Evaluation of recirculating hydroponic systems for eucalyptus cutting production.

Abstract

In South Africa, Eucalyptus grandis and hybrids of this genus are planted over 196000 ha of Mondi Forests land. The temperate and cold tolerant species (E. grandis, and E. grandis X E. nitens) occupy 142000 ha and the subtropical species (E. grandis, E. grandis X E. camaldulensis and E. grandis X E. urophylla) occupy a further 54000 ha. In Southern Africa, until 1998, there had not been any significant research undertaken into the development and application of hydroponics to cutting production in nurseries. Since then, research in Mondi Forests has focused on the evaluation of recirculating hydroponic systems for cutting production of the genus Eucalyptus. South American forestry companies have achieved noteworthy success in improving the rooting of cuttings of selected clonal Eucalyptus harvested from hydroponic gardens. Verifiable increases of up to 300% in rooting have been achieved. (JANSE, 2001 pers. comm.1). Towards this aim, protocols for the production of Eucalyptus cuttings in hydroponic systems were established, and implemented in a commercial hydroponic production system. Seven nutrient solutions with different concentrations of macro- and micro-nutrients were tested on three different substrates, viz perlite, sand and 6.2 mm dolomitic gravel. A pure nutrient film system (NFT) was also tested. Nutrients (1.2 -1.5 mS/cm) applied cyclically over a seven day period, followed by two days of leaching with clean water, were found to be suitable for cutting production. Excessive nutrient supply resulted in soft foliage, susceptibility to powdery mildew (Oidium eucalypti) and Botrytis infection. Foliar analyses showed that macro-element levels in leaves were similar to those of soil-derived plants, whilst micro-element levels were generally higher in the 'hydro-ramets.' Most significant were the higher concentrations of boron and calcium. Sand and perlite had the best water holding capacities, requiring hydration every 96 hours, whilst gravel sustained plants for up to 48 hours without any visible stress. The modified NFT unit, however, was the most efficient system in terms of managing cutting production. Some post-establishment mortalities occurred across all treatments and were attributed to pathological attack by Pylhium sp. Light source and intensity were critical. Initial low light levels (615 Lux) did not result in optimum growth and plants showed signs of being stressed, thus contributing to an increase in powdery mildew infection. Plant growth improved with increasing light levels (> 3000 Lux). Following this research, operational systems were developed. From first harvests, rooting of cuttings from the hydroponically-grown parent material were higher than those from field hedges. Mean rooting from the best performing nutrient and substrate combination was 67%, compared to 45% obtained from the field hedge. In addition, the hydroponic system was found to offer other advantages: 1) hydroponic hedges are less costly to maintain than field hedges; 2) hydroponic systems offer greater control of nutrients than can be achieved in field hedges; 3) hydroponically-grown hedge plants can be better shielded from environmental variables which impact on the productivity of field hedge plants; 4) the threat on the environment in terms of water use and effluent disposal is alleviated to a large degree.