Masters Degrees (Horticulture)

Permanent URI for this collection

https://scholar.sun.ac.za/handle/10019.1/558

Browse

Browsing Masters Degrees (Horticulture) by Author "Broeksma, Cornelis Reitz"

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    In-season mineral nutrient management of citrus by early (spring/summer) season sampling of leaves, fruitlets, and flowers
    (Stellenbosch : Stellenbosch University, 2022-04) Broeksma, Cornelis Reitz; Raath, Pieter; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: Tree nutrient supply must be optimal to ensure the production of high yields of quality fruit. Fertiliser schedules are adjusted based on the results of various diagnostic tools – one such tool is analysis of plant parts. In the South African citrus industry, leaves from fruit bearing shoots are sampled and analysed during autumn, at which time the season’s fertiliser schedule has often been implemented either entirely or with only a small portion outstanding. The results of autumn analyses are used to adjust the fertiliser schedule of the following season. This is a retrospective method of managing tree nutrient status and there is a need for a more proactive method. The latter could potentially be achieved by sampling earlier (spring/summer) in the season, thereby allowing for adjustment of current season fertiliser schedules. In this study, the relationship between early season nutrient concentration in leaves, fruitlets, and flowers and final fruit nutrient concentration at harvest, was evaluated. Five treatments were applied, viz. the normal fertiliser schedule (control), 200% N of the control, 200% P of the control, 200% K of the control, and 200% Mg of the control. The two trial sites used were in separate geographic regions of South Africa; Nelspruit of the Mpumalanga province which has a sub-tropical climate and summer rainfall, in a ‘Midknight’ Valencia orchard and De Wet of the Western Cape province which has a mediterranean-type climate and winter rainfall, in an ‘Orri’ mandarin orchard. Leaf, fruitlet, and flower nutrient analysis results did not indicate significant reactions to the nutrient treatments. However, the N- and K-treatments did induce a response. The N-treatment produced an immediate response in leaf and fruitlet N concentration, which was maintained during the second season. A response to the K-treatment was only observed in leaf K content during the second season. ‘Orri’ mandarins were less responsive to the trial treatments compared to the ‘Midknight’ Valencias, with only the K-treatment producing a response in leaf nutrient content and only during the second season. Correlations between nutrient analysis of different plant parts and final fruit nutrient concentration was not strong. The effect of excessive application rates of nutrients on final fruit quality, and the relationship between early season analysis of leaves, fruitlets, and flowers and that of final fruit quality were also evaluated. The treatments produced significant differences within quality parameters (rind colour, TSS, TA, fruit size) for the control and the N- and K-treatments of both the ‘Midknight’ trial and the ‘Orri’ trial. Most notably, in the ‘Orri’ trial where N was liberally applied and K was moderately applied, the K-treatment response on final fruit quality was strongly expressed during the second season. Nutrient content of leaves from fruit bearing and non-fruit bearing shoots were compared. In both cultivars, leaves from non-fruit bearing shoots had significantly higher concentrations of N, P and K, whereas the Ca levels were higher in the leaves from fruit bearing shoots. Additionally, leaves from non-fruit bearing shoots were more responsive to changes in nutrient supply. This study suggests that the accepted sufficiency ranges of leaf nutrient concentration might be too wide, and that the expected treatment responses might only occur at lower nutrient application rates. It is expected that correlations with final fruit nutrient concentration will be stronger at lower leaf, fruitlet, and flower nutrient concentrations, due to the law of diminishing returns which states that at a certain point additional changes to a system will yield smaller and smaller improvements resulting. In addition, it was found that, a significant change in nutrient concentration within leaves and fruit does not necessarily have to precede a significant fruit quality response, suggesting that current leaf nutrient norms might be too wide to predict nutrient related fruit quality responses under all circumstances.