Certified Crop Advisor Practice Exam

Which of the following is responsible for decreased P availability at high soil pH?

1. Calcium phosphate precipitates

2. Iron oxides

3. Organic matter

4. Potassium ions

The correct answer is: Calcium phosphate precipitates

The correct answer highlights that calcium phosphate precipitates play a significant role in reducing phosphorus (P) availability in soils with high pH levels. When soil pH rises above neutral (around pH 7), calcium phosphate compounds tend to form and precipitate out of the soil solution. These compounds are less soluble than other phosphorus forms and can lead to a decrease in the amount of available phosphorus for plant uptake. At higher pH levels, the solubility of phosphorus is affected because phosphorus tends to bond with calcium, forming stable compounds that are not readily available to plants. This process reduces the free phosphorus in the soil, which is crucial for plant health, as it is a vital nutrient involved in energy transfer, photosynthesis, and cell division. The influence of calcium phosphate precipitates underscores the importance of soil pH management in agricultural practices, as maintaining an optimal pH range can help ensure adequate phosphorus availability for crops. The other options mentioned do not play a significant direct role in this specific context of phosphorus availability at high pH levels.