SOIL pH IS a basic soil property that affects many chemical and biological activities in the soil. The degree of acidity or alkalinity in the soil is known as the “Soil Reaction” or soil pH, measured by pH (potential Hydrogen ion, H+) concentration in the soil. It can be thought of as a measure of the amount of lime (calcium) contained in the soil and the type of soil. In a broad generalization, soils in moist climates tend to be acid and those in dry climates are alkaline. However, the material from which the soil was formed and the way it has been used both affect pH, so that in any area, or at different depths below ground, the measurements may vary widely.
Soil pH is expressed on a logarithmic scale between 0. 0 - 14.0 . A soil with a pH lower than 7.0 is an acid soil and one with a pH higher than 7.0 is alkaline. A pH of 7.0 is neutral.
Lime Index or Buffer pH is a measure of the soil’s capacity to resist pH change after lime has been added. Two soils with the same soil pH may have quite different buffer pH’s, and thus one will require significantly more limestone than the other to obtain an optimal soil pH. The extent to which the buffer pH is lower than 6.8 is proportional to the amount of limestone needed. The soil lab makes this calculation if needed.
SOIL pH EFFECTS INCLUDE
Nutrient availability: Most plants grow best in mineral soils with a slightly acid reaction. In this range, most plant nutrients are at or near their high 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.
Soil functions: Soil pH can also influence plant growth by its effects on the activity of beneficial soil based microorganisms that build soil structure, cycle organic matter and fix nitrogen in the soil.
Pesticides: Soil pH can also have a significant effect on the performance and breakdown of many pesticides. In alkaline soils some herbicides persist for many months longer than expected and may injure future crops. Some soil-applied insecticides become ineffective in alkaline soil.
Soil organic matter: Bacteria that decompose soil organic matter are hindered in strongly acid soils. This prevents organic matter from breaking down, resulting in an accumulation of organic matter and the tie-up of nutrients particularly Nitrogen, that are held in the organic matter. Phosphorus, Potassium, Calcium and Magnesium also become insoluble in the high acid soils.
Some plants do well in moderate to high acid soils, while other plants are more tolerant of alkaline soils. Alkaline tolerant plants do not do well in acid soils, nor do acid-loving plants thrive in high pH soils. In both cases they tend to become chlorotic, or pale, often yellowish between the greener leaf veins, due to the inability to absorb sufficient nutrients. It is usually best to choose plants adapted to existing soil conditions rather than trying to change the soil, but for many reasons, farmers and gardeners have long worked at amending the soil to suit the plants they want to grow.
This article gives more information about iron chlorosis in trees and shrubs: University of Illinois Extension bulletin 603.pdf
This site was last updated 11/22/06