Forward Osmosis Priced Honestly: A Regeneration Bound That Closes the Seawater Claim, and the Fertilizer-Draw Irrigation Design That Survives It

The minimum work of regenerating a forward-osmosis draw is derived and shown to exceed the minimum work of desalinating the feed directly at every recovery: for seawater at 0.50 recovery the bound is 1.53 kWh/m3 against a direct minimum of 1.06 kWh/m3, and a stated 2.0 mol/L NaCl draw requires 2.08 kWh/m3 at the minimum, totalling 6.09 kWh/m3 against 3.04 kWh/m3 for reverse osmosis at a common second-law efficiency of 0.35. A thermolytic NH3-CO2 draw requires 56.49 kWh of heat per m3 at 60 degC under the same accounting, and the comparison flips only if that heat is supplied free. A fertilizer-draw design that needs no regeneration is fully sized (1,484 m2 of membrane for 50 m3/day from 2.0 g/L brackish groundwater), and its nutrient balance shows full-rate application over-delivers nitrogen by 18.7-65.6x while nitrogen-matched spreading makes the unit the complete fertilizer source for 32.8 ha of medium-demand crops. SIMULATED, NOT MEASURED: a reproducible computation, no unit built or tested.