Posted by: 0 in Untagged on Feb 16, 2010
In South Australia we have dry alkaline soils that tend to lock up phosphorus (P). The nutrient forms strong bonds with iron and aluminium, and stays in the soil but unavailable to plants. In lucky years when we have enough rain, any free P may be washed down through the soil profile, below the reach of surface roots.
The problem of P availability in alkaline soils is well known and agronomists have traditionally calculated P additions up to ten times the requirement of the crop to account for lockup. This meant that farmers were paying for a lot more phosphorus than they export from the paddock in produce, but super-phosphate was cheap and until the 1980’s it was subsidised.
I started reading about shortage of phosphorus supplies in The Ecologist magazine, but reports on short supply of P rarely made it into farming papers or magazines.
We have become familiar with the concept of peak oil, but what about forecasts of "peak P".
Recent price increases for fertilisers have perhaps focused our attention a little on efficiency of use of P, but availability still rarely rates a mention.
Richard Simpson of CSIRO is finally getting some press with his pleas for attention to the urgency of the problem. He is urging efficiency of use of P, and ultimately suggesting that we have to balance inputs and outputs of P.
As usual the researchers can find many ways of improving efficiency of use of P. What about selecting P-efficient varieties for example? Common rye-grass varieties for instance can vary in efficiency of P utilisation by up to 400 times. Researchers are now looking at breeding mechanisms from P-efficient plants into crop and pasture species.
An example cited by Richard Simpson is white lupin, which excretes citric acid into the soil from its roots to solubilise locked-up P.
In the bushland setting Banksias utilise P very well, with assistance from mycchorizal fungi. Leaf litter from the banksias then released some of this P back to less efficient plants (if they could get it before it was locked up again).
Microbiology is the key to solving the Peak P problem. Genetic modification (using conventional plant breeding) may be part of the solution, but soil systems using biology work much more directly on the problem. There is an enormous enzymatic powerhouse at work in soil organisms, and an amazing complexity of species to explore and work with.
The first step is to improve soil organic matter content. When we do that everything in the soil starts to work better, carbon is sequestered and trace element utilisation also improves.
It will help to lay off all chemicals as well. Microbial systems for acquiring P are ultimately efficient but the plant does need to invest some energy it making it work. Offered free (to the plant but not the farmer) water-soluble P or N or whatever, the plant tends to accept the easily available form and turn off the biological system. Think of this like your body allowing the now-unused appendix to wither gradually away. But ultimately the biological systems are more efficient. The rainforest or bushland works entirely on biology, and is completely immune to Peak P.
Explore organic to deliver many different tools on how to encourage soil biology. Our latest favourite garden product is called Fishers Creek Rock Dust or more simply FCRD. It is a basaltic rock, so it contains some P, but it works principally because the abundant silica forms even stronger and more lasting bonds with iron, releasing some of the previously locked up P. FCRD also contains a spread of minor and micro-nutrients.
FCRD is only one of many tools in the organic armoury.