5. The effect of the sulphur form used on the ratio of S to P required
Background
Pasture plants need to take up 7 kg of sulphur (S) for every 10 kg of phosphorus (P). So, if both nutrients are being applied in reasonably efficient forms, a fertiliser containing 9% P should only need to contain 9×0.7 = 6.3% S. Near the coast, where there is significant input of sulphate in rainwater, the amount of S required is less or even zero. But most people know that single superphosphate or ‘super’ contains 9% P and a fixed 11% S, almost twice the calculated amount of S required. Furthermore, in higher rainfall areas and low-medium P retention soils, even this amount is insufficient, and super has to be amended with elemental S to make sulphur-super. Why is this? It is simply because much of the sulphate-S gets leached from the soil before it can be used by the pasture (taking valuable cations such as calcium, magnesium and potassium with it). Even though 10-35% of applied soluble P becomes fixed in progressively unavailable forms in our soils, this is still more efficient than is sulphate-S, except in very dry conditions.
The science of S fertilisation
It has been known for decades that, except in very dry conditions and on very high P retention soils, finely ground elemental S is far more efficient than sulphate -S. This is why elemental S rather than say gypsum (calcium sulphate) is added to straight super if more S is required, and elemental S is the ‘go-to’ product for adding to the likes of TSP, DAP and MAP. It is no surprise therefore that by far the most common blend of RPR and elemental S sold by Quinphos was 92% RPR and 8% fine elemental S. With 12.7% P in the RPR, which also contained about 0.8% S as sulphate, this gave a product containing 8.7% S and 11.7% P ; an S:P ratio of – you guessed it – 0.7 to one.
A big proviso is that the elemental S used has to be truly fine – mainly less the 250 microns and all less than 500 mm. This can be irritating to the eyes if not dampened. These days, the cost of prilling molten sulphur with bentonite clay is far less than it used to be. These small water-dispersable hemi-spherical prills contain hundreds if tiny sulphur particles in each prill, which disperse easily in the soil. The only situations where sulphate is likely to be needed are on extremely low rainfall areas or in cold late winter/early spring if soil S levels are low. Sulphate of ammonia provides a good N boost and takes care of the S requirement in the latter case.