SOIL CRUSTING: POSSIBLY THE BIGGEST SOIL PROBLEM IN SOUTH AFRICA
The 5th of December was World Soil Day. On this day the importance of soil as a critical component of the natural system and as a vital contributor to human well-being is celebrated. Events around World Soil Day aim to connect people with soils and raise awareness about their critical importance in our lives.
The United Nations have declared 2015 – 2025 as the International Decade of Soil. This decision was taken because there is great concern about the condition of soil and the rate against which soil is degrading worldwide. Generally, the public is also not aware of the impact of this problem on food security.
Soil crusting, that is surface sealing of soil, is a widespread and very serious problem throughout South Africa. This is due to the fact that large parts of the country are covered by soils that are inherently very susceptible to crust formation. It is particularly severe where the surface of vulnerable soils is left bare due to injudicious farming practices. It is found under all kinds of farming. In extensive rangelands, under both livestock farming and in game parks or reserves, it is caused by overgrazing. Under dryland cropping, the worst situations occur where non-arable soils are cultivated and then abandoned because cropping has failed. Soil crusting is a serious problem under overhead sprinkler irrigation. It is a widespread and very serious problem in high value crops, like fruit orchards, under drip or micro-sprinkler irrigation where the soil surface under the canopy in the row is kept bare by use of herbicides.
Soil crusting has several serious consequences. It causes poor water infiltration into soils and thus excessive runoff. The increased runoff leads to increased soil erosion and also to floods, even under normal rainfall events. Due to the poor water infiltration, little water enters the soil and causes drought conditions to prevail even under adequate rain. Poor water infiltration under irrigation leads to poor irrigation water use efficiency. A dense crust also leads to poor soil aeration, and thus poor germination, and causes a high mechanical resistance against seedling emergence. This leads to a poor crop stand, especially of small-seeded crops such as wheat, as well as dicotyledons, including various types of beans. It also prevents recovery of bare overgrazed areas and abandoned cultivated fields, even after more than 40 years since withdrawal.
Much research has been done in South Africa on the mitigation of soil crusting and amelioration of crusted soils. In rangelands, a dense grass cover should be maintained by avoiding overgrazing, especially of vulnerable areas. This is a large and difficult challenge in game parks. Cultivation of vulnerable non-arable areas must be avoided. In cultivated areas, maintenance of a dense stubble mulch is essential. In irrigated areas under high value crops, application of organic soil conditioners or mulching are both very effective.
