Weeds are the major biotic constraint to rice production. Field observations have suggested that certain fertilizer regimes could enhance infestations of particular weed species emerging with rice. The study objective was to determine the effect of surface-applied calcium phosphate on weed growth in flooded California rice systems. In field and pot studies, triple superphosphate (TSP) applied to the soil surface increased weed emergence. Surface-applied TSP increased the number of sedge and broadleaf weeds, including smallflower umbrella sedge, blue-flowered ducksalad, redstem, ricefield bulrush, waterhyssop, and California arrowhead. A laboratory study measured germination of smallflower umbrella sedge and ricefield bulrush in response to the application of phosphorus (P) and calcium (Ca), which comprise 20 and 15% of TSP, respectively. Calcium stimulated smallflower umbrella sedge germination and had no effect on ricefield bulrush germination. Phosphorus did not stimulate either smallflower umbrella sedge or ricefield bulrush germination. Results indicate that surface applications of calcium phosphate increase the growth of certain weed species and that Ca may stimulate germination of smallflower umbrella sedge. By incorporating preplant applications of calcium phosphate into the soil profile, growers can reduce weed pressure from certain species. Alternatively, surface applications of calcium phosphate may be useful to stimulate weed emergence in stale-seedbed management.

Nomenclature: Blue-flowered ducksalad, Heteranthera rotundifolia (Kunth) Griseb.; California arrowhead, Sagittaria montevidensis Cham. & Schlecht.; redstem, Ammannia coccinea Rottb.; ricefield bulrush, Schoenoplectus mucronatus (L.) Palla; waterhyssop, Bacopa spp. L.; smallflower umbrella sedge, Cyperus difformis L.; rice, Oryza sativa L.