High Phosphorus Fertilizer Does NOT Mean More Blooms

Phosphorus for Blooming Plants

Mitchell, C.C.; Pinkston, C.B.; and Wheeler, E.J.

Garden centers and retail fertilizer companies often promote high P fertilizers for blooming plants with no regard to soil test P levels. Master Gardeners in Cullman and Marshall Counties in northern Alabama wanted to determine if applying a high P fertilizer to a soil that was high in P would actually improve flowering for summer annual flowers. Identical experiments were conducted in 2006 at two locations in North Alabama. Soil test prior to establishment indicated that both sites tested very high in extractable P; no additional P was recommended. Ten treatments replicated 4 times were applied that included various N: P ratios, two commercial fertilizers for blooming plants and poultry broiler litter (a 3-3-2 grade fertilizer). The highest N and P rate applied was equivalent to 120 lb. N and 120 lb. P2O5 per acre applied twice during the growing season. Annual blooming plants were planted in each plot and monitored during the growing season. Additional P fertilization had no effect on number of blooms, bloom quality or vegetative quality. Nitrogen fertilization alone up to 120 lb. N per acre produced healthier plants and more blooms at both locations. This applied research activity provided local Master Gardeners with an opportunity to contribute to agricultural experiment station research.