From clay to crops: improving water productivity in rosemary and lavender under sandy soil conditions in South Africa

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Background Increasing irrigation water productivity is a key strategy for achieving sustainable agricultural production. This study evaluated the performance of self-regulating low-energy clay-based irrigation (SLECI) as a sustainable alternative to surface and subsurface drip irrigation under water-scarce sandy field conditions, with a particular focus on quantifying the trade-off between irrigation water productivity and fresh biomass yield. SLECI system is a subsurface irrigation technology that uses microporous clay emitters buried within the crop root zone. Water is released passively in response to soil moisture deficits, as water flows through the clay pores only when soil suction exceeds the retention force of the emitter. This self-regulating mechanism delivers water directly to plant roots without the need for pumps, sensors, or external control systems. Methods The experiment was conducted on newly established, 1-year plantings of rosemary ( Salvia rosmarinus , Tuscan Blue variety) and lavender ( Lavandula stoechas , Hollandia variety) grown under open-field conditions at the Welgevallen Experimental Farm, Stellenbosch, South Africa (33°56′33″ S, 18°51′52″ E), from November 2023 to February 2024, during the hot-dry summer season. Results The highest fresh marketable yield of rosemary was obtained under subsurface drip irrigation (906 kg/ha), while lavender achieved its highest yield under conventional surface drip irrigation (3,625 kg/ha). Although yields under SLECI were lower, this system used substantially less irrigation water (533.2 m 3 /ha) over the 3-month period, compared with surface drip (6,800 m 3 /ha) and subsurface drip irrigation (3,060 m 3 /ha). Irrigation water productivity (IWP) demonstrated the water-saving efficiency of SLECI, producing 0.529 kg of fresh marketable rosemary per m 3 of applied water, nearly 4.5 times higher than surface drip irrigation. Similarly, SLECI achieved an IWP of 1.467 kg/m 3 for lavender, representing a more than 2.7-fold increase compared to surface drip irrigation (0.115 kg/m 3 ). Irrigation technique significantly affected fresh marketable yield. Subsurface drip irrigation and SLECI increased rosemary plant diameter relative to surface drip, whereas subsurface and surface drip irrigation promoted larger lavender plant diameters. Conclusion This study contributes to understanding how clay-based irrigation systems perform under severe sandy soil conditions and provides practical insights into the trade-off between water saving and crop productivity in semi-arid environments.

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