The Effect of Phosphorus Fertilizer Placement on Weed and Algae Growth in Rice Systems.

The effect of fertilizer management practices on weed growth and abundance in California rice systems is not known. The objective of this study was to evaluate the effect of phosphorus (P) fertilizer and its placement in the soil on weed growth, cover, and abundance. Studies were conducted in greenhouse and on-farm. In ten farmers' fields, we compared the effect of surface applied P to zero P on weed cover at mid-tillering. In plots with surface applied P there was greater cover of waterhyssop (300%), ducksalad (100%), smallflower (100%), bulrush (90%), watergrass (130%), and redstem (100%) as compared to plots with no P fertilizer. In a controlled pot study, we compared weed abundance and biomass in pots with three P treatments: zero P, surface applied P, and P buried 2.5 cm. Algae, harvested from the flooded pots 8 days after planting, yielded greater biomass in pots with surface P (100%) and buried P (30%) than in pots with no P. Weeds harvested between 21-26 days resulted in significantly higher percentages of smallflower (257%), waterhyssop (82%), the grouping of ducksalad, monochoria, and arrowhead (150%), and the grouping of redstem and smartweed (363%) in the surface P treatments relative to treatments with zero P. Weed counts in the buried P treatment were between the zero P and surface P treatment counts. With the exception of watergrass, the weed species that responded to P fertilizer in the flooded pots were similar to the weeds observed in the on-farm study. In conclusion, the placement of P fertilizer in the soil has a large effect on weed populations based on both pot and field studies. Pinpointing the effect of P fertilizer on weed populations has the potential to indicate management practices that reduce weed populations in California rice without an escalation of herbicide use.