The ecological sustainability of potato production in the sandveld region of the Western Cape: nutrient and water use efficiencies

Thesis (MScAgric)--Stellenbosch University, 2019.

ENGLISH ABSTRACT: Uncertainty regarding the rate at which water and nutrients move and are distributed throughout the soil profile is key in managing potato production systems in the Sandveld region of the Western Cape. The sandy soils with low nutrient and water holding capacities complicate irrigation water management and fertiliser practices. Information on efficient water management practices is scarce due to the difficulties of measuring water losses to the environment. Thus, the aim of this study was to quantify inputs and losses in potato production systems in the Sandveld region to close the gap in knowledge with regards to water and nutrient leaching under current management practices. The study was conducted on nine potato fields (processing cultivar FL2108 and table cultivar Sifra) between March 2018 and March 2019 under centre-pivot irrigation systems. Water inputs were monitored with flow meters and pressure transducers. Nutrient and water losses (drainage and leaching) was assessed using drainage lysimeters and soil water movement throughout the profile was monitored with the use of capacitance probes. Tuber yield was determined when the crop was mature, and soil-water balance components as well as water and nutrient-use efficiencies were calculated. The regular evaluation of irrigation systems is recommended to prevent over or under application of water to combat inefficiencies and meet the evapotranspiration demands of the crop. The simulation of evapotranspiration through adjusted basal crop coefficient curves to meet the demands of the specific areas was indicated to be a good measure of crop water use. Evapotranspiration values obtained ranged from 188 to 647 mm. Irrigation is generally not adjusted to crop physiological needs, resulting in over application of water, particularly during winter due to the effect that rainfall has on the increased potential of drainage. The rainfall recorded ranged from 54 to 271 mm. Substantial drainage occurred in summer planted crops as a result of irrigation water exceeding crop requirements. However, as a result of the rapid depletion of water in the soil profiles due to low water holding capacities, farmers cannot leave substantial room in the profile for rainfall. Weather station data and soil capacitance probes provided good information regarding the potential occurrence of drainage events and are recommended as management tools. Large nutrient losses were associated with substantial drainage, occurring on average at 70 kg N ha-1, 52 kg P ha-1 and 138 kg K ha-1. Drainage collected ranged from 4 to 302 mm per season. Water use efficiency observed was average (65.4 to 122.2 kg mm-1), which is accredited to low yields and high drainage losses in winter. Yields ranged from 34.7 to 118.2 t ha-1. Relatively low yields in winter and autumn resulted from cool temperatures and less available solar radiation in these periods. Yields during winter where below 60 t ha-1, compared to summer crops, which yielded 59.0 and 118.2 t ha-1. Keywords: water-use efficiency, nutrient use efficiency, nutrient leaching, drainage lysimeter, soil water balance, evapotranspiration.