15 December 2014
CRP trials show Chatham rock phosphate highly effective fertiliser
New glasshouse trials show the performance of direct application Chatham Rise rock phosphate compares favourably with
superphosphate fertiliser.
Chatham Rock Phosphate (CRP) commissioned Lincoln University and AgResearch to undertake trials in a glasshouse over a
period of four months to assess the effectiveness on plant productivity of rock phosphate from the Chatham Rise, east of
New Zealand. The trials also tested rock phosphate sourced from a Mexican marine phosphate deposit.
The trial found the Chatham phosphate delivered on average 85% of the productivity of low-sulphur triple superphosphate.
The tests used clover grown on five contrasting pasture soils – one from Chatham Island, two from the South Island and
two from the North Island. The fertilisers were added at three phosphorus application rates and three harvests were
taken over the four month period, with plant yield and phosphorus uptake in the clover used to measure performance.
The findings of the study achieved comparable results to extensive field trials conducted during the 1980s which also
found Chatham Rise rock phosphate to be an effective pastoral fertiliser.
Due to the presence of calcium carbonate in the Chatham phosphate rock the standard citric acid solubility test used to
assess the chemical reactivity of a phosphate rocks grossly underestimates the potential of Chatham phosphate rock as a
direct application fertiliser. For example although the citric acid solubility of the Chatham phosphate rock was only
half that of the Mexican phosphate rock, its average relative agronomic performance across all five soils and three
rates of application in the glasshouse study exceeded that of the Mexican phosphate rock (85% compared with 81%). These
findings highlight that chemical solubility alone is an inadequate measure of short-term agronomic performance of
sparingly soluble phosphate rock fertilisers in low pH and low phosphate fertility soils under ideal growing conditions.
Chatham rock phosphate offers a number of environmental benefits including a lower phosphate loss risk in surface
run-off from soils into waterways and extremely low levels of the heavy metal cadmium. Located within New Zealand’s
exclusive economic zone, it offers a reduced carbon footprint, given its location relative to current phosphate sources,
mostly in Africa and the Middle East. It is also of strategic importance, offering security of supply, given it is New
Zealand’s only phosphate resource.
CRP is planning to undertake field trials beginning in 2015, having just qualified for a Callaghan Innovation growth
grant to support spending on research and development. The field trials will extend the evaluation of the just completed
glasshouse trials.
They will be conducted in a range of environments and pastoral systems across the country to update and confirm the
applicability of the product to modern farming practices. The field programme is also designed to look more closely at
phosphorus use efficiency, as well as providing contemporary confirmation of the agronomic findings of field trials
conducted in the 1980s. The emphasis will be on New Zealand farming conditions but the results are expected to be
applicable to global markets.
ENDS