potassium held by soil colloids are replaced by hydrogen ions. Soils formed under conditions of high annual rainfall are more acidic than are soils formed under more arid conditions. Thus, most southeastern soils are inherently more acidic than soils of the Midwest and far West.
Soils formed under low rainfall conditions tend to be basic with soil pH readings around 7.0. Intensive farming over a number of years with nitrogen fertilizers or manures can result in soil acidification. In the wheat-growing regions of Kansas and Oklahoma, for example, which have soil pH of 5.0 and below, aluminum toxicity in wheat and good response to liming have been documented in recent years.

Factors Affecting Soil Acidity

Rainfall

Rainfall contributes to a soil’s acidity. Water (H₂O) combines with carbon dioxide (CO₂) to form a weak acid — carbonic acid (H₂CO₃). The weak acid ionizes, releasing hydrogen (H+) and bicarbonate (HCO₃). The released hydrogen ions replace the calcium ions held by soil colloids, causing the soil to become acidic. The displaced calcium (Ca++) ions combine with the bicarbonate ions to form calcium bicarbonate, which, be ing soluble, is leached from the soil. The net effect is increased soil acidity.

Subsoil Acidity

Even if the top 6 inches of soil show a pH above 6.0, the subsoil may be extremely acidic. When subsoil pH's drop below 5.0, aluminum and manganese in the soil become much more soluble, and in some soils may be toxic to plant growth. Cotton and, to some extent, soybeans are examples of crops that are sensitive to highly soluble aluminum levels in the subsoil, and crop yields may be reduced under conditions of low subsoil pH. If you’ve observed areas of stunted plants in your field, take a subsoil sample in these areas. If the soil pH is extremely acidic (below 5.2), lime should be applied early in the fall and turned as deeply as possible.

Liming Soil Pays

Correcting soil acidity by the use of lime is the foundation of a good soil fertility program. Lime does more than just correct soil acidity. It also:

• Supplies essential plant nutrients, Ca and Mg, if dolomitic lime is used
• Makes other essential nutrients more available
• Prevents elements such as Mn and Al from being toxic to plant growth.

Limestone Increases Fertilizer Efficiency and Decreases Soil Acids

Soil Acidity	Nitrogen	Phosphate	Potash	Fertilizer Wasted
Extremely Acid — 4.5 pH	30%	23%	33%	71.34%
Very Strong Acid — 5.0 pH	53%	34%	52%	53.67%
Strongly Acid — 5.5 pH	77%	48%	77%	32.69%
Medium Acid — 6.0 pH	89%	52%	100%	19.67%
Neutral — 7.0 pH	100%	100%	100%	00.0%

Liming Material	Composition	Calcium Carbonate Equivalent (CCE)
Calcitic Limestone	CaCO3	85-100
Dolomitic Limestone	CaCO3; Mg CO3	95-108
Oyster Shells	CaCO3	90-110
Marls	CaCO3	50-90
Hydrated Lime	Ca(OH)2	120-135
Basic Slag	CaSiO3	50-70
Gypsum	CaSO4	None

Liming Materials

Liming materials contain calcium and/or magnesium in forms, which when dissolved, will neutralize soil acidity. Not all materials containing calcium and magnesium are capable of reducing soil acidity. For instance, gypsum (CaSO₄) contains Ca in appreciable amounts, but does not reduce soil acidity. Because it hydrolyzes in the soil, gypsum converts to a strong base and a strong acid as shown in the following equation:
CaSO4 + 2H2O = Ca (OH)2 + H2SO4 The formed Ca (OH₂) and H₂SO₄ neutralize each other, resulting in a neutral soil effect. On the other hand, when calcitic (CaCO₃) or dolomitic lime (Ca Mg (CO₃)₂) is added to the soil, it hydrolyzes (dissolves in water) to a strong base and a weak acid.
CaCO3 + 2H2O = Ca (OH)2 + H2CO3 Calcium hydroxide is a strong base and rapidly ionizes to Ca++ and OH- ions. The calcium ions replace absorbed H ions on the soil colloid and thereby neutralize soil acidity. The carbonic acid formed (H₂CO₃) is a weak acid and partially ionizes to H+ and CO₂^-2 ions. Therefore, the net effect is that more ca than H ions are released in the soil, and consequently, soil acidity is neutralized.

