A greenhouse investigation to evaluate the effects on plant growth of incorporation of high rates (2.5% or 10% by weight of soil) of paper mill sludge material from several primary and secondary treatment processes was conducted. Sludges from secondary biological treatment processes contained sufficient N in relation to organic C (range of C/N values was 12:1–50:1) so that N was not limiting to plant growth. However, sludges from primary treatment processes contained high organic C/N ratios (ca. 150:1) and would require considerable supplementary N fertilizer if cereal crops or grasses were to be planted soon after sludge application. At the high sludge rate, plant growth was diminished even though sufficient fertilizer N, P and K had been applied.