Sources of P other than the rare rock
From the November 2008 edition of Agriculture Today.
First it was a lack of water, and now lack of phosphorus (P) threatens global food supplies.
Phosphate rock is mined in only a few places, including China, the US and Western Sahara.
Due to the limited supply and higher energy costs, superphosphate, manufactured from phosphate rock, is becoming expensive, so there is great interest in other sources of P.
University of Adelaide researchers Chris Penfold and Annie McNeill suggest three options for organic broadacre farmers who are not allowed to use superphosphate.
One option is to increase the P use efficiency of crops and pasture species.
Field grown plants in dryland systems vary widely in their ability to extract P from soil and fertiliser, so it may be possible to breed crops with higher P use efficiency and greater capacity to use less soluble forms of P.
Trials have found that rye and triticale appear more P efficient than wheat.
Some legume species excrete solubilising compounds that can make P more available.
A second option, difficult to do in semi-arid climates, is to increase soil organic matter.
The third option is manures and composts such as deep litter from pigs and chickens.
The rising cost of superphosphate and N fertilisers such as urea now make the P and N in manures and composts more valuable and more cost efficient to transport.
Fortifying compost with rock phosphate can increase plant availability of P because the organic acids and humic substances produced by bacteria and fungi in the compost help release P from the rock.
Anaerobic digestion of manures leaves a residue rich in P and N and produces methane, which can be a fuel source.
Burning manures with minimal oxygen (pyrolysis) will similarly produce energy (gas or oil) and a char with varying levels of available P and N.
At Wagga, NSW Department of Primary Industries’ Jeffrey Evans is looking at ways to make P in reactive phosphate rock more available to plants grown organically.
Elemental sulphur mixed with the pulverised rock helps dissolve the rock and makes the P more available, but there are still problems to address to make this strategy more effective, including preventing the rock and sulphur separating on application.
The University of Technology Sydney’s Sustainable Phosphorus Futures program is investigating P levels in human excreta and technologies to safely recycle it.
"According to some studies in Sweden and Zimbabwe, the nutrients in one person’s urine are sufficient to produce 50-100 per cent of the food requirements for another person," the University’s website says.
"Combined with other organic sources like manure and food waste, the phosphorus value in urine and faeces can essentially replace the demand for phosphate rock.
"In 2000, the global population produced three million tonnes of phosphorus from urine and faeces alone."
Jeffrey Evans says a combination of processes and use of recycled organic inputs and cheaper mineral P sources will help farmers in future.
Mr Evans says these include breeding plants that can better access P already in the soil.
For example, plants with bigger root systems, and plants that can survive in low P soils, using P-solubilising microbial inoculants, and using green and composted manures in combination with modified rock phosphates, are likely to have an increasing role in securing P for agriculture.
- Read about Mr Evans’ research at www.bfa.com.au/_files/x06aoj_036p.pdf
- Read the Penfold and McNeill paper at http://orgprints.org/12466/
- Find the UTS Phosphorus Futures website at http://phosphorusfutures.net/