Calcitic Limestone

Ground limestone contains mostly calcium carbonate and generally has less than 1 to 6 percent magnesium. Its neutralizing value depends on its purity and fineness of grinding.

Fluid Lime

A liming material commonly referred to as fluid lime generally consists of finely ground limestone suspended in water at a ratio of about 50 percent water to 50 percent limestone. In most instances, producers of fluid lime utilize very finely ground limestone – most of which will pass a 200-mesh screen. Fluid lime is capable of changing soil pH in a relatively short period of time. This is a distinct advantage in situations in which liming has been delayed to just before planting, or in situations in which low soil pH is discovered after a crop is planted. Keep in mind, since fluid lime contains approximately 50 percent water, this means that a farmer applying fluid lime at the rate of 1,000 pounds per acre would be applying only 500 pounds of limestone.

Pelletized Lime

Pelletized lime is finely ground agricultural limestone that is pelletized with the aid of clay or synthetic binders to produce pellets in the 5- to 14-mesh range. Usually, about 70 percent of the initial limestone, prior to pelletizing, passes 100- to 200-mesh sieves. It may be spread with conventional spinner fertilizer spreaders, which makes it attractive to use. Unpublished research indicates that pelletized lime should be allowed to react with a good rainfall or irrigation on the soil surface to disperse the pellets before it is mixed with the soil. If rates of 250 to 500 pounds of this liming material are mixed with the soil before the pellet "melts" down, a limited soil volume may be affected by each pellet, and desirable pH adjustment of the plow layer may not be achieved.

Use of Fluid Lime and Pelletized Lime

Fluid and pelletized lime are excellent sources of lime to be used under certain circumstances such as:

• Correction of a low soil pH condition after a crop is planted
• A rapid change in soil pH if liming is delayed to just before planting a crop
• For maintaining pH in the optimal range for plant growth and yield.

However, these two liming materials should not be depended upon to maintain the soil pH during the full crop-growing season if applied at one-fourth of the recommended lime rate.

Fineness of Grinding is Important in Selecting Liming Materials

Lime quality is measured by how effectively it neutralizes soil acidity. This is determined largely by its chemical purity and size of particles.
The purity of lime is expressed as calcium carbonate equivalent (CCE). This is a measure of how much of the material can react with the soil to neutralize acidity under ideal conditions

compared to pure calcium carbonate. Limestone should have a neutralizing value of at least 90 percent. Even if the CCE of lime is satisfactory, it will not neutralize soil acidity unless the limestone is finely ground.
In an attempt to arrive at a more accurate lime rating to measure liming material effectiveness,

some states' soil test laboratories have adopted effective calcium carbonate content for rating liming materials. An efficiency rating is arrived at by multiplying the calcium carbonate equivalent times the effective calcium carbonate content, which is based on the fineness of the liming material.

Efficiency Factors for Liming Materials

The following example of the "effective neutralizing value" (ENV) calculation, used by the University of Illinois, serves to illustrate the importance of lime particle size in potential soil acidity neutralization.

ENV = Total fineness efficiency x (% calcium carbonate equivalent / 100)

Example

Assume that a liming material has a 96 percent calcium carbonate equivalent. After screening, the liming material is found to have the following particle size distribution:
+8 mesh = 4%
–8 to +30 = 25%
–30 to +60 mesh = 26%
–60 mesh = 45%
The total fineness efficiency factor may be calculated as follows for the example material:
+8 mesh efficiency is 5%, so .04 x 5 = 0.20
–8 to 30 mesh efficiency is 20%, so .25 x 20 = 5.00
–30 to +60 mesh efficiency is 50%, so .26 x 50 = 13.00
–60 mesh efficiency is 100%, so .45 x 100 = 45.00
Total Fineness Efficiency for 1st Year = 63.20
Therefore, the effective calcium carbonate content of ENV = 63.20 x (96/100) = 60.67 for this example of liming material for the first year.
These calculations enable a grower to determine the shorter- and longer-term value of the liming material being considered for purchase.

Particle Size	Within 1 Year of Application	After 4 Years of Application
Greater than 8 mesh	5	15
Between 8 and 30	20	45
Between 30 and 60	50	100
Smaller than 60 mesh	100	100