The solution to run-off of phosphate into waterways lies in more use of
direct application rock phosphate fertiliser, according to Chatham Rock
Phosphate chief executive Chris Castle.
Mr Castle said a range of scientific studies over many years has shown direct application rock phosphate offers strong environmental benefits.
CRP has evaluated some of the studies undertaken which compare the use of rock phosphate and super phosphate on New Zealand and international farmland.
They show that when applied directly reactive rock phosphate (RPR) is both a very effective sustained release fertiliser and resistant to leaching.
The findings of the studies – some of which go back several years – are supported by Dr Bert Quin, probably New Zealand’s pre-eminent expert on the use of rock phosphate as a fertiliser, who first conducted extensive field trials while working as a scientist for government agencies during the 1980s.
Dr Quin believes nutrients continue to enter waterways from agricultural land, simply because of the type of fertilisers we use. He says the traditional fertilisers used in New Zealand have been single superphosphate (‘super’), which supplies phosphorus (P) and sulphur (S), and urea for nitrogen (N).He says their biggest problems are that they are ‘leaky’ fertilisers.
“Super is prone to run-off of applied phosphorus into waterways in the weeks after application, leaching into shallow sub-surface drains and water bodies on dairy farms, and being leached right through soils with low phosphorus retention such as those in Northland and the West Coast. Urea is prone to volatilisation (evaporation) losses as ammonia gas to the atmosphere, nitrate leaching and nitrous oxide GHG emission.
“By far the most cost-effective option for phosphorus is reactive phosphate rock or ‘RPR’. This is a natural mineral, formed on the sea floor originally, which is a very effective source of sustained-release phosphorus, ideal for maintaining high-producing pasture and extremely resistant to leaching.
Dr Quin estimates switching from super phosphate to RPR and RPR/DAP (diamonium phosphate) blends would reduce average run-off losses of P into waterways by 80-90%.
“This would take P losses below the trigger levels necessary to keep our lakes in a eutrophied state. In 5-10 years, water quality in the Rotorua lakes, for example, would be massively improved.
“Sulphur requirements are easily met by adding in elemental S. Like RPR, this is a sustained release fertiliser. The water-soluble sulphate form of S in super is very easily leached from many soils. As this happens, it takes valuable cations such as calcium and magnesium with it.”
Dr Quin says the time is right for people with the political will and determination to save New Zealand’s environment to stand up and be counted, and force change.
“If we do not, we will come to be viewed as the ‘gutless generation’ by our children and grandchildren.”
He also says anecdotal evidence over the 25 years-plus that some farmers
have been using RPR shows eventually, maintenance requirements for
phosphorus start to drop significantly because of reduced run-off and
reduced fixation onto soil clay particles.
He believes there is a need for specific advice for farmers regarding managing or minimising any lag in production following a switch to RPR.
Other New Zealand and international research
The other research into waterway leaching by New Zealand and international scientists considered by CRP:
1. RPR revisited (1): Research, recommendations, promotion and use in New Zealand by BF Quin and M Zaman.
2. RPR revisited (2): Long-term farmer experience helps define the role of RPR in grazed pastures by M Zaman and BF Quin.
3. Phosphorus fertiliser form affects phosphorus loss to waterways: a paired catchment study by R. W. McDowell, R. P. Littlejohn and J. D.. Blennerhassett.
4. Evaluation of two management options to improve the water quality of Lake Brunner, New Zealand by RW McDowell.
5. Potential phosphorus losses in overland flow
from pastoral soils receiving long-term applications of either
superphosphate orreactive phosphate rock by RW McDowell, RM Monaghan and PL Carey.
6. Rainfall intensity and phosphorus source effects on phosphorus transport in surface runoff from soil trays by Francirose Shigaki, Andrew Sharpley and Luis Ignacio Prochnow.
7. Phosphorus Leaching in an Acid Tropical and Triple Superphosphate by E Gikonyo, AR Zaharah, MM Hanafi, and R Anuar.
8. Effectiveness of rock phosphate, coastal superphosphate and single superphosphate for pasture on deep sandy soils by MDA Bolland, MF Clarke, and JS Yeates.
Contact Chris Castle on 021 55 82 85 or email@example.com