How Far Does One Buried Clay Pot Really Reach? A Wetted-Volume Bound for Pitcher Irrigation, and a Test of the Coverage Claims Made for It

Buried porous clay pots — pitcher irrigation — are promoted with generous coverage figures: a single pot is said in widely repeated promotional accounts to serve several square metres of crop, or a stated half-dozen plants. This paper tests those figures against two elementary bounds computed from stated assumptions for a 15-litre pot (12 litres usable) seeping at a nominal 1.0 litres per day (stated range 0.5 to 2.0). The first bound is volumetric: seeped water raises soil from its initial moisture to field capacity, and in the most generous possible geometry — a spherical bulb, no drainage below it, no evaporation, no root extraction during wetting — the wetted radius after 7 days is 0.322 m in sand, 0.241 m in loam, and 0.234 m in clay; after 30 days it reaches only 0.523, 0.391, and 0.381 m. Reaching a plant 0.5 m away takes 26.2 days in sand and 62.8 days in loam; reaching 1.0 m takes 209.4 and 502.7 days. The second bound is a water balance: at a crop demand of 5.0 mm per day, one pot sustainably serves an area of just 0.2 m² — a square grid spacing of 0.447 m, or 5.0 pots per square metre — rising to only 0.4 m² even at the top of the seepage range. Against these bounds, the claim that one pot serves 4 m² fails by a factor of 20 on supply (600 litres demanded over 30 days against 30 litres supplied; twenty pots, not one, are required), the claim of six plants at 0.5 m spacing fails by a factor of 7.5, and the claim of a 1 m × 1 m bed fails by a factor of 5.0. What survives is modest and real: one plant rooted within about 0.25 m of the pot is exactly balanced at nominal seepage, and two plants within 0.3 m are served only at the top of the seepage range, and then only after the wetting front takes 13.6 days to reach them in loam. The refill interval for the usable 12 litres is 12.0 days at nominal seepage, 6.0 days at the top of the range. Pitcher irrigation is a sound way to water individual plants; it is not, by an order of magnitude, a way to water beds. The study is simulated, not measured.