Watermelons [Citrullus lanatus (Thunb.) Matsum. & Nakai] were grown with three rates each of lime, gypsum, and K during two seasons to evaluate their effects on fruit production and mineral concentration. The first experimental site was a recently cleared Sparr fine sand with an initial pH of 5.4 and Mehlich I extractable K of 32 ppm (low) and 948 ppm Ca. The second site was a virgin Pomona fine sand with a pH of 4.8, 28 ppm K, and 612 ppm Ca. `Crimson Sweet' fruit yield was reduced 10% with an increase in lime rate from to 4.48 Mt·ha-1 in the first season. In the second season, lime rate had no effect on yield. In both seasons, fruit yields were reduced 14% by an increase in Ca from gypsum from 0 to 1.12 Mt·ha-1. Fruit yields were not influenced by K rates from 90 to 224 kg·ha-1. Application of lime and gypsum increased leaf tissue Ca concentrations and decreased K. An increase in K application significantly increased leaf K and decreased Mg in the first season but not significantly in the second season. Fruit firmness and soluble solid content were not consistently affected by treatment.