Theme 3 blog update: Australian sugarcane field sites 2016-2025

In Australia alone, the sugarcane industry generates approximately $2 billion per year in export earnings. Application of crushed basalt to soil under sugarcane could enhance production, improve runoff water quality and reduce greenhouse gas emissions alongside carbon sequestration benefits. Our Australian trials were set up to test these ideas. 

Our field sites in Australia originally started out as the Hesp site and the Howe site, along with a pot trial, in North Queensland in Northern Australia, growing sugarcane crops. Both of the field sites were designed identically, except that Hesp is a loamy soil type, and Howe is more clayey. Catch up with progress below, including relevant publications, since our inception in 2016.

Jump to:

• 2016-2017
• 2017-2018
• 2018-2019
• 2019-2020
• 2020-2021
• 2021-2022
• 2022-2023
• 2023-2024
• 2024-2025
• Papers featuring our Australian trials

2016-2017

Much of our set-up year, starting in July 2016, was spent designing and agreeing the design of the pot trial and finding suitable sites for the field trials. We’ll be assessing the effect of basalt particle size and application rate (2 rates plus control) in a trial with large pots and two soils, which will be run for approximately 10 months. 

After assessing many potential sites for the sugarcane field trials we settled on two sites that are about to be replanted, which will allow incorporation of the basalt. The Hesp site is high rainfall with highly weathered loamy soil and the Howe site is lower rainfall with irrigation and has clayey soil. The trials will test the effect of basalt application rate (two rates plus control) over four years (plant crop plus three ratoons). In all trials, we’ll be assessing the weathering rate by measuring dissolved inorganic carbon in deep drainage during the trial, and silicon and cations in the soil and plants at the end of each crop.

Solute flux in deep drainage will be measured directly in the pot trial, and estimated in the field trial, from measured soil concentrations and modelled deep drainage flux. The liming effect of applied basalt, and consequent avoided CO₂ emission from use of limestone, will also be determined in all trials. 

Dr Kalu Davies was appointed in June 2017 to run the trials. 

2017-2018

The pot trial was set up to assess the effect of basalt particle size (<4.75 mm and fines), application rate (0, 20 and 50 t/ha) and soil type (Kandosol and Hydrosol/Tenosol). In May 2018, we built 52 pots (700 mm high by 380 mm diameter) from PVC pipe, that were filled with soil and planted with sugarcane. 

The sites for the field trials were chosen and the first field trial (Hesp) commenced. Pre-treatment soil samples were taken (0-0.25 and 0.25-0.5 m depth) and basalt (140 tonnes) was applied at three rates (0, 20 and 50 t/ha) over 0.5-ha plots (550 x 9.1 m, 4 replicates) in May 2018. 

All trials will be instrumented and monitored. Suction lysimeters will be deployed in the field trials and soil pore water will be sampled through the wet season. A crop model will be parameterised so that deep drainage can be estimated. The cane crops will be harvested approximately 12 months after planting, whereupon the crop and soil will be analysed and weathering rates determined. The cane will then be allowed to re-sprout, basalt will be re-applied and harvest and analysis will be repeated, and so on for a total trial length of four years.

Crushed basalt being delivered to one of the sugarcane field trial sites (Hesp) in Australia in May 2018 (Photo: Kalu Davies).

Crushed basalt being spread on the Hesp field trial plots in May 2018 prior to cultivation and planting of sugarcane (Photo: Paul Nelson).

2018-2019

Our large pot trial with sugarcane has continued over the past 12 months to assess the effect of basalt particle size (<4.75mm and <2mm), application rate (0, 20 and 50 t/ha) and soil type on ERW and carbon capture. The soils are a Kandosol from the Hesp trial site soil (Hesp) and a highly weathered Tenosol with very low available silicon from the Daintree area, identified in previous research on weathering by Suzanne Berthelsen (Reynolds). Leachate from the trial has been monitored for dissolved silicon. which tended to be higher in all basalt treatments, indicative of enhanced weathering, but was substantially affected by the source of irrigation water and the soil type.

In the pot trial, our results suggest that the Hesp soil, in particular, is immobilising the silicon released during the weathering process. The soil from each pot has been sampled and is currently being prepared for further analysis of weathering rates. Leachate was also measured for pH and bicarbonate and a small increase in pH was observed for the Reynolds [1] soil [2] only but no differences in bicarbonate for either soil. Non-destructive measurements of cane growth indicate there may be a positive benefit to the cane yield on the Reynolds soil. Destructive harvest of the cane will take place in the coming months. The pot trial will continue for at least another 12 months following the cane harvest, with basalt to be reapplied.

