Organic and mineral fertilizer field trial on rooibos tea under Northern Cape climatic conditions
Thesis (MScAgric)--Stellenbosch University, 2019.
ENGLISH ABSTRACT: Prior to this study there was little or no information regarding fertilizer recommendations for cultivated rooibos tea under Northern Cape growing conditions. The first aim of the study was to examine the effect of application rate of various commercial brands of compound NPK organic and mineral fertilizers on one-year old rooibos plants under Nieuwoudtville field conditions in the Northern Cape. The second aim was to study the availability of NPK in the soil during the winter months, when rooibos plants usually accumulate nutrients for the dry summer growth period. The third aim was to examine the effect of vermicompost tea (VCT) solutions and water on rooibos nutrient uptake and tea yields and rhizosphere microbial diversity. The final aim was to assess the economic feasibility of organic and mineral fertilizer application on the farms used in the field trials. Field trials were established, during a prolonged drought, at Rogland and Blomfontein Farms in the Nieuwoudtville region in June 2017. One-year old rooibos plants were fertilised as a completely randomised design at three different NPK ratios (2:3:2; 3:1:5; 3:2:4) and application rates based on a phosphorus (P) application of 3.3, 10, 30 and 60 mg P/kg, as rooibos is known to be sensitive to P toxicity. The fertilizers and application rates were selected based on previous rooibos greenhouse and field trials. A field trial was conducted at Rogland to determine the effect of the application of VCT solutions and water on one-year old rooibos plant growth and rhizosphere microbial diversity. Parameters measured include climate, soil water content and temperature, soil pH, electrical conductivity (EC), exchangeable cations, Bray II P, micronutrients, total carbon and nitrogen, foliar macro and micronutrient concentrations, plant survival and tea yields. Economic costs relating to rooibos production, such as labour; pesticides, fertilizers, transport and harvesting were used to determine the gross margin and economic feasibility of fertilizer application on rooibos production. In the wet winter months mineral fertilizers were more effective for plant nutrient uptake as organic fertilizers appeared not to mineralise effectively. The lower N and K fertilizer application rates proved to be most plant efficient, as higher applications were likely lost through leaching or weed interception, while P fertilizer application had a low efficiency level. Foliar NPK concentrations peaked in July and decreased in August due to plant growth stimulated by warm and dry drought conditions. The application rate of 60 mg P/kg and above 87 (N) and 110 (K) mg/kg of organic and mineral fertilizers significantly affected soil fertility by significantly increasing soil pH, ECEC and K, while the chicken-manure based fertilizer enhanced soil and plant uptake of micronutrients. The application of P at a rate of 3.3 – 10 mg/kg stimulated above-ground plant growth in the organic and mineral fertilizer treatments with higher P applications having a negative effect on rooibos production. No significant interactive effect of N and K on biomass production was found. The application of P increased soil Bray II P, however failed to reach the intended target levels likely due to P fixation occurring in the acid soils. The increase in soil Bray II P failed to significantly increase foliar P likely due to drought conditions and weed competition, which could have led to lower availability for rooibos plant uptake. The application rate of 3.3 and 30 mg P/kg in the organic and 30 and 60 mg P/kg in the mineral fertilizers significantly increased plant survival indicating that one-year old rooibos plants are more resistant to P fertilization than shown in previous field studies on rooibos seedlings. The application of 30 N; 10 P; and 50 K mg/kg in the 3:1:5 mineral fertilizer produced the highest survival-adjusted yield by increasing yield by 64.5 % compared to the control treatment. However, the highest gross margin was achieved in the first and second treatment level of the mineral 3:2:4 and organic 2:3:2 fertilizers which increased the gross margin by 40.2 and 20.8 %, respectively. The application of VCT solutions had no significant effect on rooibos survival or tea yields compared to the same volume of tap water. Thus it was shown that the application of tap water resulted in significant increases in plant survival (23 – 31 % higher), resulting in lower tea yields due to higher plant density (38 – 66 % lower) compared to the control. The application of water and VCT solutions increased soil pH, which appeared to negatively affect Mn foliar levels. The application of VCT solutions significantly enhanced foliar P and reduced foliar Al, likely due to organic acids it contained. Bacterial species richness and diversity was significantly enhanced by the application of VCT in the winter months, whereas fungal communities were unaffected. The study suggest that under drought conditions the application of mineral fertilizers of up to 30 N and 50 K mg/kg be applied with 10 mg P/kg to one-year old rooibos plants to increase tea yields and gross margin. This information can be applied to improve management of soil fertility in rooibos tea production and to increase the production yield in an economically sustainable manner without compromising soil quality and the surrounding environment.
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