Can Phosphate requirements be reduced?

Posted on April 2, 2012 by Murray Kimpton
Dr Bert Quin

Farming today faces new pressures. On the one hand, good production levels are necessary for farm profitability. On the other hand, the levels of fertiliser input this requires can lead to nutrient enrichment of streams, rivers and lakes. What to do?

We have to focus more on just what happens to fertiliser nutrients after application, and find out what loss mechanisms can be reduced. Ironically, with phosphate (P) we have problems at both ends of the spectrum. On many soils, and particularly the allophanic ash soils, P is ‘fixed’ onto aluminium (Al) on the surface of soil clay particles. These soils used to be called highly ‘P-retentive’. It is fashionable in some quarters today to say they have a ‘high P storage capacity’, but this is a misnomer. As your dictionary says, a store is somewhere you put something until you need it, and can then take it out of.

However the P fixed on clay particles, especially allophone, just gets stuck, retained, or fixed (any of these words are accurate), more and more tightly, more and more deeply under Al, over time. Even when you stop putting maintenance P on, the rate at which P is ‘desorbed’ into plant-available form is far, far to slow to maintain good production. “Stored’ indeed!

The great Joe Karlovsky from MAF’s Ruakura Research Station calculated over 40 years ago that to maintain a high level of production on a dairy farm on allophanic ash soil required 15 kg P per year more than it took to maintain exactly the same level of production on a low P-retention soil. Today, Overseer gives about the same difference in requirements. There is anecdotal evidence that the rate of P fixation is slowly declining, but if it is, it is very, very slow!

Unfortunately, while applying higher rates of fertiliser P to these soils helps offsets fixation losses, it also means a lot more fertiliser granules sitting on the surface after application. As these slowly dissolve into the soil over a few weeks, they are susceptible to some of the P – either dissolved or as particles – being washed into waterways by rainfall-induced run-off. The more P that has been applied in one hit, the bigger the risk.

Small, frequent applications of P would greatly reduce the risk of run-off, but at prohibitive cost. A far better solution is to find ways to reduce the amount of P that gets fixed in the soil, so we don’t have to put as much on in the first place. Very exciting research in this area is being done in several countries, with very promising results. In Vietnam for example, where I have been involved in research into improving fertiliser efficiency for 8 years, the treatment of DAP with ‘anti-fixing’ additives has literally halved P requirements for rice grown on acid soils. Another option is to use part of the P in slow-release form such as RPR. The P from RPR is not soluble in water, so is far less prone to loss in run-off. This fact is recognized in Overseer. Another part of the solution is to apply fertiliser in fluidised form, so it quickly gets below the surface.

As in most field of endeavour, necessity is the mother of invention. I think our environment, our water, is telling us that it is high time we took some of these ‘inventions’ a lot more seriously.