Effects of Litsea glutinosa (Lour.) C.B. Rob. plant biowaste-derived media on plant growth and development of thyme and rocket.
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Enhancing and sustaining agricultural productivity is critical, as soil quality in many parts of the world deteriorates becoming unsuitable for agriculture. Plant bio-waste derived from composted alien invasives could be recycled and reused to enrich media used for plant production. This bio-waste could improve soil fertility and thereby enhance agricultural productivity. KwaZulu-Natal (KZN) is threatened by numerous alien invasive plants which negatively impact on the natural environment, human welfare and quality of life. Biological plant invasion is a natural process; however, human intervention has accelerated the rate of spread and naturalisation of many species across a multitude of landscapes. Composting some species of such alien invasives into bio-waste has been reported as a viable source of nutrients and organic matter. Farmers can, therefore, use these outputs as livestock-feed products and/or fertilizer for crops. The purpose of this study was to assess the effects of compost, derived from the IAPs (invasive alien plant species) - Litsea glutinosa – (Lour.) C.B. Rob., as enrichment for plant growth and development of two herb species, Thymus vulgaris (thyme) and Eruca sativa (rocket). This experimental study was conducted using three media into which rocket and thyme were planted: control medium (Gromor® Potting Soil, PS); experimental medium (composted Litsea, EM) and a combination (1:1) of control and enriched medium (PSEM). This study was carried out over three growing periods: eight-week experiments between April and May (autumn to winter), between September and October (winter into spring) and between February and March (summer into autumn). Composting of Litsea glutinosa plants was started at a vacant site in Verulam (KZN) before being moved to the experimental study site, at the Durban University of Technology Horticultural Practical Centre. Five replicates per treatment of the rocket and thyme plants were planted in the three media (PS, EM and PSEM). The following measurements were taken to assess plant growth and development: leaf diameter and plant height (rocket) and length of side shoot and plant height (thyme). Fresh and dry mass (g) were determined and the concentrations of total chlorophylls and carotenoids were measured spectrophotometrically. The growth of the thyme plants was positively influenced by cultivating the plants in EM and PSEM media resulting in increased plant height and length of side shoots, growth parameters significant for the culinary and cosmetic thyme industry. The leaf diameter of rocket was positively influenced when grown in the winter to spring period, particularly when cultivated in the PSEM medium compared with PS. Rocket displayed the most vigorous growth (fresh and dry mass of rocket leaves) during the winter to spring period when grown in PSEM. Results showed that herbs grew similarly in PS and PSEM media. It is, therefore, feasible to use PSEM as a medium for thyme production. Thyme grew best in EM in the autumn season (April-May), while PSEM performed best when used in summer/autumn (February-March). Thyme, therefore, grows well in this composted IAP in the summer and autumn months, rather than in winter or spring. The chlorophyll concentration of rocket plants was also affected by the season (highest concentration in plants grown during summer months) and medium (highest concentration in plants grown in PS) compared with PSEM and EM, as plants grew slowly and showed low values of pigment concentrations. Growing rocket and thyme in the composted Litsea glutinosa did not affect the taste and texture of the leaves determined by the consumer evaluation panel. Litsea glutinosa compost used to enrich potting soil (PSEM) was beneficial to the growth and development of rocket as well as thyme. Therefore, this study recommends the use of composted IAPs mixed in a 1:1 ratio with a general potting soil which would benefit the environment, the ornamental industry, as well as nurseries/wholesalers. A higher dosage of the composted Litsea glutinosa in a PSEM medium should be experimented with to grow thyme plants, while the potting soil is better suited to grow rocket plants. This study, therefore, highlights the usefulness of composted plant bio-waste derived from alien invasive plants as enrichment of media for growing herbs for human consumption.