Excess phosphorus (P) loading has been identified as an important cause of poor water quality in Lake Simcoe. A small (∼4%) but important source of P is the marshland that was developed for agriculture in and near the Holland Marsh. Applying the optimum rate of P fertilizer is an economical and environmentally friendly approach to crop management. In Ontario, P fertilizer recommendations are made based on the Olsen P soil test that is accredited by the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). The current study was a response to concern about high application rates of P fertilizer and grower and industry suggestions that the recommended rates for P were too low. The marketable yield of yellow cooking onions was assessed in relation to different rates of P fertilizer in high organic matter (muck) soils. Organic matter ranged from 38% to 86%. Small- (6 site-years) and large-scale (21 site-years) field trials were conducted in the Holland Marsh area from 2009 to 2011 on muck soils that varied in pre-plant P (17–312 mg L⁻¹). The study demonstrated that there was more plant-available P in the soil than was required for optimum yield in most fields; thus, P fertilizer applications had no effect on marketable yield of the onions. These results supported the OMAFRA P recommendations for onion production on muck soils, especially for soil test P levels over 61 mg L⁻¹ where a response to applied P is rated as “low, rare, or no response”.