Soils: acid and alkaline

Primary symptoms

Acid soils: Roots and plants are stunted. Roots may be discolored (black or brown).
Alkaline soils: Maize grown in  a high pH soil with high levels of free carbonates shows symptoms of severe iron deficiency.  

Confirm the problem by checking the tables below.


Causes of pH problemsAdditional evidence required
Inherent property of the parent material / climate combination. Review soil maps of the area. Problems of low pH are common in aged, highly leacheds soils with high levels of Al and Fe oxides. Problems of high pH are common to soils which are derived from limestone parent materials. If pH is low, have soil tested for presence of free Al+++.
Application of acidifying fertilizer. Question farmer about fertilizer products applied. For example, continued use of ammonium sulfate tends to lower soil pH.
Over-liming to correct a soil acidity problem. Question farmer about liming practices.
Application of alkaline irrigation water. Check pH of irrigation source.

The maize crop generally grows well over a pH range of 5.5- 7.8. The effect of soil pH outside this range is usually to make certain elements more or less available, so toxicity or deficiency develops. Below a soil pH of 5.5, AI and Mn toxicities are frequently problems, and deficiencies of P and Mg are common. At a pH above 8 (or above 7 on calcareous soils), deficiencies of Fe, Mn, and Zn tend to occur. The field symptoms of unsuitable pH thus generally resemble those of micronutrient problems. (See Figure).

Effect of pH on availability of common elements in soils

Figure: Effect of pH on availability of common elements in soils. The width of the bar indicates degree of availability. Source: Booker Tropical Soil Manual. J.R. Landon (ed), 1984, Pitman Press Limited, Great Britain.

Is soil pH a problem?

Evidence: measurements.

Collect a soil sample and test it for pH. Sample the plow layer and subsoil separately. Kits for doing reliable field tests are available. If pH is below 5.5 or above 7.8, it is probably outside the range for good growth of maize.

A quick test can be used to diagnose the presence of free carbonates in high pH soils. Add a few drops of 10% HCI solution to the soil. If the soil fizzes, the soil is calcareous and deficiencies of Zn, Fe, and Mn are possible.

Evidence: observations.

Look over the field for symptoms of micronutrient deficiencies (See mineral nutrition). If leaf symptoms are present, yield is probably being reduced. If the pH is low, you should examine the roots of some plants for symptoms of AI toxicity. The roots of AI-affected plants will be stunted and short; they will often look like thick clubs rather than like normal, slender roots. The roots may be brownish or black in color. Low pH soils may also exhibit Mn toxicity, causing small reddish-brown spots on older leaves.

Possible solutions

  • Apply lime to raise the pH of the soil.
  • Apply micronutrients to foliage or soil to correct pH-induced deficiencies.
    • Change to a non-acidifying fertilizer source if the pH is low, or to an acidifying source if pH is high.
    • Change liming practices to avoid over-liming.
    • Change to a tolerant variety.
    • Apply elemental sulfur to lower soil pH.
    • Reduce irrigation with alkaline water source.