When soil phosphorus levels become excessive the capacity of the soil to fix it may be exceeded and levels of soluble phosphorus in the soil may rise and result in increased phosphorus losses through surface runoff and, to a much smaller degree, leaching. This commonly happens when animal production is concentrated and the manure that is generated is repeatedly applied to the same area. Table 1 summarizes some common misconceptions about phosphorus. Soil tests can provide producers with information on levels of phosphorus in their soils. Most of the phosphorus that is lost from soils is lost through either erosion or runoff.
Erosion carries soil particles enriched with fixed phosphorus into lakes and rivers.
As water flows over the soil surface it can dissolve phosphorus in a very thin layer at the soil surface and run off into surface waters where the phosphorus can cause eutrophication . Some soluble phosphorus may be leached out of the surface layers of the soil. Although most of this phosphorus becomes fixed to soil particles, some may be transported below the root zone where it is no longer available to plants. Research in Delaware's Inland Bays area, in fields where chicken litter has been repeatedly applied, has demonstrated greater downward movement of phosphorus than in comparable, uncultivated soils on the field borders. One reason why phosphorus builds up in soils receiving manure is because there is an imbalance between manure nutrient composition and plant nutrient requirements.