Water and nutrient retention under Swiss chard (Beta vulgaris Var. cicla) and cabbage (Brassica oleracea Var. capitata L) cultivated in soil amended with zeolite

Abstract

Poor soil fertility and irrigation water shortages are major challenges to vegetable production. Farmers have used different methods to address these challenges. Some methods such as the application of organic materials on soils have fallen short in providing stable non-decomposable soil amendments. A greenhouse pot experiment was conducted at the Agricultural Research Council (ARC), Infruitec-Nietvoorbij, Stellenbosch, South Africa to evaluate the effects of zeolite on the growth and yield of cabbage cv. Copenhagen (Brassica oleracea Var. capitata L), and Swiss chard cv. Ford Hook (Beta vulgaris Var. cicla). The experiment investigated the water and nutrient retention ability of zeolite amended sandy soils as influenced by zeolite. Six-weeks old seedlings were planted one seedling per pot during transplanting. The study was conducted over 2 growing seasons; the first being late autumn through to late spring 2018 for both vegetables; the second, early autumn to early spring 2019 for Swiss chard and winter/spring 2019 for cabbage. The experiment was laid out in a randomised complete block design (RCBD) and replicated six times. Prior to the commencement of the study, a baseline composite soil sample was collected for soil analyses to determine the macronutrients, electrical resistance, cation exchange capacity (CEC), exchangeable cations, pH and trace elements in the soil. At the end of each growing season, representative soil samples were also analysed for the same parameters. Data collection on growth parameters commenced on the third week after transplanting (WAT) for cabbage and the fourth WAT for Swiss chard. Measurements were recorded once a week for leaf plant height, width, leaf length and leaf area in both vegetables. At eight WAT, harvesting of the leaves and stem of Swiss chard commenced, it was a continuous harvest which was carried out at three weeks intervals. Five harvests were done in total. Cabbage head was harvested at maturity (19th WAT). Fresh mass of both cabbage and Swiss chard yield were recorded at harvest while the dry mass was determined after oven-drying at 70oC until constant weight. The dry samples were separately milled, stored in marked air-tight containers and refrigerated for the determination of their nutritional composition. Soil water content was regularly monitored gravimetrically and through weighing of pots. Soil moisture in each pot was maintained between 50% and 70% field capacity throughout the period of the experiment. All data were statistically analysed by the Biometric Department of the ARC Infruitec-Nietvoorbij, Stellenbosch. The data were subjected to analysis of variance (ANOVA) to detect treatment and where necessary, seasonal effects. For interactions that were not significant at p<0.05, Fisher’s least significant difference was used to compare treatment means. In the first season, the leaf area and plant height of cabbage decreased (p<0.05) with increased zeolite. Whereas, in the second season, these parameters increased as zeolite application increased. The same trend was also observed with Swiss chard. The first harvest for Swiss chard yielded higher biomass on the non-amended treatment. Thereafter, the zeolite amended treatments continued to show improved Swiss chard yields. Cabbage did not show any significant yield response (p>0.05) to zeolite in the first season. However, in the second season, higher yields were recorded in the zeolite amended treatments. The nutritional composition of cabbage head showed no significant difference (p>0.05) in terms of the proximate analysis, although the results were comparable with previous cabbage studies. Mineral composition of cabbage showed that the non-amended treatment had higher (p<0.05) Ca, Zn and B contents, with a lower level of Na. For Swiss chard, Ca, Mn, Zn and Fe contents were all higher (p<0.05) on the non-amended treatment compared to the zeolite amended treatments. The demand for water by cabbage significantly (p<0.05) reduced with increased zeolite rates, in the first season. However, in the second season, 30% zeolite required the least (p<0.05) irrigation followed by the non-amended treatment, while 10% and 20% zeolite treatments utilized the most water. Swiss chard irrigation in both seasons showed that the non-amended treatment had less irrigation water requirement compared to the zeolite amended treatments. On the other hand, soil chemical composition showed that zeolite application increased (p<0.05) cation exchangeability, pH and soluble S. Soil chemical composition further indicated that there could be a limit to zeolite application with N availability. Zeolite showed potential in ameliorating agricultural soil acidity and improving soil water retention. However, there is a need to carry out these experiments under field conditions to see if these benefits can be sustained, especially at smallholder farmers’ level.