What makes synthetic phosphorus fertilizers effective?
Written by
Nguyen Minh
Reviewed by
Prof. Charles Hartman, Ph.D.Synthetic phosphorus fertilizers provide instant nutrition to plants that are deficient in phosphorus, through water-soluble forms that plants can quickly absorb. The success of both MAP and DAP fertilizers depends entirely on the proper matching of soil with fertilizer: MAP is excellent in acidic soils while DAP is best in neutral pH conditions. Liquid formulations will correct an urgent deficiency in a short time.
Monoammonium Phosphate (MAP)
- 11-52-0 NPK ratio with high phosphorus content
- Acidic reaction lowers soil pH upon application
- Best for soils below pH 7.0 with phosphorus deficiency
- Avoid in alkaline conditions where it causes ammonia toxicity
Diammonium Phosphate (DAP)
- 18-46-0 NPK ratio with balanced nitrogen
- Alkaline reaction raises soil pH slightly
- Ideal for neutral soils between pH 6.5-7.5
- Causes phosphorus fixation in acidic conditions below pH 6.0
Liquid Polyphosphates
- 30-50% orthophosphate for immediate availability
- Works in cold soils down to 40°F (4°C)
- Foliar application corrects deficiency within 48 hours
- Higher cost per unit of phosphorus than dry options
The matching of soil pH is what makes synthetic fertilizers effective. The acidity of MAP will benefit soils that are naturally acidic (those with a pH below approximately 6.5), and the slight alkalinity of DAP ensures it will always be suitable for neutral soils. On the other hand, DAP applied to acidic soil will cause phosphorus lock-up. In contrast, MAP on alkaline soils will induce ammonia toxicity to roots. Always test the pH before making a choice.
Methods of application can significantly impact performance; band applications unambiguously place phosphorus adjacent to roots, where it is readily available to plants. Foliar applications do not have the soil fixation issue, but need to be repeated for any meaningful efficacy. Recommend incorporating fertilizers into the top 6 inches of soil to ensure adequate contact with the roots.
Properly managing for environmental concerns is important. Phosphorus, even synthetic phosphorus, is easily transported to our waters where it can foster algal blooms. Utilizing the incorporation of fertilizer is preferable to surface broadcasting. Applications of any fertilizer should not occur before heavy rainfall. Buffer strips, which help filter runoff, should be used adjacent to surface water.
Use synthetic starters with organic amendments to get balanced nutrition. Use MAP or DAP at planting to get the best immediate availability. Consider using bone meal as a supplemental organic amendment to have sustained release of nutrition. Overall, this approach provides both immediate and long-term benefits for the plant's needs, serving to correct the immediate issue and promote the long-term health of the soil.
Read the full article: Phosphorus Deficiency Plants: Signs and Solutions
