Effects of soil & foliar nutrient application strategies for improving fruit quality for 'Zesy002' kiwifruit : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Sciences at Massey University, Palmerston North, New Zealand

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Massey University
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Nutrient management research work has been conducted in established kiwifruit varieties such as ‘Hayward’ and ‘Hort16A’. But less is known about such research on the newly commercialised ‘Zesy002’ kiwifruit cultivar. A goal is to optimise sustainable production of quality fruits with high fruit dry matter percentage (FDM, a quality predictor at-harvest), without driving excessive vine vigour. The project is aimed to (i) increase calcium (Ca) nutrition and to (ii) optimise potassium (K) fertilisation to produce high quality ‘Zesy002’ kiwifruit at-harvest. The fundamental knowledge of the role of Ca and K nutrients and their interactions was used to increase the photosynthetic performance and improve at-harvest fruit quality. The study was based on the hypothesis that the competitive effect of high K supply on the uptake of Ca may be reduced at soil level by using (i) spatial and (ii) temporal separation of soil-applied Ca and K fertilisers, and (iii) foliar Ca applications after fruit set through to fruit maturity during the growing season. The study also tested the hypothesis that K application at different rates and growth stages through (i) soil-applied and (ii) foliar-applied fertilisation may better meet the nutrient demand of kiwifruit vines and developing fruit and therefore, may improve the potential of the fruit to compete for dry matter import. The results showed that both spatial and temporal separation of soil-applied Ca and K fertilisers reduced the competitive effect of high K input on the availability and uptake of Ca and therefore significantly increased leaf and fruit Ca concentration in ‘Zesy002’ vines compared to true control, not spatial and not temporal strategies and grower practice in each orchard. Likewise, foliar Ca significantly increased Ca concentration in leaf and fruit tissues in ‘Zesy002’ vines and significantly improved photosynthetic performance, nutrient uptake (vine physiology) and at-harvest fruit quality (fruit size and firmness). Both soil and foliar-applied nutrient strategies, which were used mainly to increase Ca nutrition, showed promising improvements in fruit quality; larger and firmer fruit with high soluble solids (SSC) and fruit dry matter concentration (FDM) at-harvest and after thirty-day storage. The results also showed that three split applications of soil K fertilisers (each 100 Kg K ha⁻¹) from bud-break (BB) through to 90 days after full bloom (DAFB) compared to one application either at BB, FB or 90 DAFB, improved at-harvest fruit quality in commercial orchards at Bay of Plenty and Hawke’s Bay, New Zealand. Foliar application of K fertilisers from fruit set through to harvest also improved vine physiology and at-harvest fruit quality. The optimisation of K by using both soil and foliar fertilisation strategies improved fruit growth rate during the growing season, at-harvest fruit weight, SSC and FDM compared to true control. The soil and foliar-applied nutrient (Ca and K) fertilisation strategies employed here were specifically targeted using ‘the right time’, ‘right amount’ and ‘right plant organ’ principles and successfully, delivered larger fruit with higher SSC and FDM at-harvest. For growers, this translates to a lower fruit count per tray (size class) and a higher taste Zespri grade, triggering increased premium payments and orchard gate returns. The fertiliser input used in foliar strategies was 3-8 Kg. ha⁻¹ and only a fraction of soil-applied fertilisation 50-300 Kg. ha⁻¹. Therefore, the foliar nutrient fertiliser application strategies employed in this study were inherently more sustainable approach compared to the soil-applied fertilisation. These research findings have significant implications in horticultural fruit crops to increase leaf chlorophyll, net photosynthesis and stomatal conductance, and increase leaf and fruit Ca and K concentration to deliver high quality fruit at-harvest. The research strategies used in this study can be very easily adapted to the existing fertilisation programs and applied in the commercial orchards. For example, fertiliser spreaders can be modified to side dress K to the weed-strip or broadcast K to alternate rows and broadcast Ca over the whole orchard floor to implement spatial separation of Ca and K fertilisers. Temporal separation of Ca and K can be very easily implemented by applying soil Ca fertiliser at BB and by delaying application of soil K fertiliser until closer to FB. Foliar Ca and K applications can also be easily included in the early season spray programs. For future research, there is a true potential to further maximise fruit quality gains by supplementing soil-applied fertilisation strategies with specific timely foliar applications.
Kiwifruit, Nutrition, Fertilizers, Quality, New Zealand