Sustainable production of baby leaf lettuce and spinach through the use of compost and its extracts

Resumo

Resumen de la tesis: The increase in organic waste and by-products from the agro-food industry generates environmental and economic problems. An alternative for its use is composting, which is a natural biological process of decomposing organic waste material. Compost is the product of the controlled biological decomposition of the above materials. Compost and its biological extracts can be used, among other uses, as a source of nutrients for plants, improving crop productivity. In addition, compost can contribute to improve the quality of crops and favour the biological control of pathogens, due to the suppression capacity that some of them have. All the above contributes to the reduction of the use of chemical fertilizers and pesticides, making agriculture more sustainable and promoting the circular economy. At present, minimally processed vegetables are a group of foods that generate a great interest by the consumers, leading to a great demand in the market. Within this group, baby leaf lettuce and spinach are important vegetable products in the industry. Their production has been traditionally carried out in soil, but in recent years they are being grown in soilless cultivation systems, mainly hydroponics. Hydroponic cultivation allows a better control of the plant growth during its entire cycle, leading to greater automation of crops. In addition, growth uniformity, shortening of cycles, efficient use of fertilizers, and higher yield and quality of the final product are achieved. Among the hydroponic cultivation systems, the floating system allows correcting and controlling important parameters in plant, such as nutritional compounds (vitamins, phenols and antioxidants) and anti-nutritional compounds (nitrates and oxalates). The trays in this system are normally filled with peat, which is a material that increases the plant susceptibility to some diseases caused by fungi or oomycetes such as Pythium spp. causing production losses. Furthermore, the demand for peat has been growing in recent years, increasing its extraction excessively and causing a negative effect on the environment. To counteract this effect, alternative substrates to peat are being studied, such as compost, which can favour not only the productivity of crops but also their quality in an environmentally friendly way. The main objective of this thesis was to demonstrate that certain types of composts could be used as alternative substrates to peat in floating system, as well as organic amendments in cultivation of baby leaf vegetables in soil. For this purpose, it was studied whether compost and its biological extracts can be considered added value agricultural products, minimizing and/or limiting the incidence of pathogens and improving the accumulation of nutritional compounds in baby leaf lettuce and spinach. In order to achieve this objective, first an agroindustrial compost, composed of tomato (71%), onion (17%) and vineyard residues (12%), was studied during two cycles of red baby leaf lettuce cultivated in floating system. Two simultaneous experiments were carried out: i) the influence of two organic substrates (peat and compost) on the germination, growth and nutritional quality of lettuce was evaluated, ii) a suppression bioassay was carried out using the two above substrates inoculated with Pythium irregulare. The results showed that compost improved the percentage of germinated seeds and increased the nutritional quality of the lettuce compared to peat. Likewise, plants grown with compost inoculated with P. irregulare showed a higher survival rate and a higher crop yield. Secondly, the way of application of a compost extract, added to the nutritive solution directly (CENS) or by microsprinkler (CEMP), was evaluated on the yield and quality of the lettuce and on the incidence of P. irregulare during two growing cycles of baby leaf red lettuce grown in floating system. The CENS treatment showed a positive effect on plant growth under both inoculated and non-inoculated conditions. In addition, the CEMP treatment in non-inoculation conditions considerably reduced the nitrate content and showed a positive effect on total phenols, flavonoids and antioxidant capacity. Finally, the CENS treatment significantly reduced the population of P. irregulare in the water and had no negative effect on the microbial load of the final product. Finally, it was studied the possible beneficial effect of the application of a compost tea produced from onion and vineyard residues, alone (CT) or in combination with the beneficial microorganism Trichoderma harzianum T78 (CT+Th), on the quality and yield of baby leaf spinach, during two cycles of cultivation in soil. The CT and CT+Th treatments showed a higher crop yield than the control but did not produce an increase in soil of dehydrogenase activity. Moreover, CT+Th treatment improved the quality of the crop, showing an increase of total phenols, flavonoids and antioxidant capacity. The results of this investigation show that compost and compost extract obtained from agroindustrial wastes can be a promising alternative to peat in a sustainable soil-less production system for baby leaf red lettuce, as they are not only able to control the effect of the pathogen P. irregulare, but also to improve the yield and quality of the product. In addition, compost tea enriched with T. harzianum is shown to be a sustainable alternative for improving the yield and quality of soil-produced baby leaf spinach. http://repositorio.bib.upct.es/dspace/