A Ravensdown spokesman said with superphosphate specifically, the trials had shown the difference in pasture production was 40 per cent to 50 per cent less, as well as poorer quality feed with little or no legume or higher fertility grasses.
"This translates to a lot less animal production and lower returns" he said.
New Zealand, with its historically low levels of phosphorus (P), has been importing phosphate rock — which can include fossilised and mineralised fish bones and teeth — for more than a century.
Dunedin's Ravensbourne plant, which opened in 1881, was the first plant in the country to manufacture superphosphate, meaning the rock is ground up and treated with sulphuric acid.
It is now one of five sites — three owned by Ravensdown and two by Ballance — producing the fertiliser, designed to give soils an injection of phosphorus and sulphur.
The companies have traditionally sourced the raw material — rock — from Christmas Island, Nauru, South Africa, Morocco and the Western Sahara.
With the mines at both Christmas Island and Nauru no longer viable, the largest source of commercial grade phosphate rock is Morocco and Western Sahara.
At issue is disagreement among the local Sahrawi as to who owns what.
That has not slowed trade, however, with both fertiliser manufacturers deferring to the United Nations ruling that the current trade in the Western Sahara is internationally acceptable.
Numbers from the Fertiliser Association of New Zealand show New Zealand shipped in about 450 kilo-tonnes from the Phoboucraa mine in Morocco last year, about 22 per cent of its overall production.
The mine is owned by Moroccan state-owned company OCP, which as the global leader in phosphate production and export in all its forms, employs about 21,000 people.
It has exclusive access to the world's largest phosphate reserves, all in Morocco.
The US geological survey estimates global phosphate reserves at about 70 billion tonnes, of which Morocco holds approximately 50 billion tonnes.
A spokesman for Ravensdown said while the Western Saharan situation was "complex" the company was "not immune" to the humanitarian dimension.
"We've been exploring alternatives and this remains a challenge due to New Zealand's particular agronomic requirements" he said.
According to the Fertiliser Quality Council, the western Saharan rock was difficult to replace as it had specific chemical properties, including high P levels, that made it an important part of the superphosphate blend.
While Ravensdown and Ballance were looking for alternative sources, there were other challenges, including contamination from cadmium, dust and other environmental issues as well as quality and supply risks, they said.
"There are no alternatives that offer the same performance" according to a report from the Fertiliser Quality Council.
Soil consultants, however, say there are other options, such as reactive phosphate rock (RPR), diammonium phosphate (DAP) and other products made of bird guano, the latter primarily imported from small mines in Indonesia.
Quinfert consultant Dr Bert Quin said there was an emerging group of "biological farmers" who did not like over-reliance on superphosphate and wanted to get back to natural fertilisers that did not use acid-based products.
He said for many, Algerian-sourced RPR represented a viable option as it did not require processing and could be measured and applied directly.
However, the fertiliser association, said the problem was that the fertilisers could take years to take effect, because of their unprocessed nature.
Quin believed that was misleading.
"RPR is cheaper overall and it mitigates against run-off into waterways and leaching into groundwater, which are major emerging themes as farmers continue to add superphosphate when they don't need to.
"For much of our farmland, superphosphate is now effectively obsolete as farming fertility is at or above the maintenance level of phosphate. So additional soluble fertilisers are becoming less effective".
Quin said farmers were also adapting well to the use of nitrogen rich DAP, of which New Zealand had imported around 500,000 tonnes last year.
The fertiliser association, however, says it is concerned the manufacturing process for DAP results in a higher greenhouse gas footprint than other fertilisers.
"Whatever the soil inputs are, we need more innovative thinking from our major farmer-owned co-operatives, both to ensure efficient and safe food production but also to mitigate against the environmental impacts of what we put in our soil".
The fertiliser association went one step further, describing rock from the region as "a strategically crucial input for New Zealand's food creation".
