Which factor most increases the risk of water contamination by phosphorus from agricultural fields?

The correct choice focuses on high soil test phosphorus levels as the primary factor that increases the risk of water contamination by phosphorus from agricultural fields. When soil test phosphorus levels are elevated, it indicates that there is an ample supply of phosphorus available, which can easily be transported to water bodies through various mechanisms like surface runoff, leaching, or soil erosion.

In agricultural practices, phosphorus can be mobile, especially when soils are saturated or during heavy rainfall. As the phosphorus levels in soil increase, the potential for losses via runoff also rises since any excess phosphorus that isn't absorbed by plant roots is at risk of being washed away, contributing to water contamination.

While the incorporation of phosphorus into the soil is typically designed to enhance soil fertility, it does not directly correlate with increasing contamination risk unless the levels are excessively high. Low soil erosion rates generally help reduce the movement of all soil constituents, including phosphorus, thereby minimizing contamination risk. Application of phosphorus in the spring may align with plant growth cycles, but if phosphorus levels are already high, that practice could exacerbate contamination risks, but it is not the foremost factor compared to high existing levels of phosphorus in the soil.

Therefore, high soil test phosphorus levels unequivocally signify a concerning extent of phosphorus that can lead to increased