Impacts of soil on grazing

Grazing in drier areas | Grazing in wetter areas | Soil conservation | Invertebrates and soil creation |

Grazing in drier areas

Cattle grazing is the most widespread land use in northern Australia                                I. Fordyce

The major land use in the north of Australia is grazing. This is largely because the soils are lacking in sufficient nutrients and organic matter to support horticulture. The long dry season and short but intense wet season further limit production potential.

There are exceptions: the Ord River in the Kimberley for example, around Katherine in the Northern Territory and more southern and eastern areas of North East Queensland. In general the low levels of potassium and nitrogen are of primary importance in the savannas. Nitrogen is more of a limiting factor in inland areas, potassium in those areas with higher rainfall: Cape York, Kimberley, Arnhem Land-Kakadu and Darwin. The lack of nutrients is compounded by active microflora which together with leaching restrict available nutrients.

Grazing in wetter areas

For graziers in the wetter regions, the nutrient quality of pasture is of major concern, whereas in drier inland areas it is the quantity of feed which is the limiting factor. There have been many attempts at making use of leguminous pastures which add nitrogen to the soil, although in areas where the soils are already mildly acidic such as the Mitchell grass region, this approach may act to effect the soil pH in the long term.

Some estimate that 90 per cent of annual biomass production occurs in the wet season, while at the same time nutrient levels in the soil fall from leaching. Over the dry, biomass decreases and nutrients in the soil increase. Fire can act to remove some of the nutrients but a significant percentage remains in charred fragments. Indeed in the wetter areas, the grasses relocate the nutrients stored in the plant to the root region, so that while standing matter is less nutritious, the impact of fire is mitigated somewhat.

Soil conservation

The conservation of soil is of great importance in the sustainability of land management in the tropical savannas. Soil erosion is a very serious problem, and occurs quite simply because of a lack of plant cover. Combined effects of overgrazing, drought and /or fire can severely impact on the amount of ground cover. Intense fires late in the dry season for example can contribute greatly to soil erosion because the storms that follow will strike bare soil, and thus wash substantial quantities of both soil, and the ash which contains valuable nutrients, into streams and rivers. Not only then are the nutrients lost, but stream water quality will decline as a result.

Invertebrates and soil creation

Invertebrates are critical in both soil creation and nutrient recycling and indeed much of the soil now present in northern Australia is said to owe its existence to several thousand years of termite activity.