English: Maize growing in highly weathered, acid soils on Cedara Research Farm, KwaZulu-Natal. The soil in the background has been limed, but the plants in the foreground are growing in highly acidic soils (Acid saturation 60-70%; pH(water) 4.4; pH(KCl) 3.9), and are suffering from aluminium (Al) toxicity. This is the most important soil-acidity related disorder affecting maize in highly weathered acid soils in the tropics and subtropics. The primary effect of Al toxicity is the reduction of root growth; roots are often malformed, thickened and stubby.

As a result of the poor root growth, secondary effects are:

• small plant size and low crop yield;
• strong competition from acid-tolerant weeds
• wilting (water stress) as roots are unable to take up sufficient water;
• phosphorus-deficiency symptoms (good root growth is critical for P uptake, especially in these low-P soils);
• nitrogen-deficiency symptoms (nitrate is often rapidly leached to below the poor root system and acid-tolerant weeds compete strongly for plant-available N);
• and magnesium-deficiency symptoms (acid soils often have low Mg reserves, and Al toxicity reduces plant uptake of Mg).