Abstract

The extent of macronutrient leaching from soils treated with sewage sludge has been examined using large undisturbed soil lysimeters of 5 different New Zealand soils (2 pasture and 3 forest soils). Nutrient leaching from the soils was monitored continuously over a period of 3 years following a single application of sewage sludge. Application of sewage sludge resulted in increased concentrations and total amounts of SO₄^2–, NO₃^–, Ca, Mg, Na, and K in drainage leachates, but had little effect on Cl^– and dissolved organic carbon concentrations. The quality of the analytical data in the study (several hundred individual analyses) was confirmed by the excellent charge balance between the main anions and cations in the leachates. The peaking of some cation and anion concentrations well before 1 pore volume of water had passed through the lysimeters indicated that macropore flow could be an important process facilitating nutrient leaching. However, the extent of anion and cation leaching varied greatly between soils and appeared dependent on a number of soil properties and other factors. For example, the amount of drainage and the presence of growing plants clearly had a marked effect on the extent of macronutrient leaching, as did the way in which the sludge was applied to the soil. Sulfur (as SO₄^2–) was the dominant anion and Na⁺ the dominant cation leached, but concentrations of both these nutrients in the sludge used for this study were higher than would normally be expected in New Zealand sludges. The effect of sludge application on Mg²⁺ leaching was particularly interesting, in that the high Ca : Mg ratio of the sludge appeared to promote the leaching of native Mg from the soil.