Assessing the water productivity of sweet potato (Ipomoea batatas (L.) Lam.)

Abstract

In water-stressed countries like South Africa, the reliable quantification of actual crop evapotranspiration (ETA) and yield across a wide range of environments is important for improved agricultural water management. In addition, researchers are shifting their primary focus from well-studied major crops to neglected and underutilised crops. Orange-fleshed sweet potato (Ipomoea batatas (L). Lam.) remains an underutilised root and tuber crop (RTC) in South Africa, despite its potential as being nutrient-dense, high yielding and water use efficient, as reported in local literature. When compared to conventional crops, knowledge is limited on the water use and yield of RTCs under rainfed and precision agricultural production in South Africa. It is therefore important to further investigate whether the water use of orangefleshed sweet potato (OFSP) will hinder its production at the commercial scale. This study attempted to contribute towards the limited research on the crop water productivity (CWP) of OFSP. A rainfed field trial with optimum fertilisation was conducted at Fountainhill Estate (KwaZulu-Natal, South Africa) to estimate seasonal ETA, yield and CWP. The soil water balance method was used to determine ETA accumulated over the growing season from 14 December 2021 to 11 April 2022. Total ETA for OFSP was estimated at 468.13 mm, which was used to calculate fresh and dry CWP values of 7.45 and 2.59 kg m-3 , based on final fresh and dry tuber yields of 34.89 and 12.12 t ha-1 , respectively. Harvested tuber and above-ground biomass samples were sent to a laboratory to analyse nutrient content (NC). The nutritional water productivity (NWP) was determined as the product of CWP and NC, highlighting the potential of OFSP to alleviate malnutrition, especially if grown in rural communities. Field observations were used to partially calibrate the Soil Water Balance (SWB) and AquaCrop models. These models were used to simulate ETA, yield and biomass accumulation, from which CWP and NWP were calculated. Compared to observations, AquaCrop provided a better estimate of CWP (2.55 kg m-3 ) relative to the SWB model (1.16 kg m-3 ). However, AquaCrop simulated higher soil water content relative to measurements from volumetric soil water content sensors. This study showed that under suitable management practices, OFSP has the potential to be grown commercially, since the crop can produce high yields and nutrient contents under rainfed agricultural production. However, to improve production, future studies need to conduct research to improve tuber yield and biomass accumulation. Furthermore, the AquaCrop and SWB models should be calibrated and validated across different agroecological zones in South Africa.