Technical Yearbook 2023

SEPTEMBER

Winery wastewater for irrigation (Part 2): Evaluation of catch crops on open land By Carolyn Howell, Karen Freitag & Lester Sassman Wineries in South Africa are estimated to generate between 2 190 million and 15 330 million litres of wastewater per year. 1 In a drought prone country, re-using this water for irrigation makes economic and environmental sense. However, this water has a high chemical oxygen demand (COD) with high levels of potassium (K) and sodium (Na). 2 Using it for irrigation adds substantial amounts of elements such as K and Na to the soil, 3,4 therefore maximising element removal by means of a catch crop is important.

Introduction In a previous study where diluted winery wastewater was used for vineyard irrigation, oats and Pearl millet removed substantial amounts of K from a sandy soil. 1 However, minimal amounts of Na were taken up by these two crops. In a pot study, Fodder beet absorbed 38% of Na applied via Na-enriched irrigation water compared to soil without plants. 5 Furthermore, the Fodder beet reduced exchangeable soil K by 50%, thereby indicating that it could also absorb K applied via winery wastewater. Taking above-mentioned into consideration, the aim of this study was: 1. to determine whether irrigation of open land with diluted winery wastewater affected the performance and nutrient content of selected catch crops on loamy sand to sandy clay loam soils, and 2. to determine the ability of the selected catch crops to intercept sufficient amounts of Na and K from the soil. This is the second article in a series on the evaluation of selected catch crops where winery wastewater is re-used for Diluted winery wastewater was applied to open land on the Nietvoorbij research farm where 10 different fodder producing catch crops were compared to a control where no catch crop was cultivated in the summer. Details of the irrigation application and water quality have been given previously. 6 The dry matter production (DMP) of the catch crops was estimated by sampling the above ground vegetative growth in a 0.5 m 2 sub-plot randomly chosen in each plot. Samples were oven-dried for 48 hours at 105 ° C. A sample was collected from the sub-plot and analysed by a commercial laboratory for macro- and micro irrigation. Methods

elements. The amounts of the different macro- and micro elements intercepted by the cover crops were estimated by multiplying the DMP of the species with the concentration of the different elements in the samples harvested for analyses. Results and discussion The DMP, which is commonly used as an indication of how well a catch/cover crop grows under prescribed conditions, of the catch crops over the four years of the study is given in table 1. These results show that the DMP of the 10 catch crops showed variability over the study period with some showing higher and lower values. However, looking globally and taking the DMP of all the years into consideration, the Vetiver grass (T8) produced the highest amount of DMP followed by Dolichos beans (T7) and Chicory (T6). Vetiver grass can be cultivated as a perennial grass and has been shown to be tolerant of elevated Na. 7 Furthermore, Dolichos beans is a drought tolerant crop and tolerates a wide range of soil types from sandy soils to heavy clays. An average of 147 bales per hectare was produced by Vetiver grass (T8) and could generate an income of as much as R11 000 when sold (R75 per bale). Expenses against this income would be the cost of the 28 kg of fertiliser N and the 10 kg per ha of P applied to promote catch crop growth. This will increase the input cost by R1 624/ha and R1 587/ ha, respectively. The net income would be approximately R7 800 where 2.2 t/ha DMP of Vetiver grass is produced. On average over the four seasons of the study, Cow peas (T5), Chicory (T6) and Dolichos beans (T7) accumulated the most N in their foliage (figure 1). Dolichos beans (T7) and Bottle brush grass (T9) accumulated the most P and Chicory (T6) and Pearl millet (T1) accumulated the most

47

TECHNICAL YEARBOOK 2023