A number of researchers have studied the phenomenon of soil drying 16, 17, 18, however, during past years, it has been recognised that water vapour flow plays a crucial role by transporting water and vapour of chemicals. Due to the scarcity of water resources, knowledge about water liquid and vapour movement regarding the developing and extending of dryland farming, has gained importance. Several researchers noted that the loss of water due to evaporation from bare soil is between 50 to 70% of the annual rainfall 13, 14, 15. Soil evaporation (E) in semi-arid or arid regions causes the greatest loss of water 10 and the evaporation demand is usually greater than the ability of soil to conduct water in the liquid phase 11, 12. Fallow efficiency after long dry summers is, however, still unknown in the case of rooibos that is normally planted during winter, between June and August. winter wheat in a summer rainfall area 9. Fallowing is mostly practised in areas and for crops where rainfall precedes, the planting of a new crop e.g. Fallow efficiency (FE) is improved, amongst other factors, by deeper soils and a higher amount of plant residue on the soil surface 8. The negative effect of temporary seasonal drought can be mitigated by fallowing, a practice that can increase stored soil water by conserving rainfall. It was reported by Stassen 7, who did research on soil properties related to rooibos cultivation, that rooibos prefers soils which are deep and cooler with higher soil water storage (SWS). Since its commercialisation, there has been a greater research focus on the health benefits and quality of rooibos tea than on the specific soils where rooibos likes to grow. Over the last ~ 18 years, the production of rooibos has varied between 10,000 and 18,000 tons per year, all of which occurred under dryland production 6. ![]() Although, this special plant had no commercial interest at the beginning of the twentieth century, the cultivated area reached 95 000 ha in 2016 5. The rooibos plant ( Aspalathus linearis) is a leguminous shrub 1, belonging to the Fabaceae family 2, and grows in the Mediterranean climate 3 of the Cape Floristic Region of the Western- and Northern Cape provinces of South Africa 4. This study revealed that unfertilised plants on deeper soils result in higher rooibos production under drought conditions. ![]() ![]() The highest shoot biomass with the longest taproot resulted from the unfertilised treatment on the deep soil thanks to higher soil water content, whereas the shoot and root biomass of the fertilised treatment at both sites were low due to high P soil concentration. A further survival method was proposed, involving moisture moved through evaporation from the deeper soil layers into the drying-front in the ~ 10−30 cm soil layer where a condensation-evaporation cycle enables rooibos to harvest small amounts of water. It was concluded that rooibos plants could survive through an adapted root system. With the exception of the 20−30 cm root zone of the planted plots at the deep site, the water content decreased to levels below the permanent wilting point in the soil profile during summer. At the end of the study, the profile soil water content and evapotranspiration of the bare and planted plots were similar which prove that fallowing (water harvesting) is not an option in the sandy soils of this region. All treatments were replicated on shallow and deep soils sites and plant growth was determined at the end of the season. Soil water content of plots planted to unfertilised and fertilised plants as well as that of bare soil were determined over the duration of the 2016/17 season. Consequently, the effect of inorganic fertilisation and soil depth on soil water dynamics in a young rooibos plantation at Nardouwsberg, Western Cape were studied. Being low-rainfall regions, information on the soil water balance of rooibos is essential, but such data is limited. Rooibos ( Aspalathus linearis) is endemic to certain regions of the Western- and Northern Cape of South Africa, where it is also commercially grown.
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