Soil pH – A Basic Soil Property

Soil pH is a basic soil property that affects many chemical and biological activities in the soil. The degree of acidity or alkalinity in a soil is known as the “Soil Reaction”. In the laboratory, the “Soil Reaction” or “soil pH” are determined by the hydrogen ion (H+) concentration in the soil solution. An acid soil has more H+ than OH- ions, while a basic or alkaline soil has more OH- than H+ ions. The term “Soil pH” was coined to characterize this reaction in a soil.

Soil pH is expressed on a logarithmic scale. A soil with a pH of 6.0 has ten times as many hydrogen ions (H+) present as a soil with a soil pH of 7.0. The relative acidity or alkalinity of soils across a wide range of soil pH values is shown below.

Soil pH Range Soil Reaction
3.0 – 4.0 Very Strongly Acid
4.0 – 5.0 Strongly Acid
6.0 – 7.0 Slightly Acid
7.0 – 8.0 Slightly Alkaline
8.0 – 9.0 Moderately Alkaline
9.0 – 10.0 Very Strongly Alkaline

soilphpicMost plants grow best in soils with a slightly acid reaction. In this range, most plant nutrients are at or near their highest solubility. Generally, plants take up nutrients only if they are dissolved in the soil solution, so if the nutrients are in the soil solution they are available for plant uptake. In soils which are strongly acid (pH 4.0-5.0), aluminum is very soluble and dissolves more readily (aluminum is a component of clay and silt particles). As the level of dissolved aluminum increases, a chemical reaction occurs with phosphorus compounds, making them insoluble and unavailable to plants. If the dissolved aluminum levels become too high, toxic levels are reached and plants die.

When soil pH is moderately alkaline (pH>8.0), the solubility of many nutrients decreases. Phosphorus is an example of a nutrient that becomes less soluble as soil pH increases. Additional phosphorus is added to compensate for adverse affects caused by high soil pH. It is uneconomical to lower soil pH for most crops in production agriculture.

Soil pH also affects the activity of soil organisms that build soil structure, cycle organic matter or fix nitrogen in the nodules of legumes. Soil pH can have a significant effect on the performance and breakdown of many pesticides.

In the laboratory there are many accepted methods for soil pH determination. The methods for soil pH determination offered by AGVISE Laboratories are listed below. If you need a method that is not listed below, please give us a call. We will be happy to work up any accepted method you request.

Soil pH Method
Calcium Chloride Method
Potassium Chloride Method
Water Method (1:1) Method
Saturated Paste (1:5) Method