Two large-scale field trials have now been established (Hesp and Howe), each measuring three application rates of basalt (0, 20 and 50 t/ha) in a randomised block design with 4 replicates of each treatment plot (approximately 0.5 ha each). The basalt has been characterised by XRF, pre-treatment soil samples have been analysed and field instrumentation installation is underway to parameterise a crop model to estimate deep drainage. Harvest of both field trials will occur in late 2019, at which time the crop will be analysed for sugar and nutrients and further determinations on the weathering in the soil.

Soil sampling at the Howe field trial site in 2018 (Photo: Kalu Davies).

2019-2020

In the two field trials, which commenced in mid-2018, we carried out the first harvest and plant sampling (August at Howe, September at Hesp), followed by the first soil sampling and second basalt application. The second basalt application was a repeat of the first (0, 20 and 50 t/ha). A provisional indicator of increased carbon capture is provided by increases in soil extractable alkalinity at the Howe site, especially at 25-50 cm and in the inter-row, but there was no effect in the more acidic Hesp soil. 

Concentration of alkalinity in soil extracts from the two sugarcane field trials (Hesp and Howe) at two depths, at the three rates of basalt application, one year after application.

We found no significant accumulation of total soil carbonate at either site nor any significant effect on other measured soil properties. Base cation content of soil and related properties, and sugarcane, is greater at Howe than Hesp. Neither cane yield nor commercial cane sugar (CCS) was significantly affected by basalt application. 

With an increasing basalt rate there was an upward but statistically insignificant trend in yield at Howe and CCS at Hesp. Variability was large at Howe due to ‘Yellow Canopy Syndrome’, a disorder whose causal agent has not yet been explained, over part of the trial. In the pot trial, which had equivalent rates of basalt, there was a significant increase in soil pH with basalt rate irrespective of particle size or soil type, especially near the surface.

Measuring cane biomass in the Hesp field trial in 2019 (Photo: Kalu Davies).

Harvesting the Howe field trial in 2020 (Photo: Paul Nelson).

2020-2021

We increased soil sampling at the two field trials to biannual in the past year; once in July 2020 in the dry season and again in March 2021 in the wet season. The field trials were harvested for the second time (September 2020 at Howe, October 2020 at Hesp) following the second round of plant sampling. The basalt was applied for the third time at both field sites (October 2020 at Howe, November 2020 at Hesp) and was a repeat of the previous two applications (0, 20, and 50 t/ha). We deployed an intensive lysimeter sampling regime during the wet season (December 2020 – April 2021). In November 2020 we mapped soil variability across both field trials using an electromagnetic induction meter (EM38).

The dry season soil sampling showed a statistically significant increase in the exchangeable cations Na⁺, Ca²⁺ and Mg²⁺ in one or more soil depths between the control and one or more basalt treatments at the Hesp site only. There was no statistically significant difference in any exchangeable cations at the Howe site or in K⁺ at either site. Cation exchange capacity significantly increased with increasing basalt application at all depths at the Hesp site, and there was no difference at the Howe site. Soil extractable alkalinity increased with basalt application rate at Howe and differences were statistically significant for the 25-50 cm depth. The same trend was evident at Hesp but the overall values and differences were substantially smaller and differences were not significant. Soil pH was significantly higher in the 50 t/ha basalt application compared to the 0 and 20 t/ha applications for all depths at Hesp. Soil pH increased with increasing basalt application rate at all depths at Howe but differences were not statistically significant.

Harvest results were consistent with the first harvest, with neither cane yield nor commercial cane sugar (CCS) significantly affected by basalt application. With increasing basalt rate there was an upward but statistically insignificant trend in yield at Howe and CCS at Hesp.

