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By Richard Simpson 26 June 2019 6 min read

Triple-superphosphate; an example of a mineral phosphorus fertiliser developed from phosphate rock to provide the nutrient in a form that is accessed more readily by plants and soil biota. The granular form of the fertiliser improves the evenness of fertiliser spreading and also increases the efficiency with which plants can access the nutrient.

THIS year marks 350 years since the accidental isolation of phosphorus from urine by Hennig Brandt, a German alchemist. He was searching for the elusive “philosopher’s stone”, a mythical substance thought to transform base metals into gold.

Although Brandt didn’t find what he was looking for, he had stumbled across one of the key elements essential for life on earth – from us as humans, to the food we eat.

Phosphorus is a component of nucleic acids (DNA and RNA), essential for cell membranes, for energy transactions in all living cells, and critical for teeth and bones.

Some regions of the world have naturally phosphorus-fertile soils, or are oversupplied with phosphorus, for example in western Europe. Phosphorus fertiliser use in these areas is often highly regulated or in decline because oversupply has led to polluted waterways.

However, other regions, including about 30 per cent of the world’s arable land, have low phosphorus availability and are dependent on phosphorus fertiliser to maintain high crop and pasture production (e.g. Australia, New Zealand, Africa, to name a few).

Assessments lead to dire warnings and new solutions

Ten years ago, an assessment of the known global phosphorus “reserves” - the rock phosphate deposits, loosely defined as economically viable to mine for fertiliser production - indicated that their longevity was very limited (decades).

This sent shockwaves around the world. People began taking stock of how critical phosphorus is for global food production, and whether we could cope with shortages in supply.

New audits of the global phosphorus reserves followed the initial assessment and quickly dispelled the immediacy of the apparent crisis. It was re-estimated that we have about 200-300 years of supply at current rates of use.

While it’s unlikely that phosphorus use will remain static (it may increase as demand for food grows), technology is also not static. The world also has vast phosphorus “resources” - deposits that are currently not economic to mine. It’s entirely feasible that increases in the price paid for phosphorus and improved mining and processing technologies will open up these resources.

These assessments heightened global awareness of the importance of phosphorus for food security and it’s recognised that the efficient use of phosphorus must be improved. The effectiveness of phosphorus use in food production and recycling it for reuse in agriculture are, consequently, key areas for research.

Experiments at CSIRO, Canberra, examining serradella and subterranean clover varieties as part of a national project to develop more phosphorus-efficient pastures.

One area of current research is investigating new crop and pasture varieties. For example, researchers at CSIRO and NSW Department of Primary Industry demonstrated that alternative pasture legumes – serradellas – are likely to require up to 30 per cent less phosphorus fertiliser than subterranean clover which has been a mainstay of southern Australian agriculture for over 100 years.

Serradellas have long, fine roots with long root hairs that allow them to access phosphorus in soil at substantially lower concentrations than clovers. They can be just as productive, and broadening their use would complement pastures presently based on subterranean clover.

Australia has already made progress. Serradellas are prized for pasture production on light, acidic soils and in rotation with crops. The key to wider use of serradella is development of cultivars that regenerate reliably each year and persist at high plant densities in Australia’s permanent grass-legume pasture systems on heavier soils.

Phosphorus fertilisers are a significant cost to Australian farmers. Any improvements in phosphorus efficiency will also improve farm profitability and help farmers cope with future increases in phosphorus fertiliser costs.

The natural phosphorus cycle

Although phosphorus is a relatively abundant element (about 0.1 per cent of the Earth’s crust), only a relatively small proportion is available for uptake by plants. Phosphorus is slowly released from rocks by weathering during soil formation and becomes available to plants when soil minerals weather further.

Animals, including humans, consume phosphorus by eating plants and/or other animals. Phosphorus returns to the soil in excreta and when plants and animals die, thus completing the terrestrial phosphorus cycle. The cycle timeframe is days to decades.

Phosphorus leaves the terrestrial cycle in leachate and runoff, or when soil is eroded. It then enters streams and ultimately the oceans. The aquatic cycle is as complex as phosphorus cycling on land. If conditions are favourable, phosphorus accumulates in sediments on the ocean floor. The sediments may eventually become phosphate-enriched rocks. Uplift over geological time (millennia) is required to make rock phosphate accessible again in the terrestrial environment.

The timeframe is vast and hard to comprehend, but the implications are clear. Phosphorus that escapes rapid cycling in the terrestrial environment is effectively out-of-reach for re-use on land. This is why the world’s reserves of high-grade rock phosphate are considered a “finite” resource.

A brief history of phosphorus fertiliser use

In medieval times, manures were the primary phosphorus fertiliser and were gathered from grazing animals and applied to crops. Boosting crop production by concentrating organic nutrient sources on part of the farm required a much wider land area for “nutrient foraging”, than the area on which the crop was grown.

Mineral phosphorus fertilisers have been manufactured since the 1830s: firstly using bones, then guano, and now mostly from rock phosphate (rock deposits with approximately 8 per cent phosphorus composition). The global use of mineral fertilisers escalated after the 1940s, when the food requirements of an expanding human population began to exceed our capacity to grow enough crop using organic nutrient sources alone.

Good agronomy and scientifically-informed soil fertility management have guided Australia’s phosphorus fertiliser use. Wheat yields have increased three-fold since the 1880s. Phosphorus fertiliser was an essential component of this revolution, along with legumes, better crop varieties and improving farm technology. The productivity of Australian pastures has also been transformed by the combined use of superphosphate and pasture legumes such as subterranean clover.

Getting the balance right

Used strategically, organic or mineral phosphorus fertilisers applied to crops boost the natural phosphorus cycle in nutrient poor soils and ensure higher crop yields per hectare of farmland. High yields minimise the amount of land that must be devoted to food production, maximise the efficient use of scarce rainfall resources, and help to keep food supplies stable and relatively cheap. This is important in an increasingly crowded world.

Researchers harvesting experiments testing more phosphorus-efficient pasture legumes.

Done well and responsibly, these strategic interventions are a key component of safe and sustainable resource management on farms. When done poorly, however, phosphorus over-use can exacerbate losses to the aquatic phosphorus cycle and this can be a factor in the eutrophication of lakes, rivers and seascapes.

The good news is that Australian farmers, through their co-investments in agricultural research, are already getting ahead of the game. They use soil tests to plan and monitor efficient fertiliser use, and the critical phosphorus requirements of many crops and pastures have been determined to ensure appropriate and targeted soil fertility management. Farmers now know a lot about where and how to place fertilisers for maximum effectiveness and efficient food production.

Dr Richard Simpson is a pasture agronomist with CSIRO.

Funding acknowledgement:

Development of phosphorus efficient pastures is a project supported by the Australian Government Department of Agriculture as part of its Rural R&D for Profit program, Meat and Livestock Australia, Dairy Australia, Australian Wool Innovations Ltd, and participating research organisations and farmer groups.

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