Soil Water Amendments Priced by Water Held per Dollar: A Common Yardstick, a Ranking Across Soil Textures, and the Break-Even Rainfall Deficit

Dryland farmers lose crops to short dry spells, and biochar, synthetic hydrogels, bentonite clay and compost are all sold as water-holding fixes with no common price tag. This paper builds one computed yardstick - litres of additional plant-available water delivered in the root zone per dollar spent per hectare over the amendment's working life - and runs six options (three biochar classes, synthetic hydrogel, bentonite, compost) through it on sand, loam and clay, with break-even rainfall deficits against a maize water value. SIMULATED, NOT MEASURED: all figures are outputs of the accompanying computation. On sand, bentonite ranks first (2,931.4 L/$) and low-temperature wood biochar second (2,687.1 L/$). Synthetic hydrogel collapses: 1,050 L/ha per fill (0.10 mm), 13.2 L/$, 222.1 times less than the best option. Only bentonite and low-temperature wood biochar on sand ever pay for maize on water alone, at break-even deficits of 19.4 and 16.7 mm/yr.