How does soil pH affect phosphorus availability?
Written by
Nguyen Minh
Reviewed by
Prof. Charles Hartman, Ph.D.Soil pH has a major effect on the amount of phosphorus a plant can access. If soil pH is between 6.0-7.0, phosphorus remains in a soluble form and available for uptake. If the soil pH falls outside that range, phosphorus will bind to soil particles and become unavailable for plant uptake. This is crucial to know when attempting to avoid phosphorus deficiency, as simply applying enough fertilizer may not be enough.
Acidic Soils (pH <6.0)
- Aluminum and iron bind phosphorus into insoluble compounds
- Rock phosphate dissolves effectively below pH 6.5
- Requires lime application to raise pH
- MAP fertilizer works best in these conditions
Neutral Range (6.0-7.0)
- Phosphorus remains soluble and plant-available
- Minimal chemical binding occurs
- Both organic and synthetic sources work effectively
- Ideal for most crops and gardens
Alkaline Soils (pH >7.0)
- Calcium binds phosphorus into calcium phosphates
- Requires sulfur application to lower pH
- Chelated phosphorus sources bypass fixation
- DAP fertilizer becomes ineffective above pH 7.5
Acidic soils with a pH below 6.0 combine phosphorus with aluminum and iron, which means your plants will not get any nutrients, regardless of how much you add to the soil. Rock phosphate is only soluble in these conditions. You will want to apply lime in small amounts to increase the pH without over-liming, which can cause calcium fixation.
Alkaline soils with a pH greater than 7.5 bind phosphorus with calcium, forming insoluble calcium phosphates that plants cannot utilize. Phosphorus fertilizers in chelated forms can be used to avoid fixation. Applications of elemental sulfur can also reduce pH over a period of months. Test the soil every 3 months to check the progress of the correction.
Organic matter provides an effective buffer to fluctuations in pH. Compost helps stabilize soil conditions in the preferred pH range of 6.0 to 7.0. Regulating soil chemistry is typically achieved by utilizing the relatively stable microorganisms and earthworms already present in the soil. A reasonable recommendation would be to apply 2 to 3 inches of compost annually to maximize phosphorus availability under various conditions.
Test the pH level seasonally, as fertilizers and rain can alter soil acidity. The spring test reflects the effects of leaching during the winter, and the fall tests reflect any changes that occurred during the summer. Calcareous soils should be tested at least twice a year. To manage existing phosphorus available on the soil surface, use the pH readings obtained when determining fertilizer options.
Read the full article: Phosphorus Deficiency Plants: Signs and Solutions
