Rehabilitation of mined-out dune land at Tronox KZN Sands for sugarcane production
Van Jaarsveld, Corlina Margaretha
MetadataShow full item record
Titanium-rich minerals were extracted from the soil of the Hillendale mine near Empangeni in the KwaZulu Natal province of South Africa (28° 50’ S and 31° 56’E) by using a hydraulic open-cast procedure. A mechanical process was employed to separate the heavy minerals from the sand and slimes (silt and clay fractions combined) in the soil. The mining process resulted in a physical and chemical disturbance of the soil and the mining company (Tronox KZN Sands) has a legal obligation to rehabilitate the post-mined soil back to sustainable sugarcane production. The sand and slimes fractions (void of heavy minerals) were mixed together to a ratio of 70:30 sand:slimes to create a reconstituted soil. The reconstituted soil was subsequently deposited on sand to a depth of 1.8 m. The objective of this study was to test specific soil amendments for their potential to improve the reconstituted soil physically and chemically for successful sugarcane production. These amendments included gypsum, filtercake and inorganic phosphorus (P). Waterlogging was a constraint that affected the tested amendments under field conditions. Data collected from field experiments revealed that waterlogging affected sugarcane growth both directly and indirectly. The most noteworthy effect of waterlogging on sugarcane was iron (Fe) toxicity which was observed in treatments where filtercake was applied in the plant crop. Iron toxicity is a rare phenomenon in sugarcane production. In addition, magnesium (Mg)-related dispersion was identified in the reconstituted soil which increased soil erodibility. It was also noted that the reconstituted soil had high potential for hardsetting and compaction. The reconstituted soil lacks structure. Hence, gypsum, as a source of calcium (Ca), was tested as a potential flocculent to improve soil structure in both field and pot experiments. The gypsum application rate varied from 0 and 16 t/ha. Gypsum successfully improved the Ca:Mg ratio of the soil to the ideal range for optimum crop growth of 1.5 to 4.5 or higher. Improvement of the Ca:Mg ratio is important in counteracting Mg-related dispersion. Gypsum application however did not significantly improve mean weight diameter (a parameter associated with soil structure) and sugarcane growth and yield relative to the control treatment without gypsum. Organic matter has the potential to improve soils both physically and chemically. Filtercake (FC), an organic waste product from the sugar milling process, was tested as a potential organic amendment of the soil at rates that varied from 0 to 100 t/ha. Applying FC at a rate of 100 t/ha was very effective in increasing the electrical conductivity (EC) of the soil and in increasing the P content of the soil. However, FC application in the field was associated with negative effects on sugarcane growth including nitrogen (N) immobilization and Fe toxicity (as already mentioned). Yet, in the corresponding pot experiment, where waterlogging and N immobilization were absent or minimal, sugarcane growth was not significantly improved where FC was applied compared to the control treatment without FC. Mean weight diameter was also not significant improved by FC application in the field. Thus, FC did not appear to be a suitable amendment in this study and might have to be replaced by a different source of organic matter. The post-mined soil was low in P which is an essential plant nutrient and therefore the effect of inorganic P application was tested in the field and in pots. Inorganic P application rates varied between 0 and 70 kg/ha in these two experiments. Sugarcane growth and nutrient uptake was significantly improved by inorganic P application in the plant crop of the field experiment. In the first ratoon crop, waterlogging was likely responsible for reduced sugarcane growth and nutrient uptake. In the pot experiment, application of inorganic P was associated with increased aboveground, root and whole plant biomass. Inorganic P application also significantly improved uptake of P and Ca. When pooling the data for the three pot experiments (which was done in similar soil and over similar period of time), it revealed that sugarcane whole plant biomass (WPBM) had a significant and positive relationship (p<0.01) with inorganic Ca (r2 = 0.63), P (r2 = 0.66) and K (r2 = 0.62), as well as the Ca:Mg ratio of the soil (r2 = 0.58). Whole plant biomass also correlated negatively (p<0.01) with the Mg:K ratio in the soil (r2 = -0.48). The minimum inorganic Ca, P and K application rates and Ca:Mg ratio for optimum WPBM, as extrapolated from the data, were 608 kg/ha, 74 kg/ha, 287 kg/ha and 0.9, respectively. These application rates will have to be tested under field conditions. The results of this study confirmed that amending the reconstituted soil both physically and chemically is imperative for successful sugarcane production. More research is required to fine-tune the amendments tested in this study in order to sustain sugarcane production in the reconstituted soil in the long run. Soil erosion, waterlogging, hardsetting, compaction, nutrient deficiencies or toxicities and poor yield can be expected if the soil is not rehabilitated successfully. One of the most important aspects of the soil rehabilitation at Hillendale will be to improve soil structure which in turn will improve water drainage. With improved water drainage, the risk for sugarcane production being compromised by the above mentioned constraints will be reduced. Recommendations were made with regards to management of the reconstituted soil and for future research at Hillendale.