Browsing by Author "Van Schalkwyk, Roeline"
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- ItemSoil water balance and root development in Rooibos (Aspalathus linearis) plantations under Clanwilliam field conditions(Stellenbosch : Stellenbosch University, 2018-03) Van Schalkwyk, Roeline; Hoffman, J. E.; Hardie-Pieters, Ailsa G.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.ENGLISH ABSTRACT: Rooibos (Aspalathus linearis) can only grow in certain parts of the Western– and Northern Capes, whereas the production declines every year. If the global demand trend continues to increase, the production will be unable to meet the world demand. Currently, research of Rooibos is focused mainly on its health benefits and not on the agricultural production aspects. The aim of this study was to investigate the effect of fertilisation and soil depth on the soil water balance (SWB), root development and biomass water use efficiency (WUEB) of Rooibos. The experimental trial was conducted during 2016 and 2017 at Vaalkrans farm, Nardouwsberg, Clanwillliam in the Western Cape. The following three treatments were evaluated on shallow (≤ 30 cm) and deep (≥ 80 cm) soils: (1) unfertilised planted soil, (2) planted soil receiving moderate NPK fertiliser treatment (20 mg.kg-1 N, 30 mg.kg-1 P and 20 mg.kg-1 K) and (3) bare, unplanted soil. The soil water content (SWC) was monitored at weekly intervals during the growing season (July 2016 until April 2017) and during the fallow periods (bare treatment) using a Diviner 2000 soil moisture meter. The Diviner 2000 was used to record in 10 cm increments up to 30 and 80 cm soil depths. At the end of the 2016/17 growing season, the SWB, the total biomass and biomass WUE was determined. Volumetric water content and soil temperature at the deep site was monitored every 10 minutes using ECH2O sensors. Root growth, N-fixing nodules count, taproot length and root system characteristics were measured on the plants at various growth stages. The cumulative evapotranspiration (ET) of the unfertilised treatment was 110.4 and 121.2 mm, and the fertilised treatment was 108.4 and 115.8 mm on shallow and deep soils, respectively. The cumulative evaporation (E) of the bare treatment was 108.6 and 116.1 mm on shallow and deep soils, respectively. The ET and E was lower at the shallow soil sites due to less soil water storage (less water availability) compared to the deep soils. During the winter season, the SWC in the 10-20 cm soil layer of unfertilised and fertilised treatments was higher than the other soil layers. This is likely due to higher soil organic carbon of 0.18–0.19%, low bulk density (1.45–1.54 g.cm-3) and high root concentration in the 10-20 cm layer compared to the 20–40 cm soil layer. Fallow efficiency on deeper soils was higher than the shallow soils due to higher SWC. Soil temperature fluctuations were significant in the 0-10 cm soil layer of all treatments, but less so at the deeper soil layers. This was due to poor energy transfer in the dry sandy soil. The diffusivity coefficient in the 10-20 cm soil layer was exceptionally low due to the drought conditions and varied between ca. 0.072-0.090 mm2.day-1 over duration of the 2016/17 season. The deeper soils had higher shoot biomass compared to the shallow soils. The lower root biomass and thinner taproot were caused by the P concentration. Cluster roots of Rooibos was found in the 10–20 cm soil layer which were where nutrient acquisition mainly occurred. The growth of the cluster roots in the 10–20 cm soil layer was due to low bulk density, low soil temperature and high SWC. Plants of the unfertilised treatment at the shallow site did has a high WUEB, but the water usage was higher than at the deep site. Overall, the WUEB was found to be inconclusive due to the stoppage of the SWB on April 2017 whilst the plants were still immature. The study indicates that young Rooibos plants growing in deeper soils with higher soil water storage will result in higher yields.