Browsing by Author "Lourenco, Marcello Rufino"
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- ItemEffect of NPK application on rooibos (Aspalathus linearis) under Clanwilliam field conditions(Stellenbosch : Stellenbosch University, 2018-03) Lourenco, Marcello Rufino; Hardie-Pieters, Ailsa G.; Valentine, A. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.ENGLISH ABSTRACT: Currently no macronutrient fertiliser recommendations have been established for rooibos plants under field conditions. The aim of this study was to examine the interactive effect of NPK (nitrogen, phosphorus, potassium) on young rooibos plants’ growth and survival, and soil chemistry and fertiliser leaching under Clanwilliam field conditions with an aim to establishing soil and foliar nutrient norms for optimum rooibos production. A field trial was established at Vaalkrans Farm, Clanwilliam district in June 2016. Rooibos seedlings were fertilised at planting as a completely randomised design in combinations of various levels of N (0, 20, 40, 60 mg/kg) as NBPT-coated urea, P (0, 15, 30, 45, 60 mg/kg) as triple superphosphate and K (0, 20, 40, 60, 80 mg/kg) as potassium chloride (KCl). The N and K applications were split, 50% at planting and the remainder top-dressed after 2 months. The fertilisers and application rates were selected based on previous seedling greenhouse trials. Parameters measured during the trial included: soil pH, electrical conductivity (EC), total carbon (C) and N, mineral N, Bray II P, exchangeable cations, micronutrients, soil enzyme activity, plant height, survival, biomass, and foliar nutrient content. The movement of the applied fertilizer was also determined on selected treatments, and a pot trial to determine the effect of lime application of rooibos seedling growth was performed. Initially, during the wet winter months, the application of P at 15 and 30 mg/kg stimulated biomass production. However, after the dry summer it was observed that all P applications suppressed plant growth and decreased plant survival, and this effect was more pronounced as P application rate increased. Foliar P and shoot biomass were negatively correlated (R2=0.5929). No interactive effect between N and P on biomass response was found, and N application could not help rooibos to overcome P-toxicity, contrary to previous studies. The highest above-ground biomass yields were recorded at K application rates of 20 – 40 mg/kg. When yield was adjusted according to mortality, the 20 mg/kg K treatment had the largest yield (597 kg/ha), nearly double that of the unfertilised control. Due to the low intensity rainfall experienced in Clanwilliam during the field trial, the fertiliser had not leached significantly in the soil profile, and the majority remained where it was initially placed at planting (20 – 30 cm) and on surface (0 – 20 cm). Rooibos seedling biomass responded positively to lime application at all rates up to an equivalent of 1.29 t/ha in a greenhouse pot trial. Application rates of 1 – 1.3 t/ha nearly doubled the mass of rooibos seedlings after two months. The ideal pH for rooibos seedling growth in this study was found to be around pH (KCl) 7.4. This study highlights the importance of field trials, as opposed to short-term greenhouse trials, as the effect of nutrients combined with climate can have deleterious effects. It is recommended that young rooibos plants do not receive any P fertilisers at planting, but receive up to 20 mg/kg of N and between 20 – 60 mg/kg of K (applied as split application).