Soil water balance and root development in Rooibos (Aspalathus linearis) plantations under Clanwilliam field conditions

Van Schalkwyk, Roeline (2018-03)

Thesis (MScAgric)--Stellenbosch University, 2018.

Thesis

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.

AFRIKAANSE OPSOMMING: Die globale aanvraag vir Rooibos (Aspalathus linearis) het in die afgelope paar jare verhoog danksy die gesondheidsvoordele van die tee, maar die produksie van Rooibos verminder elke jaar. Rooibos groei net in sekere areas in Wes en Noord-Kaap en indien die aanvraag verhoog, sal die produksie nie by die aanvraag volhou nie. Daar word meer gefokus op die aspek van Rooibos se gesondheid en min aandag word aan landbouproduksie aspekte gegee. Die doelwitte van hierdie studie handel oor hoe die kunsmis en gronddiepte die grondwaterbalans, biomassa waterverbruikdoeltreffenheid en wortelontwikkeling van Rooibos beïnvloed. Die proef is by Vaalkrans plaas, Nardouwsberg, Clanwilliam in Wes-Kaap gedoen in die tydperk vanaf 2016 tot 2017. Drie verskillende behandelings op vlak (≤ 30 cm) en diep (≥ 80 cm) gronde geëvalueer: (1) onbemeste grond met plante, (2) bemeste grond (20 mg.kg-1 N, 30 mg.kg-1 P en 20 mg.kg-1 K) met plante en (3) braak sonder met plante. Die grondwaterinhoud van al drie behandelings is weekliks met behulp van ‘n kapasitansie apparaat (Diviner 2000) bepaal gedurende die 2016/17 groeiseisoen (Julie 2016 tot April 2017). Die bepalings is by die vlak gronde tot by 30 cm gronddiepte in 10 cm inkremente geneem en by die diep gronde tot by 80 cm gronddiepte. Aan die einde van die 2016/17 groeiseisoen is die grondwaterbalans, die totale biomassa en biomassa waterverbruikdoeltreffenheid bepaal. Die EHC2O watermeters het die volumetriese waterinhoud en die grondtemperatuur van die diep gronde gemeet. Na elke oes, is die wortelgroei, N-fikserende nodules telling, penwortel se lengte en wortelsisteem eienskappe bestudeer. Die kumulatiewe evapotranspirasie van die onbemeste behandeling was 110.4 en 121.2 mm en vir die bemeste behandeling was dit 108.4 en 115.8 mm van die vlak en diep gronde, respektiewelik. Die braakbehandeling se kumulatiewe verdamping was 108.6 en 116.1 mm van die vlak en diep gronde, respektiewelik. Die lae kumulatiewe evapotranspirasie en verdamping van die vlak gronde was as gevolg van lae grondwaterstoring (dus minder waterbeskikbaarheid). Die grondwaterinhoud in die 10–20 cm was hoër as die ander grondlae van die onbemes- en bemeste behandelings. Dit is as gevolg van hoër grondorganiese koolstof (0.18–0.19%), lae bulkdigtheid (1.45–1.54 g.cm-3) en hoër wortelkonsentrasie in die 10–20 cm grondlaag. Die diep gronde se braakeffektiwiteit was hoër as van die vlak gronde as gevolg van die hoër grondwaterinhoud. Vir al die behandelings was die grondtemperatuur fluktuasies in die 0-10 cm grondlaag baie prominent, maar laer in die dieper grondlae. Dit was as gevolg van die droër grond se vertraagde energielading. Die diffusiwiteitkoeffisiënt in die 10-20 cm grondlaag was besonders laag as gevolg van. die droë toestande en het varieër tussen ca. 0.072-0.090 mm2.dag-1 gedurende die 2016/17 seisoen. Die onbemeste gronde se loot biomassa was hoër as die bemeste gronde. Die P konsentrasie van die bemeste plante in die grond het veroorsaak dat die wortelmassa laag was en ook dunner penwortels. Die troswortels van die Rooibos groei hoofsaaklik in die 10-20cm grondlaag en dit is ook waar die voedingstofverkryging meestal plaasvind. Die troswortels groei in die 10–20 cm grondlaag omdat die bulkdigtheid is laer, die grondtemperatuur is laer en ook hoër grondwaterinhoud in daardie grondlaag. Die onbemeste plante van die vlak gronde het die hoogste biomassa waterverbruikdoeltreffenheid gehad, maar die waterverbruik was hoog. Oor die algemeen, is die gevolgtrekking van die biomassa waterverbruikdoeltreffenheid nie geldig nie omdat die grondwaterbalans het tot by April 2017 gestop terwyl die plante nog jonk was. Resultate uit die studie dui aan dat dieper gronde met hoër grondwaterinhoud, ’n toename in produksie sal veroorsaak.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103440
This item appears in the following collections: