Phosphorus Mobility on Biosolids Amended Rangelands

The state of Utah has been concerned about excess P in soils that had received biosolids application, especially at loading rates that exceed crop nutrient requirements. The biosolids loading rates have been based on N crop requirements, which results in excess P in biosolids-amended soils. Therefore, the mobility of phosphorus in biosolids-amended soils in western Utah was investigated to address this issue. Three different types of biosolids including lime stabilized, aerobically digested, and anaerobically digested biosolids were land applied on 1/3-acre test plots at a variety of application rates (up to twenty times the estimated agronomic rate). The total P appeared to accumulate in the soil surface (i.e. 0.75 ft) for all different biosolids-amended soils. Accumulation of total P at the further soil depths (i.e. 0.75, 2, 3, 4, and 5 ft) was lower than that of the soil surface. No consistent trend was found with respect to the change of total P in terms of time (i.e. Year 1 and Year 2 following biosolids application). The accumulation of the plant available P (Olsen P) along the soil depths exhibited the same tendency with that of the total P. The P leachability from biosolids land application was low because the ratios of total P to Al+Fe were below 1 in all biosolids-amended test plots. The biomass yields at the test sites were enhanced as compared against the control. It was concluded that N-based biosolids land application did not result in significant P leachability that could pose a risk to the ground water source, human health, and the environment.