Effect of NPK application on rooibos (Aspalathus linearis) under Clanwilliam field conditions

Lourenco, Marcello Rufino (2018-03)

Thesis (MScAgric)--Stellenbosch University, 2018.

Thesis

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).

AFRIKAANSE OPSOMMING: Huidiglik is daar geen aanbevelings vir makrovoedingstof-bemesting vir rooibosplante onder veldtoestande vasgestel nie. Die doel van hierdie studie was om die interaktiewe effek van NPK (stikstof, fosfor en kalium) op jong rooibosplante se groei en oorlewing te bestudeer, asook grondchemie en bemestingloging onder Clanwilliam veldtoestande met die doel om grond- en blaarvoedingstof standaarde vas te stel vir optimale rooibos produksie. ‘n Veldproef is in Junie 2016 by Vaalkrans Plaas, Clanwilliam gestig. Rooibos-saailinge was tydens planting bemes as ‘n ewekansige ontwerp in kombinasies van verskeie vlakke van N (0, 20, 40, 60 mg/kg) as NBPT-bedekte urea, P (0, 15, 30, 45, 60 mg/kg) as trippel superfosfaat en K (0, 20, 40, 60 mg/kg) as kaliumchloried. Die N- en K-toedienings was opgedeel, 50% by die plant-proses en die oorblywende daarvan is na 2 maande oppervlakkig toegedien. Die bemestingstowwe en toedieningshoeveelhede is gekies gebaseer op vorige kweekhuis proewe op saailinge. Kriteria wat gedurende die proef gemeet was sluit die volgende in: grond pH, elektriese konduktiwiteit (EK), totale koolstof (C) en N, minerale N, Bray II P, uitruilbare katione, mikrovoedingstowwe, grondensiem-aktiwiteit, plant-hoogte, oorlewing, biomassa en blaarvoedingstof-inhoud. Die beweging van die toegediende bemestingstof was ook by geselekteerde behandelings vasgestel en ‘n potproef om die uitwerking van kalk-toediening op rooibos-saailinge se groei vas te stel is uitgevoer. Aanvanklik, gedurende die nat wintersmaande, het die toediening van P teen 15 en 30 mg/kg biomassa produksie gestimuleer. Na die droë somer was daar egter waargeneem dat al die P-toedienings plantegroei onderdruk het en plant-oorlewing verminder het, en hierdie uitwerking was duideliker soos wat die P-toedieningshoeveelheid toegeneem het. Blaar P en loot-biomassa het ‘n negatiewe verband met mekaar gehou (R2=0.5929). In teenstelling met vorige studies, is geen interaktiewe uitwerking is gevind tussen N en P op biomassa-reaksie nie en N-toediening kon nie rooibos help om P-toksisiteit te oorwin nie. Die hoogste bogrondse biomassa-opbrengs was teen K-toedieningshoeveelhede van 20-40 mg/kg waargeneem. Toe die opbrengs aangepas was volgens die sterftesyfer het die 20 mg/kg K-toediening die hoogste opbrengs gehad (597 kg/ha), byna dubbeld wat die onbemeste kontrole was. As gevolg van lae-intensiteit reënval wat in Clanwilliam ervaar is tydens die veldproef het die bemestingstof nie aansienlik in die grondprofiel geloog nie en die meerderheid daarvan het agtergebly waar dit oorspronklik geplaas is tydens die plant-proses (20 – 30 cm) en op die oppervlak (0 – 20 cm). Rooibos-saailing biomassa het positief gereageer op kalk-toediening teen alle hoeveelhede tot en met ‘n ekwivalent van 1.29 t/ha in ‘n kweekhuis potproef. Toedieningshoeveelhede van 1 – 1.3 t/ha het die massa van rooibos-saailinge naastenby verdubbel na twee maande. Die ideale pH vir rooibos-saailinge se groei in hierdie studie was bevind om by pH (KCl) 7.4 te wees. Hierdie studie beklemtoon die belangrikheid van veldproewe, in teenstelling met korttermyn kweekhuis-proewe, aangesien die uitwerking van voedingstowwe in samewerking met die klimaat nadelige nagevolge kan hê. Dit word aangeraai dat jong rooibosplante geen P-bemesting ontvang tydens planting nie, maar wel dat 20 mg/kg N en tussen 20 – 60 mg/kg K toegedien word as gesplete toedienings.

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