Dr. Paul Murphy:

The issue with phosphorus is somewhat counter-intuitive in that way. Phosphorus is strongly bound in soils and for decades it was considered that phosphorus was not lost to water. The issue with phosphorus and water quality arises because although the quantities of phosphorus lost in the run-off of a field are rather small relative to the total amount of phosphorus the farmer is applying, the receiving water systems are very sensitive to even small concentrations. This is why when the farmer applies phosphorus it can and will get bound in the soil.

Let us consider the loss rates in the graph before the committee. The rates in kilogrammes per hectare are all well below 1 kg per hectare whereas a farmer could be applying 14 kg or 15 kg of phosphorus per hectare per year. Agronomically, it is not a loss to the farmer and he will not notice it but the impact is material. Surface water systems in particular are rather sensitive to phosphorus loss. That is the reason for it.

The classification of soils into poorly-drained and well-drained soils is sometimes related to clay content. The heavy clay-rich soils in some parts of the country mean that these soils are predominately poorly drained. However, it can be due more to the landscape position and whether the water table is close to the surface as well. It is not the case that only heavy clay soils are poorly drained. It can also be the landscape position. Does that answer the question?