Yield of yellow cooking onions is not affected by added phosphorous fertilizer in muck soils with high soil test phosphorus in Ontario

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- 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’.