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CropTalk: Soil pH and Liming Soils

September 2019

Published on Sunday, September 01, 2019

Soil is the miraculous biological, chemical, and physical medium that enables farmers, growers, and gardeners alike to plant, tend, and harvest a bounty of fruits and vegetables. For our purposes, soil is defined as the upper layer of earth that may be tilled and in which seeds germinate; emerge; take root; and ultimately grow to be plants (note: weeds are plants too). When the soil is fertile it is balanced with both water and air pore space, and the essential nutrients needed to support plant growth.

A primary factor that drives nutrient availability to plants is the soil pH. As depicted in Figure 1., the availability of 12 of the 16 Plant Essential Nutrients is derived primarily from the soil and varies with soil pH. The essential nutrients supplied by are and water are carbon, hydrogen, and oxygen; Chlorine (Cl) availability is not typically affect by pH.

Soil pH

The abbreviation pH stands for the potential (p) of hydrogen ions (H+) in water and provides a reference measurement of the relative acidity or alkalinity of the soil solution. Soil pH is expressed on a 14-point scale, where a value of 7.0 is considered neutral. Lower values (less than 7.0) on the pH scale denote increasing H+ ions and acidity, while higher values (greater than 7.1) represent increasing hydroxyl (OH-) ions and alkalinity. Alkaline soils are also called basic soils. The logarithmic scale for pH means that one unit of change is a ten-fold change in hydrogen content.

Soil pH affects many physical, chemical, and biological reactions necessary for crop survival, growth and yield. This measurement is a component of all standard soil tests. An understanding of soil pH is integral to the management of nutrients in the soil; and thus, all applied fertilizers.

Lime Neutralizes Soil Acidity

When soil pH is at a level less than desired for optimum nutrient availability, liming materials are broadcast applied to the field. When soils are acidic, there are many benefits from liming. Achieving a pH of 6.0 - 6.5 can provide a better environment for beneficial bacteria and fungi in soils. As depicted in Figure 1., the availability of phosphorus is affected by soil pH. Liming to a pH of 6.5 noticeably increases the supply of soil phosphorus available to plants. An agricultural liming material contains calcium (Ca) and/or magnesium (Mg) elements capable of neutralizing soil acidity. Most soils in many states contain sufficient Ca for crop growth. Liming materials are generally not used to supply Ca, but, depending on the product, maybe a key source of Mg.

When applied to the soil, Ca and Mg dissolve out of the liming materials and displace hydrogen (H+) ions from the clay particles in the soil. It is these H+ ions that make soils acidic. The net effect of this chemical process is that the H+ converts from an ion on a clay particle to a neutral molecule of water, thereby reducing soil acidity.

Containing Ca or Mg does not mean a compound is a liming material. Gypsum is calcium sulfate (CaSO4 • H2O). When added to the soil, the Ca in the gypsum can displace the H+ on a clay particle. However, the H+ remains in the soil solution and the pH does not change. In some areas, when managing base saturation of the soil cations, quality ag lime or gypsum is used to supply Ca and effectively displace excess Mg levels. In combination, ag lime and gypsum can correct soil pH and drive the base saturation to desired levels. For additional information on base saturation in soils, you may reference Chad Kalaher’s article from the March 2019 Issue of CropTalk by visiting: http://bit.ly/BaseSaturation

Crop Optimal pH

The optimum pH for crop growth varies among crops. Most Midwestern universities divide crops into three groups. These groups are shown below in Figure 2. A general recommendation for mineral soils is to apply lime to raise the soil pH to 6.5 for Group I crops. For any crop in Group II, apply lime to raise the soil pH to 6.0. The crops listed in Group III grow best in acidic soils so no lime is needed. For organic soils (O.M. greater than 10%), raise the soil pH to 5.5.

To learn more about soil pH and liming your soils for optimum crop growth, please reference your respective state extension service bulletins on the topic.

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