When to Apply Super Phosphate to a Garden

Phosphorous fertilizers vary in their availability to plants, depending upon the phosphorous source. Sulfuric-acid-treated rock phosphate creates a readily available phosphorous called super phosphate. If your garden is deficient in phosphorous, super phosphate should be applied to the garden just prior to seeding or transplanting plants. In phosphorous-poor soil, established plants need super phosphate when they begin to emerge or show signs of new growth. When applying super phosphate or other fertilizers to your garden soil, follow manufacturer's instructions for best results. Wear goggles, gloves, dust masks and a long-sleeved shirt during application of this fertilizer.

beautiful ornamental garden with blooming flowers
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Large flower garden

Soil pH Levels Affect Phosphorous Availability

Soil pH levels affect the availability of phosphorous for plant use, with the greatest availability when soil pH is between 6.5 and 7.0. To ensure proper fertilization of your garden, use a home soil test kit or use the services of a lab to determine soil pH and phosphorous levels. If your soil needs phosphorous, correct soil pH levels prior to application of super phosphate to increase availability of this plant nutrient. For home soil testing, follow the manufacturer's instructions.

Super Phosphate Mixed into Soil

Phosphorous fertilizer tends to remain in place, instead of moving through soil the way nitrogen and certain other fertilizers move. Super phosphate benefits plants best when it is incorporated into soil at the root level, where plants can absorb the nutrient. Colorado State University Extension recommends mixing super phosphate into soil in the spring just before planting, if required.

Banding Super Phosphate Benefits Plants

Banding super phosphate prior to seeding or planting your garden provides a steady supply of phosphorous to your plants in deficient soils. To band super phosphate, avoid placing super phosphate in seed rows. Instead, open a furrow adjacent to the seeding or planting row. For rows that will be seeded, make the furrow 3 inches deep. For transplants, make furrows 6 inches deep. Distribute 1/2 cup of super phosphate for every 10 feet of row, according to Texas A&M University Extension. Cover the fertilizer with 2 inches of soil.

Super Phosphate for Established Plants

When your garden soil is low on phosphorous, established plantings benefit from super phosphate application during active growth to strengthen their stems and encourage bloom. Bulbs, tubers and flowering trees benefit from a side dressing of super phosphate in mid- or late winter when the ground is not frozen. Sprinkle super phosphate lightly on the ground 6 inches away from emerging bulbs or plants. Mix the fertilizer lightly into the soil. Chrysanthemums (Chrysanthemum spp.), hardy in U.S. Department of Agriculture plant hardiness zones 5 through 9, and dahlias (Dahlia spp.), hardy in USDA zones 7 through 10, are heavy feeders with shallow roots. Scratch a mere sprinkling of super phosphate into the soil and dig in lightly away from the plant; avoid disturbing the roots. Do not apply super phosphate to frozen ground, because it cannot be incorporated into the soil and may run off into waterways during snowmelt or rain.