How is phosphorus from fertilizers or manure most likely to be lost from crop fields?

Phosphorus from fertilizers or manure is most likely to be lost from crop fields through runoff into surface water. This is primarily due to phosphorus being a solid particle, which can easily attach to soil particles. When rainfall or irrigation occurs, these soil particles can be carried away with the water, effectively transferring phosphorus into nearby streams, lakes, or rivers. This process is particularly problematic because it can lead to eutrophication—a condition where excess nutrients in the water stimulate algal blooms, which can harm aquatic ecosystems.

While leaching into groundwater can occur, phosphorus is less soluble than other nutrients like nitrogen, making it less prone to leach away. Volatilization into the atmosphere is a process more applicable to ammonia, which can occur with certain fertilizers but is not a significant pathway for phosphorus. Therefore, runoff is the primary pathway through which phosphorus exits crop fields, making it the most accurate choice in this context.