Fifteen‐year old Monterey pine (Pinus radiata) was irrigated for 3 yr with wastewater derived from industrial and municipal sources. The wastewater contained high concentrations of Na2+ and HCO3− and was quite alkaline. Irrigation thus caused substantial increases in exchangeable Na2+, extractable P, exchangeable K+, pH, and the electrical conductivity of the soil solution. Highly colored organic compounds derived from pulp‐mill effluent apparently combined with inorganic N from municipal effluent to form organic N compounds. Nitrogen remained largely in the organic form and became concentrated in the groundwater with colored humic compounds. The greatest environmental hazard in the use of such blended wastewater for irrigation is the contamination of drainage water with colored, saline water containing high concentrations of organic N. The organic compounds appeared to inhibit nitrification in the groundwater.
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