Phosphorus fertiliser might not spring to mind as highly important to our everyday lives in Australia, but it is critical for our grazing and cropping farms. This is because the majority of Australian soils are deficient in phosphorus, a nutrient essential for plant growth.
Phosphorus fertilisers have dramatically increased farm yields in southern Australia over the last century. In the last decade the underlying cost of phosphorus fertiliser has doubled. It is likely prices will continue to rise as the world moves to mine new phosphorus reserves that are of lower quality or harder to extract.
Phosphorus is a very important resource from a global perspective. Crop yields are limited because of the lack of available phosphorus in soils on more than 30% of the world’s arable land.
The world’s phosphorus reserves underpin global food security. The need to use these resources efficiently and equitably is clear. The world’s population is projected to reach nine billion by 2050, and there will be increasing competition for land and fertiliser resources between crops and biofuel production.
Will we run out of phosphorus?
In 2009, Dr Dana Cordell, from University of Technology, Sydney, and her collaborators estimated that “peak phosphorus” (the point where global demand for phosphorus would exceed phosphorus supply) might occur within 25 to 30 years. This is a grim prospect given global population projections. Some of the world’s poorest farmers are working in areas where food production isn’t enough to meet to current needs (e.g. in Africa), and those farmers are already struggling to afford phosphorous fertilisers.
The prediction of peak phosphorus prompted renewed research into more efficient use of phosphorus, and reviews of the global supply situation. The size of global phosphorus reserves was revised upwards and it is now considered unlikely that peak phosphorus will occur in the foreseeable future.
Nevertheless, there is a heightened awareness of the importance of phosphorus for global food security and the need for more sustainable use of this finite resource.
How much phosphorus does Australia need?
In Australia phosphorus fertiliser is an affordable way for farmers to increase crop and pasture yields and use farmland and water more effectively.
Australia uses 480 kilotonnes (kt) of phosphorus each year. The majority of it, 450kt, is used in agriculture with an average efficiency rate of 25%.
This means four units of fertiliser are applied to soil to produce one unit of phosphorus in products. Those products are exported or consumed domestically. The remaining 75% (the other three units) accumulates in agricultural soils, with a small proportion also lost to waterways.
Some phosphorus is already recycled from waste streams for re-use in agriculture. Global phosphorus shortages would increase the need to recover and recycle phosphorus from waste. For some countries, recovery and recycling alone would go close to covering their phosphorus needs.
In Australia, the amount of phosphorus available for recycling from domestic consumption will cover only about 5% of the annual phosphorus requirements for agriculture. So, while there is an increasing role for phosphorus fertilisers derived from waste streams, the major avenue for addressing increases in phosphorus fertiliser costs is through improved phosphorus use efficiency on farms.
Increasing the efficiency of phosphorus use
The phosphorus balance efficiency of the major southern Australian broadacre farm enterprises varies considerably from extremely poor (5-15% for some horticultural enterprises), through poor (20-40% for grazing industries), to moderate (45-60% in cropping enterprises).
If a crop is grown with 50% phosphorus balance efficiency, it means that two units of phosphorus have been applied as fertiliser to achieve one unit of phosphorus in products sold off the farm. However, for a grazing enterprise operating at 20% efficiency, it means that it was necessary to apply five units of phosphorus to achieve one unit of phosphorus in farm products.
Australian soils tend to hold on to added phosphorus, which means plants can only access some of the phosphorus that is applied. This is why Australian farmers have to apply more phosphorous than is exported. Some phosphorus also gets incorporated into organic materials in soil that resist degradation.
The net result is accumulation of phosphorus in soil in sparingly available forms and consequently reduced efficiency in phosphorus fertiliser use. While these inefficiencies add to the costs of food production, they also mean there is an opportunity to reduce the amount of fertiliser needed for food production if more efficient plants and management technologies are developed.
Australian farmers are increasingly targeting their phosphorus fertiliser use to more effectively meet the production goals of their farms. Targeted phosphorus fertiliser use ensures maximum value from fertiliser investments. It can also mean immediate savings in fertiliser costs on farms where soil fertility levels are relatively high.
On grazing farms, this reflects a better understanding of the minimum phosphorus levels needed in soils to achieve production goals, and how to efficiently maintain these soil fertility targets.
It’s not easy, but it’s worth doing
Achieving substantial improvement in the phosphorus-balance of Australian agriculture is not an easy task, despite the clear imperative and obvious potential for production and environmental benefits. Phosphorus is such a universally important input for Australia that changes would already be implemented were there easy solutions.
However, recent grazing system research indicates that the rate of phosphorus accumulation in soil may be slowed by using pasture plants that are productive at lower soil phosphorus concentrations. This will require development of pasture legumes with roots that can explore soil more effectively.
Although there are few current examples of Australian crop and pasture species with these desirable phosphorus-efficiency traits, there are naturally occurring examples of plants with extensive fine root systems that grow well in low phosphorus soils. There are also other plants that can extract sparingly available phosphorus from soils by releasing organic acids from their roots or by releasing enzymes that release phosphorus from organic matter in soil.
Breeding crops for improved growth in low-phosphorus soils is underway internationally, with good progress in some of the species used in subsistence farming systems where phosphorus fertiliser is an unaffordable luxury.
Improvements in fertiliser design are also expected to be important; especially if they reduce reactions between soil and phosphorus applied as fertiliser and lead to more direct supply of phosphorus to crops.
Phosphorus inefficiency is presently costly for Australian agriculture, but its very existence means there is a substantial opportunity to reduce costs if more phosphorus efficient farming systems are developed.