Effect of calcium-bentonite and/or mill mud/ash mixtures on properties of a Mackay sandy soil: a preliminary study

Sandy soils occur in sugarcane-growing regions in Australia and overseas and are often thought of as being ?marginal? because of their poor physical and chemical properties. They are characterised by low cation-exchange capacities (CEC), organic-matter (OM) contents, and moisture- and nutrient-retention capabilities (Gillman et al. 1989). These factors limit plant growth and fertiliser-use efficiency. Increasing CEC and OM in sandy soils may result in benefits such as improved soil-nutrient cycling, water-holding capacity (WHC) and erosion control (Gilbert et al. 2008; Croker et al. 2004). Sources of organic matter available to sugarcane growers include mill by-products such as mill mud (MM) and mill mud/ash (MMA) mixtures. They contain high concentrations of nitrogen (N), phosphorus (P), potassium (K) and calcium (Ca) (Moody et al. 2014; Gilbert et al. 2008). In addition to yield benefits, MM and MMA applications increase CEC, water retention and plant-available water in sandy soils (Kumar et al. 1985) However, MM and MMA decompose relatively quickly in humid subtropical and tropical climates, making the amount required to maintain adequate CECs beyond the cost of an average farmer (Noble et al. 2000). Adding bentonite is an option to improve the structure and fertility of sandy soils. Calcium bentonite (Ca-bentonite) granules disperse into fine particles forming sub-microscopic, fine, clay-layered structures that have large surface areas containing reactive, negatively charged ion-exchange sites (Gillman 2011). Therefore, when applied to sandy soils with low CECs, bentonite may result in increased CECs. However, when combined with MM and MMA there is the possibility of increased retention of organic carbon (OC) due to binding to clay surfaces (Baldock 2007). A further benefit of bentonite is that it has a capacity to increase soil WHC and availability of nutrients (Noble et al. 2001; Croker et al, 2004; Satj, 2009; Suzuki et al. 2007). We evaluated the effect of Ca-bentonite and/or MM and MMA on physical and chemical properties and nutrient storage on sands. A preliminary pot trail used a sandy soil collected from a block of sugarcane in the Mackay region. Natural Ca-bentonite in granules were obtained from the Ebenezer deposit in southeastern Queensland. Soil physical and chemical parameters were measured at the start of the trial using the standard methods described by Rayment and Lyons (2011). CEC was 1.8 cmolc/kg, pH was 6.8, EC was 0.2 dS/m, soil texture was clay 7%, silt 6.5% and sand 86.5%, WHC was 20%, OM was 0.66%, and ammonium-N (NH4+-N) content was 3.74 mg/kg. The experiment consisted of 10 treatments, comprising a control with no additions (C), Ca-bentonite applied at 6 t/ha (B6) and 12 t/ha (B12), MMA at 20 t/ha (MMA20), MMA at 60 t/ha (MMA60), the combinations of B6 + MMA20, B6 + MMA60, B12 + MMA20, B12 + MMA60, and gypsum (G) at 6 t/ha (G6) as G6 + B6 and G6 + B12. Pre-dried soil was mixed with the appropriate treatment and placed into appropriate pots. The control and amended soils were maintained at field capacity for 6 weeks. There was a significant increase in soil CEC from the original 1.8 cmol(+)/kg to 5.95 cmol(+)/kg at the highest rate of application, B12 + MMA60. This was followed by a CEC of 5.35 cmol(+)/kg with B6 + MMA60, and 5.2 cmol(+)/kg with MMA60. These values represented increases in CEC by 69.7%, 66.3% and 65%, respectively, and indicated the major increase in CEC was due to MMA applications. When B6 and B12 were combined with gypsum, the CEC increases were 33% and 47%, respectively. Building OM in sandy soils is difficult due to the rapid decomposition rate and cultivation. Our results showed that OM, measured from loss-on-ignition, increased with the addition of all treatments, but the highest increase (1.51-1.82%) occurred with application of MMA60. Bentonite additions of B6 and B12 with MMA20 made no 73 difference to soil OM during the trial. Mineralisation of OM to NH4+-N and nitrate-N occurs naturally in soils and depends on the amount of organic matter and microbial activity (Schroeder et al. 2007). Application of B12 + MMA60 increased the NH4+-N content to 9.32 mg/kg compared from 3.74 mg/kg in the control. This is consistent with a soil OM increase from 0.66% in the control to 2.32%. Increased porosity due to MMA and smaller soil pores due to bentonite application improved WHC by 19.5%. Although our results suggest that in the short-term MM and MMA application may have an over-riding effect, bentonite may persist in soil over a longer term. The second phase of our study will evaluate the effects of the amendments on seedling growth and survival, especially under drought-stress conditions.