Annals of Applied Biology ISSN 0003-4746 RESEARCH ARTICLE Effects of untreated two-phase olive mill pomace on potted olive plantlets S.T. Endeshaw, E.M. Lodolini & D. Neri Department of Agricultural, Food and Environmental Sciences, Marche Polytechnic University, Ancona, Italy Keywords Olea europaea L.; olive mill waste; organic amendment; root growth; shoot growth; soil fertility. Correspondence D. Neri, Department of Agricultural, Food and Environmental Sciences, Marche Polytechnic University, Via B. Bianche, 60131 Ancona, Italy. Email: d.neri@univpm.it Received: 25 March 2014; revised version accepted: 22 December 2014; published online: 21 January 2015. doi:10.1111/aab.12200 Abstract Two-phase pomace represents an important environmental problem in Mediterranean areas. With its high organic content, direct application of two-phase pomace in the field is recommended to improve soil organic carbon levels and fertility. However, this does not consider any antagonistic effects that this application might have on root proliferation and biomass partitioning. We studied the effects of untreated two-phase pomace on properties of growth substrate, and on shoot and root growth and biomass allocation of potted olive plantlets. A pot experiment was carried out in a greenhouse over 90 days, with five levels of two-phase pomace and using two olive cultivars. The effects on shoot growth, leaf pigment content and maximum quantum efficiency of photosystem II (ΦPSII) were assessed each month. After 90 days, the shoot and root biomass of the olive plants was quantified, along with total organic matter, and carbon, nitrogen and polyphenol contents of the growth substrate and shoots, and the fine root nutritional status. Two-phase pomace increased the total organic matter, total nitrogen and polyphenol contents of the growth substrate. It significantly altered biomass partitioning in the olive plantlets, with reduced shoot dry mass and leaf area, and new shoot formation. It also increased fine and total root dry mass for all two-phase pomace levels except 40%. There were no significant differences in leaf pigment content and ΦPSII across the treatment levels. Therefore, application of untreated two-phase pomace at more than 4% induces a severe imbalance in olive plantlet biomass partitioning, and shoot and root growth. Introduction The European olive oil industry produces more than three-quarters of the olive oil worldwide (Roig et al., 2006). During the process of olive oil extraction, differ- ent bioproducts are produced, depending on the method of extraction. Currently, two processes are widely used for olive oil extraction: the three-phase and two-phase cen- trifugation systems (Ouzounidou et al., 2010). The olive mill bioproducts following the three-phase system are a water-based liquid known as olive mill wastewater and a solid waste known as pomace. The newer two-phase olive oil extraction system produces only a humid solid olive mill pomace (Roig et al., 2006; Ouzounidou et al., 2010). This two-phase pomace is a solid bioproduct with a strong odour, a doughy texture and a high moisture content (≥65%), which makes its management, transport and exploitation difficult (Rozzi & Malpi, 1996; Roig et al., 2006). However, the two-phase system allows a consid- erable reduction in water use during olive oil extraction (75% reduction over the three-phase system). This two-phase pomace represents an important envi- ronmental problem in Mediterranean areas. For instance, about 4 million tonnes only in Spain (80–100 kg of 100 kg of olives) is generated over short periods of time (Alburquerque et al., 2004). Soil application of two-phase pomace has become a serious problem particularly in Spain, Italy and Greece (Rozzi & Malpi, 1996). This is mainly because of the high phenol, lipid and organic acid levels, which make this two-phase pomace a highly phy- totoxic material. However, these bioproducts also contain 508 Ann Appl Biol 166 (2015) 508–519 © 2015 Association of Applied Biologists