When to Apply Super Phosphate to a Garden

Deficient Soil for Perennial Beds

• Superphosphate is a fertilizer that contains a high percentage of phosphorous. Phosphorous promotes root development and plant health. This fertilizer type differs from other fertilizers, because it doesn't contain nitrogen or potassium. In addition, superphosphate is applied to gardens at a different time than other fertilizers to promote root growth, promote flowering and help with phosphate deficient soil.

Planting Trees

• Add superphosphate to the soil before planting a perennial bed if soil tests indicate the soil is deficient in phosphate. Conduct a soil test on the garden area when the ground thaws. Send soil sample to your local cooperative extension office for a full analysis of your yard's make up. Before planting your perennial beds in the spring, apply a superphosphate with a NPK amount of 0-20-0 to your garden area, as suggested by Colorado State University.

Fall

• Fall is an optimal time to apply superphosphate to the garden, according to Roger Cook from This Old House. Superphosphate encourages root growth without causing the plant to produce foliage of blossoms. New growth on any plant is susceptible to winter injury. Use a superphosphate fertilizer that has a NPK amount of 0-20-0 on your perennial garden beds. Water the area after fertilizing.

When Not to Apply Superphosphate

• Avoid applying superphosphate when you lime your yard. Agricultural lime prevents phosphate from being available to plants. However, those with soil with a high pH should apply superphosphate. Phosphate is not available in alkaline soils, according to Texas A&M University. To help transplants or seeds grow in the spring in alkaline soil, make sure that you band furrows or around garden areas with superphosphate before planting. Plants decline in overall health when soil is deficient in phosphate.