Question

What will be the colour indicated by the soil having ph 6.8 as compared to the colour charts for different ph values

Answer 1

Soil pH is a measure of the acidity orbasicity (alkalinity) of a soil. pH is defined as the negative logarithm(base 10) of the activity of hydronium ions (H+
or, more precisely, H
3O+
aq) in a solution. In soils, it is measured in a slurry of soil mixed with water (or a salt solution, such as 0.01 M CaCl
2), and normally falls between 3 and 10, with 7 being neutral. Acid soils have a pH below 7 and alkaline soilshave a pH above 7. Ultra-acidic soils (pH < 3.5) and very strongly alkaline soils (pH > 9) are rare.[1][2]

Soil pH is considered a master variable in soils as it affects many chemical processes. It specifically affects plant nutrient availability by controlling the chemical forms of the different nutrients and influencing the chemical reactions they undergo. The optimum pH range for most plants is between 5.5 and 7.5;[2]however, many plants have adapted to thrive at pH values outside this range.

Classification of soil pH ranges

The United States Department of Agriculture Natural Resources Conservation Service classifies soil pH ranges as follows: [3]

Denomination

pH range

Ultra acidic

< 3.5

Extremely acidic

3.5–4.4

Very strongly acidic

4.5–5.0

Strongly acidic

5.1–5.5

Moderately acidic

5.6–6.0

Slightly acidic

6.1–6.5

Neutral

6.6–7.3

Slightly alkaline

7.4–7.8

Moderately alkaline

7.9–8.4

Strongly alkaline

8.5–9.0

Very strongly alkaline

> 9.0

Determining pH

Methods of determining pH include:

Observation of soil profile: Certain profile characteristics can be indicators of either acid, saline, or sodic conditions. Examples are:[4]

Poor incorporation of the organic surface layer with the underlying mineral layer – this can indicate strongly acidic soils;

The classic podzol horizon sequence, since podzols are strongly acidic: in these soils, a pale eluvial (E) horizon lies under the organic surface layer and overlies a dark B horizon;

Presence of a caliche layer indicates the presence of calcium carbonates, which are present in alkaline conditions;

Columnar structure can be an indicator of sodic condition