When should phosphorus fertilizer be applied?
Written by
Nguyen Minh
Reviewed by
Prof. Charles Hartman, Ph.D.Proper timing of phosphorus applications maximizes plant phosphorus uptake and helps avoid deficiency symptoms. The best time to apply phosphorus is during the early growth stages or pre-flowering, before plants develop flowers. This fertilization timing coincides with the critical development stage of the roots, and thus, when plants are at their greatest need for phosphorus. In contrast, delay phosphorus applications until after flowering has begun, as it may be ineffective to get the phosphorus into the plant to correct any possible deficiency symptoms.
Seedling Stage (0-4 weeks)
- Apply starter fertilizers at planting for root development
- Band 5-10 lbs P₂O₅/acre near seeds
- Liquid polyphosphates work best for quick absorption
- Critical for establishing strong root systems
Pre-Flowering Phase
- Apply 2-4 weeks before flowering begins
- Foliar sprays at 2-4 lbs P₂O₅/acre
- Supports bud formation and flower development
- Avoid applications after buds open
Special Cases
- Perennials: Apply in early spring during bud swell
- Winter crops: Side-dress in fall for spring growth
- Container plants: Weekly applications during growth season
- Hydroponics: Maintain constant 30-50 ppm levels
The temperature of the soil is a significant factor in terms of the availability of phosphorus. For optimum uptake of phosphorus, it is recommended to apply phosphorus fertilizer when soil temperatures reach 55°F or 13°C. It should be noted that soil temperatures below 50°F (10°C) will significantly eliminate phosphorus mobility. Water-soluble orthophosphates are the best option when soils are cold. Warm soils facilitate the microbial mineralization of organic phosphorus sources.
Account for soil pH before any applications. Soils with a pH below 6.0, or acidic soils, require lime before any phosphorus applications. Soils with a pH above 7.5 or alkaline soils will require a sulfur amendment. Be sure to test pH periodically, as rain and fertilizer application can introduce acidity. Generally, maintain a pH range of 6.0 to 7.0 for optimal phosphorus solubility.
Late applications create several problems. Phosphorus added after bloom will not fix an existing deficiency. Excess phosphorus can tie up zinc and iron in the soil. The chance of environmental contamination from runoff increases with later applications. To avoid unnecessary treatment, soil should be tested annually for optimal management.
Utilize a blend of application strategies to ensure consistent and widespread nutrition. During planting, apply banded starter fertilizers directly with the seed. During important phases of development, follow with supplemental foliar sprays to provide bi-weekly nutrient inputs. Organic amendments can be applied in the fall, in advance of their apparent nutritional benefits in the spring. This application combination helps ensure the continued phosphorus nutrition.
Read the full article: Phosphorus Deficiency Plants: Signs and Solutions
