This document discusses soil pH and how it affects nutrient availability. It begins with an introduction and objectives to observe how soil pH impacts essential nutrient availability. It then defines pH and describes soil reaction in terms of acidity and basicity. Sources of acidity and basicity are outlined. The importance of buffering capacity is discussed as it relates to nutrient availability and toxicity. Specific impacts of pH on nitrogen and phosphorus availability are described. The conclusion recommends keeping soil pH in mind when planning nutrient management programs, as soils can become more acidic or alkaline over time without proper management.

1. Presented by:
WANDILE MAYA
COURSE : AGS 321
STUDENT NO: 201310334

2. Introduction
Study purpose/ objective
Soil Reaction
Sources of acidity

3. Sources of basicity in soil
 Buffering of soil and importance of
buffering
Soil pH and availability of nutrients
Conclusion & recommendations

4. Understanding that soil pH governs the
availability of most nutrients is very important
for proper nutrient management.
However, In crop production maintaining the
proper soil pH for efficient utilisation of other
crop inputs is imperative (Miller ; 1990).

5. By a strict chemical definition, pH = -log[H+]
activity in an aqueous solution. Hence, pH
values are reported on a negative log scale.

6. Increased amount of [H+] in the soil
decreases soil pH ; thus becoming more
acidic and vice versa is true about decreased
[H+] amount.

7. The first order of
business is a quick
review of pH and
associated
terminology.
To observe serious
effects of soil pH
on the availability
of essential and
trace nutrients.

8. Soil Reaction is a term used to indicate the
degree of acidity or basicity of a soil. It is
usually expressed in terms of soil pH.
The descriptive terms used include the ones
described on the table below:

9. Descriptive term pH
Strongly acid < 5.5
Moderately acid 5.5 to 6.5
More or less neutral 6.6 to 7.4
Moderately alkaline 7.5 to 8.4
Strongly alkaline > 8.4
Table 1. Soil pH and Interpretation

10. Sources of soil acidity
Soil organic matter.
Clay and Oxide minerals.
Al and Fe polymers: Al3+ + H2O→ Al (OH)2+ + H+
Fe3+ + H2O→ Fe (OH)2+ + H+
Soluble salts: NH4
+ + 2O2 bacteria NO3
-+ 2H+ + H2O
Carbon dioxide: H2O + CO2↔ H2CO3
H2CO3 ↔ H+ + H CO3
-

11. Sources of alkalinity in soils
Any process that encourages high levels of
exchangeable bases e.g. Ca, Mg, K & Na results in
reduction in acidity and increase in alkalinity.

12. Table 2
(Adapted from Truog, USDA Yearbook of Agriculture 1943-1947)

13. This is described as a distinct resistance of change in
the pH of the soil solution, even when limed.
Importance of buffering
• Affect the availability of several plant nutrients, and
• Influence the levels of other elements that may be
toxic to higher plants and micro-organisms.

14. Farmers frequently ask that; “what effect does soil
pH have on nutrients in the soil?”
Nitrogen
High soil pH can result in significant loss of N by
volatilization, especially when urea-used fertilizers
are surface applied.

15. Phosphorus
Table 3. Soil phosphorus compound in relation to soil pH.

16. (Adapted from Truog, USDA Yearbook of Agriculture 1943-1947)

17. Ultimately, the soil pH does play a role in nutrient
availability and a most paramount one.
Should you be worried about your production on your field
or farm?
Be more aware than concerned.
How so?
Keep or bear the pH factor in mind when planning nutrient
management programme.

18. Soils tend to acidify over time, particularly when large
applications of NH4
+ based fertilizers are used.
Research have shown the pH decline occurring more
rapidly in continuously cropped, direct-seeded land
(McKenzie; 2003). Thus, a soil with an optimum pH
today may be too acid or alkaline a decade from now,
depending on producer land management.

19. “The sluggard does not plow in the autumn; he will seek at harvest and
have nothing. Proverbs 20:4”
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