Most growers know that major plant nutrients become less available as soil pH decreases. But there are four ways acid soils can affect crop nutrition. Soils are considered strongly acid when the pH is below 5.5. Serious problems are likely when the pH falls below 4.5.
1. Plant roots and nutrient uptake
Plants need a good root system to access soil nutrients. This is particularly important for less mobile nutrients like phosphorus (P). To get enough P, plants need a dense root system with lots of root hairs. In acid soils, high aluminium (Al) and manganese (Mn) can reduce root hairs and fine roots. The appearance of P deficiency in plant tops (bluish-purpling leaves) can be caused by Al toxicity that suppressing root growth.
2. Trace elements
Micronutrients are not usually a big concern in acid soils. Most trace elements become more available in acid soils. The exception is molybdenum which can become deficient when pH is below 5.5.
To fix nitrogen, legumes need rhizobia living in their root nodules. Rhizobia are less effective in acid soils and might not inoculate the roots. Most rhizobia strains are sensitive to pH below 5.5. A legume crop grown in a soil with pH 5.2 will have almost no nodules on the roots and will use more N than it will fix.
4. Reduced soil fertility
Acidic soils have a reduced cation exchange capacity (CEC). As pH decreases, the number of negative charge sites on soil colloids decreases, and the soil can’t hold as many plant available nutrients.
Mike McLaughlin is a professor of soil science at the University of Adelaide. This video was recorded at the Soil Science Australia Conference in Canberra, 2018.