Changes in Chemical Properties and Temperature during the Composting of Tobacco Waste with Other Organic Materials, and Effects of Resulting Composts on Lettuce (Lactuca sativaL.) and Spinach (Spinacea oleraceaL.)

ABSTRACT This study (i) investigated changes that take place during the composting of tobacco wastes with other organic materials, (ii) characterized the resulting composts, and (iii) evaluated their agronomic effectiveness. Four composts were made, all of which contained tobacco waste and sawdust/wood shavings but differed in the third ingredient, which was either cow dung, pig dung, poultry manure or cabbage waste. Changes in pH and electrical conductivity (EC) during composting were consistent with those generally observed, in spite of the presence of toxic tobacco wastes. Changes in temperature regime and the results of a bioassay (germination test) suggested that compost maturity was achieved after 45–59 days in all four composts. Mixing of tobacco waste with other organic wastes reduced the nicotine content of tobacco wastes from 12180 mg kg−1 to 4872 mg kg−1 by dilution while composting reduced it further to < 160 mg kg1 in the final composts. Tobacco waste had a depressing effect on microbial activity in soil but the composts stimulated it, possibly as a result of their lower EC and reduced nicotine and related alkaloid levels. All final composts had C:N ratios ≶ 17 and so were considered conducive to N mineralization and thus suitable for horticultural use. However, they had high electrical conductivity (6.0 to 9.3 mS cm−1) and may not be ideal for salt-sensitive crops or as sole components in horticultural growing media without modification. Application of the composts to soil (to supply 80 mg N kg−1) improved the growth of lettuce relative to the control and uncomposted tobacco waste but full growth potential was not attained due to inadequate nitrogen supply. A subsequent experiment with spinach designed to address the N limitation revealed that satisfactory yields could be obtained by the application of 20 t ha−1 cow dung compost or 10 t ha−1 compost in combination with NPK fertilizer supplying 50 kg N ha−1.