Many Australian soils are old and weathered. In fact, many are considered the oldest soils in the world; and the nutrients have been leached, which has resulted in soils of low fertility. For example, average Australian soil phosphorus levels are 40% lower than English soils and up to 50% lower than North American soils.
Improved pasture species allow a much higher stock-carrying capacity; but to maintain this productivity, they require a higher level of soil fertility than do native pasture species.
Fertiliser applications are required to overcome the soil’s inherent nutrient deficiencies and to replace the nutrients that are lost or removed from the soil by pasture growth, fodder cropping or conservation, and animal products, such as milk or meat.
Nutrient redistribution around the farm and the inherent ability of soils to ‘retain’ applied nutrients; so they are less available for plant uptake, are other reasons for fertiliser applications.
In addition to the loss of nutrients in fodder, grain, and animal products, a significant amount of nutrients can be lost off the farm in runoff from irrigation and rainfall – see Chapters 10.5.2 and 12.3.
There are many factors that need to be considered in working out a profitable fertiliser program for a dairy farm. For details on nutrient planning see Chapter 15.