286 Research Article Received: 29 January 2009 Revised: 23 September 2009 Accepted: 23 September 2009 Published online in Wiley Interscience: 18 November 2009 (www.interscience.wiley.com) DOI 10.1002/jsfa.3812 Comparison of neem or oregano with thiram on organic matter decomposition of a sand loam soil amended with compost, and on soil biological activity Nikolaos Gougoulias, Ioannis Vagelas, Ioannis Vasilakoglou, Fotios Gravanis, Aida Louka, Eleni Wogiatzi and Nikolaos Chouliaras * Abstract BACKGROUND: Oregano essential oil and neem have been reported to be effective against soil-borne pathogens and nematodes. The possibility of having an effect on soil properties was the aim of this investigation. Moreover, thiram, a common widely used chemical pesticide, was used for comparison. RESULTS: The effects of all three above-mentioned substances were investigated during an incubation experiment, for 15 weeks. Crushed neem pellets were added at 1.0, 2.0 or 3.0 g per 50 g of soil. Oregano dry matter was applied at 0.2, 0.4 or 0.6 g per 50 g of soil. Finally, thiram was applied at 0.1, 0.2 and 0.3 g per 50 g of soil. The addition of neem resulted in a decrease in organic carbon mineralisation, higher than that of oregano, but not as prominently as thiram. The addition of neem resulted in an increase in the content of nitrate, organic P, bioavailable P, bioavailable K and bioavailable Mn. Oregano had a slight negative effect on organic matter biodegradation, but caused an increase of nutritional mineral elements. Microbe colonies in soil were increased by the addition of neem cake granules, whereas oregano and thiram treatments caused a decrease. The addition of thiram reduced nitrate and available Mn contents, but at the lowest dose increased organic P and available K. Thiram reduced prominently available Cu at the upper rates, but it increased these forms at the lowest rate, while available Zn content was increased in two lowest rates. CONCLUSION: The results of this study indicated that neem or oregano could be applied to the soil without any extremely negative effect on the available forms of nutritional mineral elements, as thiram does. c  2009 Society of Chemical Industry Keywords: neem; oregano; thiram; soil chemical properties; soil organic matter biodegradation; soil microflora INTRODUCTION Organic materials applied in soil as fertilisers or pesticides could play an important role in the establishment of sustainability in modern low-input agricultural systems. In particular, aromatic plant tissues like basil (Ocimum basilicum L.) and oregano (Origanumvulgare L.), 1,2 or plant derivatives like azadiractin (neem), a natural product obtained from the neem tree (Azadirachta indica A. Juss) 3 could be effectively used for pest suppression. Dhima et al. 2 found that oregano tissues, used as green manure before maize planting, suppressed annual weeds. Concerning neem as a biopesticide, its basic products are neem oil and neem cake. Immaraju 3 has reported that low rates of azadirachtin act as a broad-spectrum pesticide. Also, Gravanis et al. 4 reported that applying neem to soil resulted in the production of lower number of both Meloidogyne spp. galls and egg-masses per plant and significantly reduced nematode damage. Apart from its insecticidal 5–7 and nematicidal 4,8,9 properties, neem has also been reported to exhibit antifeedant, 10,11 repellent, 12,13 growth retardant 3 and antimicrobial 14,15 properties. The effects of chemically produced pesticides on soil microflora or soil respiration have been reported as both positive and negative depending on their chemical properties. 16 Wadson et al. 17 and Pampulha et al. 18 have also reported the herbicide effects on soil properties or soil microflora. However, the impacts of the natural products on soil properties, which affect soil productivity, are not well studied, 19 although this knowledge should be very useful to farmers. The aim of this research was to investigate the effect on soil organic matter and soil microflora of the application of neem or oregano dry matter. Thiram, a synthetic chemical fungicide was used as the control. ∗ Correspondence to: Professor Nikolaos Chouliaras, Department of Crop Production, Technological Education Institute of Larissa, 41110 Larissa, Greece. E-mail: chouliar@teilar.gr Department of Plant Production, Technological Education Institute of Larissa, 41110 Larissa, Greece J Sci Food Agric 2010; 90: 286–290 www.soci.org c  2009 Society of Chemical Industry