Concept About Soil Acidity and Alkalinity
Plant nutrition, growth, and even survival depend on the chemical conditions of the soil. Nutrient-rich soils can also be infertile at times. The sole reason for this is the chemical reaction within the soil. Plants can thrive if the soil’s reaction favors their nutrition and growth. However, if the reaction is unfavorable, plant nutrition and growth become impossible. This action-reaction process within the soil occurs in a natural order.
Soil reaction primarily refers to its acidity or alkalinity. Various chemical elements exist in ionic form in the soil solution. Some ions exhibit acidic properties, while others show alkaline reactions. The activation of hydrogen ions is responsible for the acidity of the soil solution, while the activation of hydroxyl ions causes an alkaline reaction. When the concentrations of hydrogen and hydroxyl ions are equal, the soil exhibits a neutral reaction.
Soil pH:
Soil acidity or alkalinity, also known as soil pH, indicates whether the soil is acidic, alkaline, or neutral. The unit of measurement for soil acidity or alkalinity is pH. By definition, soil pH is the negative logarithm of the hydrogen ion concentration in the soil solution, expressed as pH = – log [H+].
Classification of Soil According to pH:
The pH scale ranges from 1 to 14, with 7 being neutral. Soil acidity generally falls within the range of 3 to 10. Based on pH values, soil can be categorized into three main groups:
1. Acidic soil: pH below 7.0 (< 7.0)
2. Neutral soil: pH around 7.0 (approx.)
3. Alkaline soil: pH above 7.0 (> 7.0)
To facilitate science-based agriculture using pH values, soil has been classified into different classes or categories. Notable classifications include those by Brady (1974), SRDI, and Donahue (1977). Below are summaries of Brady’s and Soil Resources Development Institute (SRDI) classifications:
Brady’s Classification:
Brady divides soil into eight categories based on pH values:
serial no reaction class Acidity levels(pH)
- Very strong acidic soil Below 3
- strong acidic soil 4-5
- Moderately acidic soil 5-6
- Slight Acidic Soil 6-7
- Neutral Soil 7-8
- Moderate Alkaline Soil 8-9
- Strong Alkaline Soil 9-10
- Very Strong Alkaline Soil Above 10
It’s noteworthy that this method doesn’t include soils with pH values outside the range of 3 to 11.
(b) Classification by Soil Resources Development Institute (SRDI).
In this classification, soil acidity (pH) depends on the relative amount of acidity or alkalinity of the soil. It is divided into seven parts. namely-
Serial Acid class Acidity levels(pH)
1 Too much acid below 4.5
2 more acidic 4.5 -5.5
3 mild acid 5.6 – 6.5
4 neutral 6.6 – 7.3
5 mild alkali 7.4 – 8.4
6 more alkaline 8.5 – 9.0
7 Excessive alkalinity Above 9.0