Over application of phosphorus is often the result of using ______ for fertilizer.

Over-application of phosphorus is particularly common when using animal manures as fertilizer. This is primarily because animal manures tend to have varying concentrations of phosphorus, which can be significantly higher than the needs of many crops. Additionally, the application rates of manure can sometimes exceed crop uptake requirements, especially if the nutrients are not adequately accounted for in the soil management plan. Unlike synthetic fertilizers, which have clearly defined nutrient ratios, and compost, which typically has lower nutrient concentrations, animal manures can vary widely based on the type of livestock, their diet, and how the manure is processed. As a result, when these manures are over-applied, there is a greater risk of phosphorus runoff into water bodies, leading to environmental issues such as eutrophication. This understanding highlights the importance of careful nutrient management practices when incorporating animal manures into crop production systems.