Doctoral Degrees (Soil Science)

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Browsing Doctoral Degrees (Soil Science) by Subject "Apples -- Irrigation"

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    Determination and modelling of evapotranspiration of bearing and non-bearing apple trees at Grabouw in the Western Cape
    (Stellenbosch : Stellenbosch University, 2024-03) Meyer, Aline; Van Zyl, J. L.; Hoffman, J. E.; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: The irrigation of apple trees is important to ensure sustainable production and good quality fruit, especially in regions where rainfall does not contribute adequately to the water demand. Effective scheduling is the key to efficient water use to ensure profitability and sustainability on farms. Quantitative knowledge of the water use and the effect of irrigation application on young apple trees will improve on-farm decision making regarding scheduling. A study was conducted on Malus domestica “Bigbucks‟ (a mutation of “Corder Gala‟) trees grown in a gravelly soil at Grabouw in the Western Cape to determine the effect of three irrigation cycles on the water use, root growth characteristic and tree performance over four growing seasons (October to May) from planting to the first year of bearing. Treatment one (T1) was a short irrigation cycle receiving ca. 15 mm of water per irrigation with an average of 42 irrigations through the growing season, treatment two (T2) was a medium irrigation cycle receiving ca. 27 mm of water with an average of 21 irrigations through the growing season and treatment three (T3) was a long irrigation cycle receiving ca. 37 mm of water with an average of 13 irrigations through the growing season. Crop evapotranspiration (ETC) was determined for all three treatments based on the soil water balance. The ETC of all three treatments increased from the first to the fourth growing season as the leaf area index (LAI) of apple trees increased. T1 had a higher consumptive water use than T2 and T3. Studies done using micro-lysimeters to determine the orchard floor evaporation revealed that T1 lost more water through evaporation compared to T2 and T3, but water loss from the soil mainly occurred through transpiration, irrespective of the treatment. In situ and destructive root studies revealed that both root length density (RLD) and the number of fine roots within the soil profile is strongly related to soil water extraction (SWE). SWE increased with an increase in RLD and the number of fine roots. These results revealed that growing roots can continuously grow into moist regions of the soil. Significantly more fine roots penetrated deeper soil layers and at a greater distance from the tree for the two drier irrigation cycles (T2 and T3). The root index (RI) of T2 and T3 was also higher in deeper soil layers compared to T1 suggesting that soil moisture conditions of T2 and T3 were more favourable in deeper soil layers. It was concluded that short irrigation cycles will favour shallow root growth while longer irrigation cycles promote roots into deeper soil layers. There were no significant differences among treatments in terms of diurnal plant water status, vegetative growth, yield and fruit quality. These results suggest that longer irrigation cycles can be used to save water while simultaneously increasing root growth to deeper soil layers without compromising tree performance. Statistical analysis performed on ETC and RLD data revealed that there is a strong, positive correlation (R² = 0.741) between ETC and RLD. The data was used to develop a statistically significant model in which ETC can accurately be predicted using RLD data or vice versa. The model can be used as a reference for apple producers in South Africa to encourage more precise and effective irrigation scheduling while simultaneously increasing RLD for better water and nutrient uptake resulting in optimal crop production and quality.