Soil pH lecture for Biology, Botany, Zoology, and Chemistry Students by Salman Saeed lecturer Botany University College of Management and Sciences Khanewal, Pakistan. About Author: Salman Saeed \ Qualification: M.SC (Botany), M. Phil (Biotechnology) from BZU Multan. M. Ed & B. Ed from GCU Faisalabad, Pakistan.

1. Course Title:
Plant Physiology and
Ecology
Course Instructor:
SALMAN SAEED
Botany department
UNIVERSITY college of management & Sciences, Khanewal,
PAKSITAN

3. Introduction
pH is:
Measure of acidity or basicity
Defined as negative logarithm of activity of H+ ion in a
solution
pH = -log [H+].
 pH ranges between 1-14.
Basic >7
Acidic <7
Neutral =7

4. pH Scale

5.  pH
 The term "pH" was first described by Danish biochemist
Søren Peter Lauritz Sørensen in 1909.
 “power of hydrogen” where
“p” is short for the German word for power, potenz
and
“H” is the element symbol for hydrogen.

7. pH can affect nutrient availability by:
 formation of low solubility compounds,
 greater retention by soil colloids (clays and organic
matter),
 conversion of soluble forms to ions that plants cannot
absorb and
 Can inhibit or stimulate microbial activity.

9. Low pH
 Soil acidity affects plant growth in several ways:
 The concentration of soluble metals, especially aluminum and
manganese may be toxic.
 Calcium may be deficient.
 When the soil pH is lower, more H+ ions are available to
“exchange” base cations, thereby removing them from exchange
sites and releasing them to the soil solution (soil water).
 Lower pH increases the solubility of Al, Mn, and Fe, which are
toxic to plants in excess. A critical effect of excess soluble Alis
the slowing or stopping of root growth.
 Low pH reduces the availability of the macro- and secondary
nutrients

10. Symbiotic N fixation in legume crops is greatly
impaired.
The symbiotic relationship requires a narrower range of
soil reaction than does the growth of plants not relying
on N fixation.
Populations and the activity of the organisms
responsible to transform N, S, and P to plant-available
forms may be reduced.
The availability of mineral elements to plants may be
affected.

11. High pH
 When the soil pH is high (i.e., more basic, low
concentration of H+), more base cations will be on the
particle exchange sites.
 As a result, exchanged nutrients are either taken up by the
plant or lost through leaching or erosion.
 Microbial activity may also be reduced or changed.