Mosaic and PotashCorp raised doubts over the rebound in potash prices despite issuing upbeat forecasts for demand, particularly following China's long-awaited agreement to import deals.
PotashCorp, while highlighting "encouraging signs in potash", forecast that "weaker pricing" would curtail its results in the nutrient this year, despite strong prospects for sales volumes.
The revival in prices it achieved on potash exports, while up $33 a tonne year on year to $250 a tonne in the January-to-March period, has slowed, with the quarter-on-quarter gain at just $4 a tonne.
In the home North American market prices have already begun declining - by $9 a tonne quarter on quarter to $349 a tonne - although values remain well above the nadir if $295 a tonne reached at the start of 2014.
US-based Mosaic revealed that the potash price it achieved in the latest quarter had fallen too, to $288 a tonne, down $7 a tonne from that achieved in the October-to-December period.
And it flagged the potential for further decline in prices in the April-to-June period, forecasting values at $265-290 a tonne, despite boosts to demand, in particular from a series of Chinese potash import contracts with major suppliers.
With China the top potash importer, the terms of its deals are closely watched by other buyers, and used as market benchmarks.
"Good demand highlighted by the recent Canpotex contract with China, low pipeline inventories and significantly improved operating costs lead to a positive outlook for our potash business this year," said Jim Prokopanko, the Mosaic chief executive.
PotashCorp said that with "China's annual potash contracts now in place and planting underway in the northern hemisphere, we expect global shipments to accelerate and market fundamentals to improve in the [April-to-June] quarter".
The Canada-based group forecast world potash shipments hitting the "upper end" of the range of 58m-60m tonnes it has guided to, and lifted to 14.0m-14.5m, from 12.5m-12.7m, its forecast for demand in China itself.
Mosaic also forecast a retreat in phosphate values from the $458 a tonne achieved in the first three months of 2015, seeing prices average $425-$450 a tonne in the April-to-June period, although it gave no reason for expecting a retreat.
"As we look ahead, strong global demand and lower raw material costs bode well for our phosphates business," Mr Prokopanko said.
The comments came as the group, like PotashCorp unveiled rising earnings for the January-to-March period - but a smaller increase than investors had expected.
Mosaic's earnings gained 35% to $295m, on revenues up 7.7% at $2.14bn. But underlying earnings per share, at $0.70, fell short of the $0.74-per-share figure that investors had expected.
Mosaic shares stood 0.2% higher at $44.125 in late deals in New York.
In Toronto, PotashCorp shares were 1.3% down at $39.26.