Effect of summer and winter rotations on selected soil properties and productivity of succeeding winter chickpea (Cicer arietinum L.)

Abstract

Chickpea (Cicer arietinum L.) is one of the minor pulse crops in South Africa, but there is hardly any commercial chickpea production in South Africa and the rest of Southern Africa despite its increasing domestic demand. The country meets its domestic demand through imports. Preliminary research has shown the potential of winter sowing of chickpea in rotation with summer crops in the dry environments of Limpopo Province, South Africa. This has provided an opportunity for development of summer and winter crop rotation in the semiarid regions of the Limpopo province, South Africa. There is evidence that crop rotation positively affects physical, chemical and biological properties of the soil consequently increasing the yields in the long run. Therefore, this study assessed the residual effect of summer and winter crop rotations on selected soil chemical properties and soil biological health indicators, and productivity of the succeeding winter chickpea. A field experiment was established during the summer of 2016/17 to determine the effects of summer and winter crop rotations on selected soil chemical properties, biological health indicators, and yield performance at the University of Venda, South Africa. The current study was based on two winter seasons i.e. 2017 and 2018 focusing on chickpea crop. Experimental materials consisted of summer crops [maize (Zea mays), bambara groundnut (Vigna subterranea), cowpea (Vigna unguiculata), mung bean (Vigna radiata)], fallow and winter crops [chickpea - mustard (Brassica rapa)] and combination treatments consisting of summer-winter crop rotations. The field experiments were laid out in a randomized complete block design (RCBD) with three replicates. Seeds were inoculated and sown manually at intra-row spacing of 10 cm and inter-row spacing of 30 cm. Single superphosphate (10.5% P) fertilizer was applied at planting at a recommended rate of 90 kg P ha-1. Supplemental irrigation was applied whenever necessary. Plots were kept weed-free throughout the growing seasons. Soil samples were collected at depths of 0–20, and 20–40 cm prior to planting (experiment I) and after harvesting (experiment II) in 2018. The following chemical properties were determined: soil reaction (pH), cation exchange capacity (CEC), exchangeable bases (M2+, Ca2+, Na+, K+), available P, and (NO3-) nitrate. Soil biological indicators that were analysed include soil active carbon (AC), organic carbon (OC), (NH4+) ammonium and potentially mineralizable nitrogen (PMN). Crop phenology (days to 50% emergence, flowering, and podding) and crop growth (plant height, canopy cover, number of primary and secondary branches) were determined at various crop growth stages. Grain yield and yield components (number of pods per plant, number of seeds per pod and 100 seed weight [100-SW]) were determined at maturity. The data (various agronomic and soil laboratory) obtained were subjected to analyses of variance (ANOVA) using the general linear model (GLM) of Genstat software version 18 to examine the effects of treatments on selected soil properties and crop productivity. Significant differences between treatment means were compared using the standard error of difference (SED) of the means at 5% level. Independent samples t-test was used to compare the differences between experiments and soil depths. Correlation analyses were conducted to assess the relationship between the various parameters measured. Crop rotations did not affect soil pH, CEC, exchangeable cations, P, K and NO3- in both experiment I and II. However, relatively greater soil NO3- and P values were recorded in experiment II compared to experiment I. Similarly, crop rotations had no significant effect on soil AC, OC, NH4+ and PMN in both experiments. Soil organic carbon remained relatively constant across all the soil depths (0-20 cm and 20-40 cm) in experiment I, however, there was an increase in experiment II. There was a statistical difference between experiments on mean soil NH4+, greater mean soil NH4+ values were reported in experiment II in comparison to experiment I. There was no response of crop phenology and growth with respect to crop rotations in both growing seasons (2017 & 2018). However, the crop flowered earlier in 2018 compared to 2017, and the tallest plants were measured in 2017 compared to 2018 growing season. In contrast, 2018 growing season resulted in greater grain yield (1654 kg ha-1) than the 2017 growing season (1319 kg ha-1); for treatments there was no statistical difference found. A similar trend was found with number of pods per plant. There was no significant difference in 100 SW and number of seeds per pod. Based on the current results it can be concluded that crop rotations are generally less effective at improving soil properties and crop productivity in the short term. Many of the chemical and biological attributes of soil quality are stable and their manifestation is experimentally verifiable only over extended periods. Therefore, a study with a more prolonged period would provide useful insight, hence the importance of long-term studies.