Evaluation of chemical amendments to control phosphorus losses from dairy slurry

Abstract

Land application of dairy slurry can result in incidental losses of phosphorus (P) to runoff in addition to increased loss of P from soil as a result of a buildup in soil test P (STP). An agitator test was used to identify the most effective amendments to reduce dissolved reactive phosphorus (DRP) loss from the soil surface after land application of chemically amended dairy cattle slurry. This test involved adding slurry mixed with various amendments (mixed in a beaker using a jar test flocculator at 100 rpm), to intact soil samples at approximate field capacity. Slurry/amended slurry was applied with a spatula, submerged with overlying water and then mixed to simulate overland flow. In order of effectiveness, at optimum application rates, ferric chloride (FeCl2) reduced the DRP in overlying water by 88%, aluminium chloride (AlCl2) by 87%, alum (Al2(SO4)3·nH2O) by 83%, lime by 81%, aluminium water treatment residuals (Al-WTR; sieved to <2 mm) by 77%, flyash by 72%, flue gas desulphurization by-product by 72% and Al-WTR sludge by 71%. Ferric chloride (€4.82/m3 treated slurry) was the most cost-effective chemical amendment. However, Al compounds are preferred owing to stability of Al–P compared with Fe–P bonds. Alum is less expensive than AlCl2 (€6.67/m3), but the risk of effervescence needs further investigation at field-scale. Phosphorus sorbing materials (PSM) were not as efficient as chemicals in reducing DRP in overlying water. The amendments all reduced P loss from dairy slurry, but the feasibility of these amendments may be limited because of the cost of treatment.