We constructed and installed new lysimeters for the 2020/21 wet season, with 16 lysimeters installed in each of the 0 and 50 t/ha treatments at both sites (32 lysimeters per site). A total of 126 samples were collected from Hesp (December 2020 – April 2021) and 119 samples from Howe (December – February 2021). Alkalinity in pore water samples was generally 10-fold higher at Howe (see a) than Hesp, and variability was very high. Howe receives very little rainfall and relies upon frequent irrigation. As a result, alkalinity generated from basalt weathering may not be detectable against the high background levels. Pore water alkalinity was significantly correlated to leachate pH (see b) and as such, the pH followed a similar trend. In contrast, Hesp experiences seasonally high rainfall hence is not irrigated and the leachate alkalinity (see c) values were generally very low. Early in the wet season there was a substantial increase in the pore water alkalinity in the 50 t/ha basalt treatment as compared to the 0 t/ha treatment, however this difference did not persist. The soil at Hesp also drains very quickly and rainfall events of at least 30 mm are required to capture pore water after the rainfall event. The result of this is that leachate generated by smaller rainfall events are missed in the sampling process. This could be mitigated using a different lysimeter system that could capture leachate generated during each rainfall event. The pore water pH at Hesp (see c) did not display much variation between treatments or between sampling occasions.

Pore water analyses from lysimeters in the Howe field trial showing a) alkalinity and b) pH, and pore water analyses and daily rainfall at the Hesp field trial showing c) alkalinity and d) pH at two rates of basalt application (0 and 50 t/ha per year over three previous years). Points are mean values and error bars are standard deviations.

Sampling soil pore water from a suction cup lysimeter at the Howe field trial in 2020 (Photo: Kalu Davies).

Mapping soil variability in the Howe field trial in November 2020 using an electromagnetic induction meter (Photo: Paul Nelson).

2021-2022

Following analysis of the results of the leachate sampling in the 2020/2021 wet season, we made several changes to the field work program. 

The field work program for the Hesp site included the fifth soil sampling in November 2021, fourth biomass sampling and yield recording and fourth basalt treatment application in November 2021. No work was carried out at the Howe site in 2021-2022. As in previous years, cane yield was unaffected by the treatments but the consistent small increase in commercial cane sugar (CCS) with basalt rate was significant this year. The concentration of most nutrients was not affected by the treatments except for a small increase in leaf P and K with increasing basalt concentration. Pore water alkalinity was not measured due to the low sample recovery and high variability encountered. 

Mean phosphorus and potassium contents of leaves and cane (% of dry matter) and commercial cane sugar content (CCS) at Hesp trial at the 2021 harvest, as a function of basalt application rate. Error bars show standard error of the mean.

To address the problems and questions raised last year (a low number of samples recovered from lysimeters, excessive variability and poor drainage at Howe, no effect of treatments on CDR) a new proposal was developed for Phase 2, with two components. The first component involved continuing the Hesp (but not Howe trial) field trial, modifying its design to a 2 x 2 factorial with two levels of basalt (0 and 50 t/ha/y) and 2 levels of soil pH (± lime in 2023), replacing the suction cup lysimeters with drainage flux meters and quantifying weathering using a ratio of Ti to mobile cations in the treated soils. The second component involved modelling of the geochemistry, pot trials to parameterise the model, and another field trial to assess potential of enhanced weathering of sugarcane mill ash for CDR. The proposal was submitted to the Australian Research Council for funding to leverage the LC3M contribution but the application was not successful so we have progressed only with the first component. The drainage flux meters have been purchased but did not arrive in time to be installed for the 2021/22 wet season. Soil samples have been sent to Yale University for the elemental analysis. We carried out a pot trial to assess the potential of raising soil pH to increase CDR from EW and the results indicated a positive effect.

2022-2023 

Prior to the onset of the 2022/23 wet season, we implemented changes at the Hesp field site as proposed in 2022. A liming treatment was added, passive-wick drainage flux meters (DFMs) were installed, and monitoring was terminated in the 20 t/ha/y treatments (except for sugarcane yield and nutrient content). Monitoring at the Howe site was terminated. The addition of the liming treatment involved incorporation of two 5m x 5m sub-plots within each of the existing main basalt treatment plots. Within each main plot, lime (2.5 t/ha) was applied to one sub-plot but not the other, modifying the experiment to a 2 x 2 factorial design. Soil sampling design was modified in 2022 to cover a standard fixed transect across the row and interrow.

Our field work included the fifth harvest, biomass sampling and soil sampling in October 2022, and the DFM installation in November 2022. Soil analyses for 2022 showed a positive effect of basalt on pH (see graph below), extractable Si, and exchangeable K and Mg, and a decrease in exchangeable Al (Table 1). Sugarcane yield was again not affected by basalt treatment, but there was an increase in leaf N, P, K, Mg and Cu contents and commercial cane sugar (CCS) content of the stalks (see Table 1). DFM leachate sampling got underway for the 2022-2023 wet season. Drainage volume, pH and alkalinity are being measured immediately and samples are being retained for ion analysis by ion chromatography. 

