Water and nutrient use efficiencies of potato-based (Solanum tuberosum L.) rotation systems in North West Province, South Africa

Abstract

Water and nutrient management has a direct impact on yield, input-use efficiencies of crops, and is crucial to the ecological sustainability of production. Moreover, water-use efficiency (WUE) and nutrient-use efficiency (NUE) are common indicators of ecological sustainability in crop production. Measuring WUEs and NUEs of potato-based (Solanum tuberosum L.) rotation systems, rather than those of potato alone, provides a true image of water and nutrient use as well as the potential environmental pollution associated with potato production. Little is known, however, on the WUE and NUE of potato-based rotation systems within South Africa. This study was conducted to quantify WUE, NUE, drainage and nutrient leaching of potato fields, as well as to evaluate nutrient carry-over effects to the subsequent crop in rotation. Six irrigated potato fields on commercial farms in the North West province were selected for monitoring. Five of the six potato fields were monitored during the 2017/18 summer season, and one field during the 2018/19 season. After the potato season, follow-up crops of paprika (Capsicum annuum L.), groundnuts (Arachis hypogaea L.) and onions (Allium cepa L.) were monitored during the 2018/19 season in three fields. Surveys and field measurements were conducted regarding fertilizer type and application rate for both potato and follow-up crops. Flow meters and pressure transducers were used to monitor irrigation amounts. Drainage lysimeters were installed in four of the six fields to monitor drainage and nutrient leaching. Soil and plant samples were taken for nutrient analysis. Final yields of potato and the follow-up crops were determined at crop harvest. Calculated WUE was based on total water inputs from rainfall and irrigation (WUER+I), potential WUE (PWUE) was based on simulated tuber yield and evapotranspiration (ET), and irrigation WUE (IWUE) was based on observed irrigation. Nutrient-use efficiency was expressed as partial factor productivity (PFP), nutrient utilization efficiency (NUtE) and nutrient harvest index (NHI). Results revealed that average nutrient rates of 300 kg ha-1 N, 220 kg ha-1 P, 386 kg ha-1 K, 580 kg ha-1 Ca, 252 kg ha-1 Mg and 99 kg ha-1 S were applied to potato fields. Irrigation water applied to potato fields varied greatly and ranged from 590 – 1011 mm (average 866 mm) per season. Fresh tuber yields ranged from 60 – 93 t ha-1 (average 83 t ha-1). Potato WUER+I, PWUE and IWUE ranged between 53 – 124, 96 – 151 and 59 – 129 kg mm-1, respectively. Drainage of 488 mm was measured in one potato field and leaching of 29 kg ha-1 N, 20 kg ha-1 K, 484 kg ha-1 Ca, 179 kg ha-1 Mg and 129 kg ha-1 S was recorded. Partial factor productivity of N, P and K was quite similar for all potato fields, with average values of 288, 379 and 229 kg fresh tuber kg-1 of applied nutrient, respectively. A great variation in PFP among fields, however, was observed for Ca, Mg and S. Nutrient utilization efficiency and NHI of potato showed small field differences. Follow-up crop total input rates ranged between 140 – 328 kg ha-1 N, 108 – 284 kg ha-1 P, 171 – 406 kg ha-1 K, 1141 – 1232 kg ha-1 Ca, 405 – 944 kg ha-1 Mg and 165 – 216 kg ha-1 S. In follow-up crops, total water inputs ranged between 805 – 1526 mm (average of 1155 mm) per season. Paprika dry fruit yield of 5.5 t ha-1, groundnut dry grain yield of 3.4 t ha-1 and onion fresh bulb yield of 75 t ha-1 were obtained. The WUER+I for paprika, groundnuts, and onion were 3.6, 3.0 and 11.9 kg dry matter yield mm-1, respectively. In conclusion, the growers achieved high potato yields, although substantial variability occurred between fields. Relatively high water and nutrient input rates were applied to potato and follow-up crops. However, these input rates were not necessarily proportional to crop yields achieved by the growers, which contributed to WUE and NUE variability among fields. The results revealed that the growers have room to improve WUEs and NUEs in potato and the follow-up crops, which can be achieved by using irrigation scheduling tools and adjust nutrient rates based on soil and irrigation water nutrient status.