Grains Research and Development

Liming

Liming may be able to reduce the severity of water repellence in acidic soils and improve crop germination, establishment and productivity. Field research conducted in the 1990’s indicated that the application of lime to acidic sand increased soil wettability by physical mechanisms and by promoting bacterial degradation of wax coatings on sand grains.

Dry lime is applied on the soil surface after the dry season and with each rainfall event dissolves and neutralises acidity as it moves down the profile — often over a number of years. This change in soil chemistry supports an environment for greater populations of wax-degrading bacteria. Lime may only reduce repellence on acidic soils where the increase in pH promotes the activity of the wax-degrading microbes. Lime is unlikely to have a significant impact on water repellence of alkaline soils because its solubility is too low.

Wax-degrading bacteria have been shown to produce bio-surfactants that release hydrophobic coatings from sand surfaces, thereby assisting the bacteria to consume the waxes.

Maintaining the soil pH (measured in 0.01M CaCl2) at the recommended levels of greater than 5.5 in the topsoil and 4.8 in the subsoil prevents a subsoil acidity problem from developing. Liming to increase the topsoil pH closer to neutral (pH>6.0 CaCl2) may promote the activity of the microbes that can reduce water repellence. It should be noted, however, that larger lime applications (up to 5 t/ha) are unlikely to completely overcome water repellence, although it may reduce its severity. Applying higher rates of lime is also costly and is likely to have implications for crop nutrition. This needs to be taken into account before undertaking additional liming to promote a reduction in water repellence.

 

A tractor pulling a spraying machine across a dry field. 

Source: Steven Davies