Soil quality of kikuyu, ryegrass and clover pasture mixtures in the Tsitsikamma
Phohlo, Motsedisi Portia
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South African soils have long been classified as being severely degraded. The state of the soils is even more pronounced in sandy soils that are managed for pasture production in the Tsitsikamma region. This is mainly due to the fact that these soils have poor soil organic matter (SOM) content and poor soil fertility. The result of this is nutrient leaching which leads to contamination of ground water; water loss through deep percolation resulting in wasteful irrigation; poor pasture yields which have a direct influence on farm efficiency and profitability. Such occurrences are more detrimental in the dairy farming industry because the quality of soil and quantity of pasture produced has an overriding influence on the main farm produce, which is milk. A system of continuous supply of nutrients and irrigation is not a sustainable system for dairy farmers as it results in enormous financial pressure. Better strategies that ensure effective use of resources need to be developed and implemented and must compliment sustainable farming. Assessment of soil quality is one of the fundamental methods that have long been identified as tools, which farmers can use in order to improve farm efficiency. Soil quality as defined by Karlen et al. (1996) is the capacity of a specific kind of soil to function within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation. Managing and understanding soil quality means evaluating and managing soil so that it functions optimally now, and in the future. Land managers should be monitoring changes in soil quality on a regular basis, and using this to adopt sustainable practices, which aims to improve the productivity of soil (Doran, 2000). Soil quality can only be measured by assessment of its indicators which vary according to cropping systems. The general consensus amongst researchers is ensuring that indicators of soil quality should reflect the soils’ chemical, physical and biological status. In this study selected indicators of soil chemistry (extractable P, exchangeable Ca, K, Mg and Na, pH (KCl)); soil physics (bulk density (BD)) and soil biology (total carbon (C), active C, total nitrogen (N), C/N ratio, PMN rate and inorganic N) were measured. The selection thereof was based on their ease and reliability of measurement, the sensitivity of the measurement to changes in soil management as well as the skills essential for interpreting the results. The study was carried out in the Tsitsikamma region of the Eastern Cape where farms distributed in the upper and lower Tsitsikamma region were selected as study sites. The farms were selected based on the criteria that they were all irrigated farms with kikuyu, ryegrass and clover pasture mixtures, they had adopted minimum tillage or no tillage practices, had pastures established for at least 6 years, and lastly had accurate records of management practices that had been implemented, especially those relating to fertiliser application. An average of 5675 soil samples were analysed across the farms. These samples were taken at increments of 0-15, 15-30, 30-45 and 45-60 cm, respectively. The samples were analysed using the Veris spectrophotometer probe otherwise known as the Veris P4000. Calibration soil samples were also taken and analysed by a commercial laboratory (BemLab, De Beers RD, Somerset West, South Africa) in order to standardize the soil samples analysed with the Veris P4000. Based on the selected indicators, the objectives of the study were structured to answer 3 principal research questions, namely: Firstly, do the farms in the Tsitsikamma differ significantly within soil depth? The soil depth comparison was done at increments of 0-15, 15-30, 30-45 and 45-60 cm respectively, while farm comparisons were done at a 0-30 cm increment; Secondly, are management practices responsible for variations observed in the Tsitsikamma region?; And lastly, which soil quality indicators play the most significant role in the variations observed? Data used for this study was presented in concentration (% or mg/kg) and in stock (kg/ha) values. The analysis of variance was measured at 99% confidence level. Values that had significant differences had p values < 0.001, whereas those that showed no significant difference had p values > 0.001. Correlations between soil quality indicators were analysed using two-tailed Pearson correlation tests at 1% and 5% level. Principal component analysis (PCA) was computed using SPSS statistical program. The findings showed that both the upper and lower Tsitsikamma followed the same trend in terms of nutrient movement through soil depth. It was observed that the most significant differences occurred within the 0-30 cm depth for all indicators except for pH (KCl) and C/N ratio. The two former indicators showed statistical significances in all depth layers with a very gradual decline with depth in both regions of the Tsitsikamma. The results further showed that farms differed significantly within this region and that management practices had a significant influence in the differences observed. This was clearly illustrated in the PCA conducted, which grouped the farms according to similar management practices with those farms that had a more biological approach falling in the same category. Incidentally, the farms that had been more chemical dependent fell into their own category. Furthermore, farms that exhibited better SOM indicators generally held more nutrients, even though no heavy applications of those nutrients were done in the sampling year. Farms that were observed to also have more concentration of nutrients in the soil, even with poor SOM content, had applied those nutrients in chemical fertilisers during the sampling year. This therefore justified as to why those farms also had more nutrients in the soil. The PCA conducted also showed that 54% of the variations observed in the Tsitsikamma region could be explained in the following order by these indicators: total N, pH, exchangeable Ca, exchangeable Mg, total C, active C, exchangeable K and BD. These findings emphasised the need for farmers to not only focus on replenishing or managing N in the soil, but also to pay careful attention to pH, exchangeable Ca, exchangeable Mg, total C, exchangeable K, and BD in order to improve soil quality. The findings also highlighted the urgent need for farmers to change their line of thinking and abandon soil management practices that enhance soil degradation, a problem that is very common in South Africa. Proper management of soil quality is vital in ensuring sustainable soil management and food security; therefore researchers along with governments need to build a better transfer of knowledge to farmers in order to ensure the former.