Certified Crop Advisor Practice Exam

At which soil pH is phosphorus most available for plant uptake?

1. 5.5

2. 6.0

3. 6.8

4. 7.5

The correct answer is: 6.8

Phosphorus availability to plants is significantly influenced by soil pH. The optimal range for phosphorus availability typically occurs between a pH of 6.0 and 7.0. At a pH of 6.8, phosphorus is in a form that is readily accessible to plant roots. When the soil pH is too low (acidic conditions), phosphorus can bind with iron and aluminum, forming insoluble compounds that plants cannot take up. Conversely, in highly alkaline soils (higher pH values), phosphorus can react with calcium, leading to similar issues with availability. Therefore, 6.8 represents a balanced pH level where phosphorus is not only soluble but also stands a higher chance of being absorbed by the plants effectively. This is why 6.8 is considered the ideal pH for maximizing phosphorus availability.