Things go better with Coke

13 Feb 2014

Last week I heard a fascinating talk on the minimal (i.e. critical) level of phosphate needed in the soil to achieve optimum economic yields of broad-acre crops. RB209 has said for years that it should be Index 2 as measured by Olsen-P. Like many things, this has become an industry truism but every truism needs to be questioned from time to time. About ten years ago, I became aware that the need to retain phosphate at Index 2 for most crops was primarily based on research from just two trial locations and so it seems prudent to test this particular truism.

So hats off to the HGCA for funding a project to investigate the critical level of P for cereals and rape on a wider range of soil types under modern cropping systems. All the results are now available and they are fascinating. I would never have believed that phosphate nutrition was so interesting!

The results indicate that, in general, Index 2 has much wider relevance than just the two sites where the research was originally carried out. There appears, however, to be an important caveat.

It seems that the long-term availability of applied phosphate (triple-superphosphate was used in the HGCA-funded experiments) may be strongly influenced by the level of extractable calcium in the soil. In this series of experiments, the long-term availability of applied phosphate was around 25% on those soils with lower levels of calcium and only 10% on thin soils over chalk and limestone i.e. after 12 months, there was a significantly higher level of lock-up of applied phosphates on chalk and limestone based soils.

Sufficient available phosphate is required in particular during crop establishment. All this suggests to me that annual applications of phosphates either combined-drilled or applied to the seedbed may be more economical than rotational applications on chalk and limestone based soils because the crops would benefit more from the freshly applied nutrient before it was ‘locked-up’. A LINK project is suggesting that such a ‘targeted’ approach may avoid the need to go through the very expensive process of trying to build these soils up to index 2. I may be getting ahead of myself with these conclusions and it will be particularly worthwhile for those working on chalky or limestone soils to read the Project Reports when they appear on the HGCA website. The publication of the Critical P report is imminent and the LINK report will be published next year.

All this talk of the need for annual application of phosphate reminds me of my experiences in Australia. I remember reading one account of a Western Australian farmer who said, in 1951, that annual applications of this nutrient transformed the yield potential of his farm. I emailed Harm van Rees (what a fantastic name for a great Aussie consultant) who confirmed that the use in many parts of Australia of annual combine-drilled applications of phosphate is because of the high levels of free calcium in the soil.Applying coke as a fertiliser

There is a particular problem with phosphate availability in some parts of the Eyre Peninsula, just west of Adelaide. These particular areas are on limestone and they have found that using fluid fertilisers based on phosphoric acid provides higher yields than granular based fertilisers. These fluid fertilisers are expensive and require specialist equipment but they can be more economical; less phosphate is required to achieve higher yields. 

It is interesting to note that in Australia the extreme problems of phosphate availability can occur on limestone soils. There is a hint in the HGCA Critical P results that the problem of phosphate ‘lock-up’ may be worse on limestone than on chalk soils.

As you may recognise, there has been a lot more research done on phosphate application in Australia. A few years ago I spoke at a conference in Bendigo, Victoria where another presentation described some of this great research effort. Novel techniques were being tested and one (totally tongue in cheek) approach was the application of 200 litres/ha of Coca-Cola. It certainly greened-up the emerging crop because of its phosphoric acid content. Based on the declared chemical content of Classic Coke, this treatment does not appear to apply sufficient phosphorus and it is expensive. On a practical point, the researcher added that it was essential that the Coke was flat, otherwise there were great problems in applying it!  So whilst in the context of phosphate nutrition “things go better with Coke”, it cannot meet the claim that “it’s the real thing”.