The two actions that will best address the problem of excess phosphorus in runoff while minimizing additional environmental impacts are:
B. Plant cover crops next to farmland and rivers.
E. Continually test soil nutrient levels and add only the needed amounts.
Planting cover crops can help reduce runoff by improving soil structure and minimizing erosion, while continually testing soil nutrient levels ensures that fertilizers are applied only as needed, reducing the risk of excess phosphorus entering waterways.