ELSEVIER 0960-8524(95)00050-X Bioresource Technology 53 (1995) 43-51 © 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved 0960-8524/95/$9.50 RE-USE OF INDUSTRIAL ORANGE WASTES AS ORGANIC FERTILIZERS C. Correia Guerrero, J. Carrasco de Brito Unidade de Ci~ncias e Tecnologias Agr(zrias, Universidade do Algarve, Campus de Gambelas, 8000-Faro, Portugal & Nuno Lapa & J. F. Santos Oliveira* Grupo de Disciplinas de ECologia da Hidrosfera, Faculdade de Ci~ncias e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2825-Monte de Caparica, Portugal (Received 23 September 1994; revised version received 22 March 1995; accepted 29 March 1995) Abstract The aim of this study was to evaluate the possibility of the re-use of industrial orange wastes as organic soil fertilizer. The assay was performed with a lettuce vari- ety widely produced and consumed in Portugal and, consequently, with great commercial interest. Lactuca sativa L. (osteolata variety) was cultivated in Mitch- erlich pots containing samples of a poor soil of the Algarve region. This soil was prepared with different amounts of either pulp or peel orange-wastes from an orange-juice industry. The wastes were applied accord- ing to an increasing amount of nitrogen. The results obtained were submitted to statistical tests, in order to find the relations between the production of fresh and dry matter, and the percentages of nutrients (nitrogen, phosphorous, potassium, calcium, magnesium and iron) that were obtained in the dry matter, with the types and amounts of wastes applied. An increase in the average production of both fresh- and dry-matter with increasing amounts of either pulp or peel wastes applied, and high positive correlation coefficients between the average percentages of nitrogen, phospho- rus and potassium obtained in the dry matter with the average production of both fresh- and dry-matter were found. No phytotoxicity was observed. Key words: Industrial orange wastes, Lactuca sativa L. (osteolata variety), soil productivity. INTRODUCTION The Algarve region is one of the most important citrus production areas in Portugal. Nowadays, citrus production in this region comprises 25% of the local Agricultural Gross Product (Mariano, 1990). Eco- nomic factors have induced an increase in the *Author to whom correspondence should be addressed. 43 orange-juice industry, which produces large quan- tities of solid wastes. The traditional re-use of pulp- and peel-orange- wastes is of high cost, which is unbearable for Portugese citrus industries (Guerrero & Carrasco de Brito, 1992). Thus, these industries have been accu- mulating pulp- and peel-wastes in the soil, with serious problems to the environment. The re-use of industrial orange-wastes as organic soil fertilizers seems to be a low-cost technology for the recycling of nutrients contained in these wastes, although the type and amount of wastes applied have to be controlled according to the species of plant cultivated and the physical and chemical prop- erties of the soil and wastes (Spotswood & Raymer, 1973; Carrasco de Brito, 1986; Domingues et al., 1990; Guerro & Carrasco de Brito, 1992). In this context, a protocol was signed between the Univer- sity of Algarve and the 'Cialbe' enterprise, with the purpose of evaluating the use of industrial orange wastes as organic fertilizers of Algarve soils. METHODS Soil The soil was collected in the Tavira region and, according to the Regional Services of Agrarian Plan- ning and Cardoso (1965), it belongs to the group of Red-Yellow Mediterranean Soils (Vtc soils) which are composed of sand without calcareous material. This soil was probably derived from gneisses, shales, schists, sandstones and colluvium. The surface sandy horizon (A horizon: 3-era depth) has a low quantity of clay, but the second horizon (B horizon: 30-60-cm depth) reduces the permeability of these soils, due to its high percentage of compacted clay. The quantity of organic matter is low and decreases with depth. Thus, Vtc soils are characterized by low