The most important time to apply super phosphate is before planting -- phosphorus is critical for seedling root development and early growth. Perform this application very soon before planting to ensure that fixation does not render the phosphate unusable by the time it is needed by seedlings. Side-dressing may be beneficial if a soil test indicates low phosphorus reserves. The best time for side-dressing super phosphate is when plants enter a stage of high nutrient demand, such as when tomatoes begin to bear or ripen fruit. For perennial crops, apply in early spring when vigorous growth ensures that plant roots can take up the phosphate before it is immobilized by fixation.