The fundamental understanding of soil pH is simple. It is simply a way to measure a condition, not so different from using a ruler. Acidity and alkalinity are measured as pH (parts Hydrogen), expressed in a logarithmic scale from zero to 14. Specifically, acidity is associated with an increase in hydrogen ions, and alkalinity is with an increase of hydroxyl ions. The differences affect how molecules (or potential plant nutrients) will interact in the soil.
A pH of 7.0, like that of pure water, is neutral, because the concentrations of hydrogen ions and hydroxyl ions are equal. As the pH number goes lower than 7, the hydrogen ions increase and the soil becomes more acidic; as the pH number goes higher than 7, the concentration of hydroxyl ions increases and the soil becomes more alkaline.
Most garden plants grow best in a slightly acidic soil, with a pH between 6.0 and 7.0. Going from one pH number to the next makes a big difference, because the numbers change by factors of 10. A pH of 6 is 10 times more acidic than a pH of 7 and pH of 5 is 100 times more acidic than a pH of 7.
What is the significance of soil pH?
Availability of Nutrients
Soil pH influences the solubility of nutrients. It also affects the activity of micro-organisms responsible for breaking down organic matter and most chemical transformations in the soil. Soil pH thus affects the availability of several plant nutrients.
A pH range of 6 to 7 is generally most favorable for plant growth because most plant nutrients are readily available in this range. However, some plants have soil pH requirements above or below this range.
Soils that have a pH below 5.5 generally have a low availability of calcium, magnesium, and phosphorus. At these low pH’s, the solubility of aluminum, iron, and boron is high; and low for molybdenum. At pH 7.8 or more, calcium and magnesium are abundant. Molybdenum is also available if it is present in the soil minerals. High pH soils may have an inadequate availability of iron, manganese, copper, zinc, and especially of phosphorus and boron.
Micro-organisms
Soil pH affects many micro-organisms. The type and population densities change with pH. A pH of 6.6 to 7.3 is favorable for microbial activities that contribute to the availability of nitrogen, sulfur, and phosphorus in soils.
Pesticide Interaction
Most pesticides are labeled for specific soil conditions. If soils have a pH outside the allowed range, the pesticides may become ineffective, changed to an undesirable form, or may not degrade as expected, which results in problems for the next crop period.
Mobility of heavy metals
Many heavy metals become more water soluble under acid conditions and can move downward with water through the soil, and in some cases move to aquifers, surface streams, or lakes.
Corrosivity
Soil pH is one of several properties used as a general indicator of soil corrosivity. Generally, soils that are either highly alkaline or highly acid are likely to be corrosive to steel. Soils that have pH of 5.5 or lower are likely to be highly corrosive to concrete.
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Factors Affecting Soil pH
The pH value of a soil is influenced by the kinds of parent materials from which the soil was formed. Soils developed from basic rocks generally have higher pH values than those formed from acid rocks.
Rainfall also affects soil pH. Water passing through the soil leaches basic nutrients such as calcium and magnesium from the soil. They are replaced by acidic elements such as aluminum and iron. For this reason, soils formed under high rainfall conditions are more acidic than those formed under arid (dry) conditions.
Application of fertilizers containing ammonium or urea speeds up the rate at which acidity develops. The decomposition of organic matter also adds to soil acidity.
How is soil pH modified?
A soil pH below about 5.6 is considered low for most crops. Generally, the ideal pH range is between 6.0 and 7.0. Liming is a common method to increase the pH. It involves adding finely ground limestone to the soil. The reaction rate for limestone increases when soil temperatures are warm and soil moisture is high. If the limestone is more finely ground, the reaction is faster.
The amount of limestone to apply depends on the amount of organic matter and clay as well as the pH. Fertility testing laboratories that have local experience make this determination.
A soil pH that is more than about 8.0 is considered high for most crops. Soils that have a pH in this range are often also calcareous.
Calcareous soils have a high content of calcium carbonate. The pH of these soils does not change until most of the calcium carbonate is removed. Acids that are added to the soil dissolve the carbonates and lower the soil pH. Treatments with acid generally are uneconomical for soils that have a content of calcium carbonate of more than about 5%. Because phosphorus, iron, copper, and zinc are less available to plants in calcareous soils, nutrient deficiencies are often apparent. Applications of these nutrients are commonly more efficient than trying to lower the pH.
When the soil pH is above 8.6, sodium often is present. These soils generally do not have gypsum or calcium carbonates, at least not in the affected soil horizons. Addition of gypsum followed by leaching using irrigation is a common reclamation practice. However, salts flushed into drainage water may contaminate downstream waters and soils.
The application of anhydrous ammonia as a nitrogen fertilizer contributes to lowering the soil pH. In some parts of the country, applications of ammonia lower the surface soil pH from ranges of 6.6 to 7.3 to below 5.6. This reduction can be easily overlooked in areas of no-till cropping unless the pH is measured in the upper 2 inches. Chemical amendments that contain sulfur generally form an acid, which lowers the soil pH.
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