The Dry-Enough Bound: Hours, Nights, and Mould Risk in Sun-Drying Maize Seed to Safe Storage Moisture

Seed dried too little moulds in store; seed dried in the sun is at the mercy of the day's humidity and the night's. This paper computes, from stated assumptions, how long sun-drying maize seed from 20% moisture (wet basis) to a safe storage target of 12% takes, what a night costs, and when the target cannot be reached at all. Using an exponential thin-layer drying law and a stated maize equilibrium-moisture table, a thin layer in full sun at 40% relative humidity reaches the target in 2.65 hours of drying, and on a raised mat in 2.09 hours; at 50% humidity it needs 4.04 hours. Layer depth is the largest controllable factor: at the same humidity, layers of 2, 3, 5, and 8 cm need 3.48, 4.56, 8.11, and 14.60 hours. The bound of the method is sharp: when daytime humidity exceeds 59.1%, the equilibrium moisture of the seed in that air is above the 12% target, so sun-drying on a tarp cannot reach safe moisture however long it continues; for a relaxed 13% target the limit is 64.8% humidity. A night left out uncovered at 90% humidity returns 4.52 moisture points to seed that had reached the target (to 16.52%), costing 2.21 extra hours of drying the next morning, against 0.33 points and 0.28 hours if the seed is brought into a house. In day-by-day simulation, a thin layer brought in at night finishes in 1 day with 1.1 hours spent in the stated mould-risk condition (above 16% moisture at 25 °C or more), while a 5 cm layer left out overnight needs 2 days and accumulates 3.6 mould-risk hours, and an 8 cm layer brought in accumulates 5.6. The practical rule derived from the arithmetic is stated in the Discussion: dry in a layer no deeper than about 2 cm, turn it at least hourly, and bring it in — not merely cover it — before the evening air rises. The study is simulated, not measured.