Comparative water use efficiency and water retention in the Mandela trials.

Abstract

Abstract With the world's growing population and our limited natural resources, there is a need to produce more food using fewer inputs, especially fresh water. Water is a critical resource in agriculture and may be more of a limiting factor than other crop growth requirements. As water availability is impacted by climate change and competition from human consumption and other industries, methods of improving crop water use efficiency (WUE) through conservation of water and the enhancement of crop growth need to be employed to meet our growing demands sustainably. The research assessed the differences in soil water status between organically farmed crops with a grass mulch and conventionally farmed crops without mulch, to use the water resource more efficiently. This research, conducted at the Mandela long-term organic farming systems research trial site at Nelson Mandela University, George Campus in South Africa's Southern Cape, is part of a larger research project (the Mandela Trials) in which various researchers have covered agronomy, microbiology and pest and disease control. The organic treatment had a significantly higher soil water content (SWC) than the conventional treatment at all soil depths over the two seasons 2016/17 and 2017/18. Theta probes showed that in both seasons, for the top 6 cm of soil the organic treatment had a higher SWC than the conventional treatment. The minimum and median SWC in the organic treatment were much higher than that of the conventional and control. However, in the second season the control treatment had a higher minimum SWC than the organic, but a lower median SWC (there was almost no plant growth on the control plots due to nutrient deficiencies). The differences in the SWC of the organic and conventional treatment for 2016/17 was significant, however, differences for 2017/18 were not significant. The capacitance probe data for the 10 cm depth show that the minimum and median SWC of the organic treatment were higher than the conventional. The differences in SWC between all three treatments are statistically significant for both seasons. The combined SWC capacitance probe data of the 0-50 cm soil profile showed that the organic treatment had a higher SWC and median than the conventional treatment. However, the control treatment had the highest SWC and median of all the treatments. There was a significant difference in the organic and conventional treatments in both seasons, but no significant difference between the organic and control treatments in the first season. Soil carbon was significantly higher in the organic treatment, than the conventional. Organic farming methods preserve and promote an increase in soil organic matter (SOM), thus improving the soil structure and increasing the soil's water holding capacity. From this research, it is recommended that organic farming practices can be used to help conserve SWC, keeping it available to crops for longer and helping farmers make more efficient use of this scarce resource. This is especially relevant for low rainfall areas which are affected by water shortages. Improved SWC availability should be coupled with good agronomic practices to increase productive water losses (plant use) and the conversion of water to yields, increasing WUE. Adding organic matter to the soil will improve resilience and help sequester carbon, and thus mitigate climate change. More research is needed to assess what proportion of the increased soil water retention can be attributed to the influence of the mulch and what effect the SOM had.