Managing the safe application of sugar by-products to soil: modelling and recommendations

Managing the safe application of sugar by-products to soil: modelling and recommendations

TPC Limited’s management is considering the installation of a molasses-based distillery to be attached to their existing sugarcane factory, which is located to the south of Moshi, Tanzania. The main issue of investigation relates to developing an understanding of the potential impacts of vinasse application, and in particular the addition of potassium, K, to sugarcane fields, some of which are saline-sodic and most are already potassium-rich.

The intention is for vinasse to be applied to sugarcane fields across the estate, via the irrigation network, utilising water from a number of sources. Irrigation water, with highly variable quality, is mainly sourced from the Kikafu River, Kikuletwa River, and groundwater reserves. Other important factors that needed to be considered are the potassium (K) rich soils, existing salinity and/or sodicity of some fields, and the depth to groundwater as well as its quality.

A comprehensive international literature review along with data supplied by TPC Ltd, was used to parameterise appropriate models to determine the impact of vinasse application, at 0.5% and 2.0%, over both the short (5, 10 and 15 years) and long term (100 years). To represent the range of irrigation water utilised across the estate, the vinasse was diluted with three differing qualities.

Applying vinasse, at these concentrations, is expected to increase pH of all soils to approximately 8.5, which would exacerbate the effect of ESP/ECR due to the precipitation of alkaline calcium carbonate. At these concentrations, the vinasse is expected to have a minimal, and manageable, impact on structural stability when using the good quality river water.

However the use of ‘median’ quality irrigation water will have mixed impacts depending on the soil. For soils, e.g. Cambisols, Luvisols and Plinthosols, which are predicted to have reductions in hydraulic conductivity, the addition of calcium (Ca), typically from gypsum and potentially supplemented by press mud, could be used as an effective management strategy. The hydraulic conductivity is not expected to decrease for some soils due to the electrical conductivity (EC). For a small number of soils adding vinasse at 2% to this water should improve chances of maintaining soil stability. However, the overriding factor is the alkalinity introduced with the irrigation water and vinasse. A sulphur source may address this if sulphate reducing microbes exist in TPC soils. Furthermore, the interaction between soil dispersion, salinity and alkalinity cannot be reliably predicted; it is soil specific and needs to be considered further by TPC on site.

Addition of vinasse, at both 0.5 and 2.0%, to poor (unsuitable) quality irrigation water dramatically aids in decreasing soil structure within 5 years for nearly all soils. Over the long term, hydraulic conductivity reductions of at least 90% would be expected in almost all cases.

Regardless of irrigation water quality, it is recommended TPC adopts a sodium and potassium management plan through the application of calcium, typically in the form of gypsum, and an alkalinity management plan.

Irrigation water quality poses the greatest concern to long-term soil structure stability and sugarcane production. Improving the quality of irrigation water applied through the increased usage of good quality river water while ceasing use of irrigation water defined as ‘poor water’ is recommended.

Press mud has the potential to meet some of the Ca requirement for TPC’s median quality irrigation water and could be used to supplement external sources of Ca. Where irrigation sources are treated to neutralise alkalinity and reduce soil ECR, then press mud nutrients can be considered available in full and factored into sugarcane nutrient budgets.

Ash management must be carefully considered as it has little, if any, overall benefit and is more likely to create negative impacts such as neutralising the positive effect from Ca in press mud via raising the alkalinity of the soil from calcium oxide and calcium carbonate, resulting in further Ca precipitation.

Finally, while a number of knowledge gaps were identified and worthy of future investigation, the impact of high groundwater, and its associated quality, is considered the most important matter to be addressed.

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