Phosphorus is not required in large amounts by grapes. With few exceptions, the level of phosphorus in Ontario soils is presently adequate. Phosphorus does have a place for sod or cover crop maintenance. A soil test is the best way to determine if there is a need to apply this nutrient to the sod cover. Phosphorus should also be applied before planting a vine-yard when it can be thoroughly incorporated in the soil if a soil test indicates a need.
Grapes require a larger quantity of potassium than tree fruits.
In cultivated vineyards, potassium may be broadcast before the first cultivation in the spring. With grapes on clay soils, it is advantageous to apply potassium in a band to reduce fixation. Potash can injure roots and trunks if applied too closely to the trunk. In dry growing seasons potassium is not readily available to the plant. Where a potassium deficiency has been confirmed, foliar applications of potassium may be beneficial.