Signs and symptoms of an acid soil

Low pH in soil is a problem for agriculture and the environment because it can affect plant growth through its effect on plant nutrients and soil biology

Change in nutrient solubility and nutrient availability to plants

Some nutrients may reach toxic levels, while others can become insoluble (therefore unavailable), leading to deficiencies. The changes in the solubility of plant nutrients associated with increasing soil acidity are:

  • Increased aluminium (Al3+) in the soil solution causing stunted root development in crops and pastures. Stunted roots result in reduced capability to access soil moisture and reduced nutrient uptake. In legumes resulting in reduced nodulation and nitrogen fixation, which may also reduce survival.
  • Increased manganese (Mn2+) in the soil solution, causing reduced growth in some plants in some soils (with toxicity symptoms in susceptible plants).
  • Reduced solubility of molybdenum, phosphorus, and access to calcium, while increased aluminium in the soil solution facilitates the movement of calcium and magnesium down the soil profile where it is less accessible.

Some plants are more sensitive to aluminium than to manganese and vice versa. For example, white clover is tolerant of aluminium but sensitive to manganese. Often the cumulative effects of low pH and various toxicities/deficiencies result in the greatest damage.

The degree to which soil acidity affects nutrient availability and therefore plant health and growth depends on pH level, soil type and soil organic matter levels.

Aluminium solubility

Aluminium solubility

Manganese solubility

Manganese solubility

Solubility and availability of other nutrients

Solubility of other nutrients

Changes in biological activity

Sometimes the effect of acidic soils on the growth and production of crops and pastures is not direct, but rather through the effect on soil micro-organisms, that in turn affect plant growth. Soil organisms are influenced by soil pH.
Soil pH influences both survival and functioning of Rhizobia.
The fungus that causes the root disease Take-all in cereals, Gaeumannomyces graminis var.tritici, is most active in soils with a pH(CaCl2) greater than 4.8, and has a low level of activity in soils with a pH less than 4.6. Liming greatly increases the activity of Take-all.  
Farmers and land managers will see evidence of the above expressed as:

  • reduced yields from acid sensitive crops and pastures
  • poor establishment of pastures
  • failure of perennial pastures to persist
  • poor legume productivity and nodulation
  • increased need for fertiliser N in crops following legume-based pastures due to poor nitrogen fixation by pasture legumes
  • poor regeneration of annual legume species
  • reduced tolerance of crops and pastures to environmental stresses such as waterlogging, drought, disease and herbicide damage
  • increased incidence of acid tolerant weeds) e.g. sorrel

Changes in biological activity

Remember general signs that may point to soil acidity can also be the result of other land degradation issues:
* Reduced yields from acid sensitive crops and pastures  
* Poor establishment of perennial pastures
* Failure of perennial pastures to persist