Free frozen phosphate reserves – part 1

Better soils
with Brett Petersen
Kiwi Fertiliser & Golden Bay Dolomite

Often when we soil test, we find two or three times the phosphate required for good plant growth. Within six weeks of application, about 75 per cent of applied soluble phosphate is lost. In fact, the Commonwealth Scientific and Industrial Research Organisation – known as the – CSIRO estimate AUS$10 billion of applied phosphate now lies locked up in Australian soils.

Phosphorus has three negative charges. This means it is strongly attracted to cations with two or more charges. When it bonds to these cations, it becomes insoluble and no longer available to the plant. In soils with a pH above 6.4, the casualties of this pairing are both calcium and phosphorus. Calcium and phosphorus are two of the key minerals for the most important process on the planet, photosynthesis. In more acidic NZ soils, the triple negative P binds with minerals like iron, manganese and aluminium.

Improving management

The first step in improving phosphorus management is choosing the most suitable phosphate fertiliser. If you’re dealing with a 100-day crop, where you need an immediate phosphate hit, along with some nitrogen to kickstart root growth and vigour, then DAP/MAP has a role. But you need to buffer the acid burn with humates and counter the lockup.

If you grow pastures, orchards or vine crops, there is little reason to use water soluble, super-unstable phosphate fertilisers. You’re much better off using a slower release phosphate source like Sechura Reactive Phosphate Rock – known RPR. If phosphate is needed for a spring flush, then apply the slower release inputs six weeks beforehand. There’s a US Department of Agriculture study, where triple-super was compared to rock phosphate in order to contrast P release during a 13-year period. RPR won hands-down. Another major problem with water-soluble phosphate is the extreme burning potential of phosphoric acid.

Many will be familiar with the evidence of acid burn on roots of young crops. Mycorrhizal fungi are similarly affected. They’re massive fungal root extensions involving a network of filaments attached to roots. The filaments increase root surface area at least ten-fold, so everything the roots are offering is multiplied many times.


This hyphal “root” offers greater access to nutrients and moisture, while constantly releasing supportive biochemicals to nurture its host. Root knot nematodes are unable to coexist on a plant colonised by mycorrhizal fungi, or AMF. The hyphae also release mild acid exudates, which break the bond between locked up calcium and phosphorus, and then transport them to the plant. The least mobile of all minerals are phosphorus and zinc. They do not go into soil solution and must be retrieved by the roots. Fungal extension allows much greater access to both these nutrients.

Finally, AMF produce a sticky, carbon-based substance called glomalin. Glomalin is the triggering mechanism for humus building in soil. Thirty per cent of all organic matter is created by the stimulatory power of glomalin. But we have lost 90 per cent of our AMF in agricultural soils across the globe. A major contributor to the demise of AMF, is unbuffered acid phosphate.


1100 kg/ha

Hard Rock Phosphate

330 kg/ha

Triple Super
























< 4

The table shows an equivalent application of phosphate units as Rock

Phosphate hugely out-performs acid phosphate. The inactivity in year

one was on alkaline soils. This does not occur on acidic NZ soils with

sulphur added. Rock Phosphate is a cheaper way to build P in the soil.

Acknowledgements: Graeme Sait, NTS.



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