Table 1. Means of soil (0-10 cm) and plant variables in 2022 that were significantly (p < 0.05) affected by basalt application (0 and 50 t/ha/y). CCS is commercial cane sugar content.

Soil samples were sent to Yale for analysis of basalt weathering rate via the ratio of mobile cations to immobile Ti. There was a clear signal for high rates of weathering of basalt at both rates of application. Our results demonstrate the efficacy of soil-based mass balance approaches to enhanced weathering monitoring, with implications for the scalability of field-based enhanced weathering; and demonstrates the importance of determining in-situ weathering rates in a range of climates and geographies to predict the global potential of enhanced weathering as a CDR technology.

Mean soil pH (0-10 and 10-25 cm depth) at Hesp trial in 2018-2023, as affected by basalt application (0 and 50 t/ha/y). Error bars show standard error of the mean. The first sampling was at 0-25 cm depth, at a slightly different location to the subsequent samplings.

Installing drainage flux meters in the Hesp field trial in 2022 (Photo: Fred Holden).

2023-2024

Sampling and analysis of the deep drainage solute flux in the Hesp field trial was completed for the 2022/23 wet season. Carbon dioxide capture was calculated and the acids responsible for basalt weathering quantified. The results were reported in a paper titled ‘In-field carbon dioxide removal via weathering of crushed basalt applied to acidic tropical agricultural soil’, submitted for publication in 2024.

The sixth sugarcane harvest in the Hesp trial was conducted in November 2023, and cane yield increased in response to basalt application for the first time, from 80.0 t/ha in the control plots to 90.3 t/ha in the plots with basalt (@50 t/ha/a). 

The 2023 harvest was the final ratoon of the crop cycle, so the access tubes for the drainage flux meters were buried in preparation for cultivation before planting of a fallow legume crop over the 2023/24 wet season. Then, in December 2023, Tropical Cyclone Jasper struck the region with severe impacts, including the most extensive flooding for over a century. This disrupted farming operations in the field with the trial, and leachate sampling was not possible during the 2023/24 wet season. The field was sown to a legume fallow crop but it did not establish well, so legume was hand-planted over the intensively monitored sub-plots. Soil atmosphere sampling chambers were installed at 10 and 30 cm depth and were sampled between February and April 2024. CO₂ concentration was 615 - 17,060 ppm at 10 cm and 1335 - 22,465 ppm at 30 cm. We carried out legume biomass sampling and further soil sampling in April 2024 prior to final cultivation in preparation for the next sugarcane planting.

Cumulative mean deep drainage flux of bicarbonate (HCO₃^-) in the two treatments (squares) and fitted generalised additive models, showing 95% confidence interval (grey shading), over the 2022-2023 wet season, with cumulative rainfall and drainage and daily rainfall also shown.

Spreading lime on sub-plots in the Hesp field trial in January 2024 (Photo: Fred Holden).

2024-2025

In late 2024 basalt was applied to the Hesp site for the 7th time (7th year) and soil was sampled for the 7th time. Leachate is being sampled in the 2024/2025 wet season. Results will incorporate the effect of soil cultivation in 2023/24, in which basalt applied over the previous few years was incorporated. 

Two new enhanced weathering projects involving LC3M scientists started in 2024. In the first, a collaboration with UNDO Carbon, a field trial has been set up on James Cook University’s Cairns campus to examine the effect of soil management on CDR, soil properties and sugarcane growth. In the second, the Australian Government has funded a collaboration between James Cook University, The University of South Australia, Tropical North Queensland Drought Hub and Carbonaught, to run 7 new enhanced weathering field trials in diverse cropping systems of eastern Australia in 2025-2028.

Papers featuring our Australian trials

• Beerling, D.J., Leake, J.R., Long, S.P., Scholes, J.D., Ton, J., Nelson, P.N., Bird, M.I., Kantzas, E., Taylor, L.L., Sarkar, B., Kelland, M., DeLucia, E., Kantola, I., Müller, C., Rau, G. &  Hansen, J. (2018) Farming with crops and rocks to address global climate, food and soil security. Nature Plants, 4, 138-147. https://doi.org/10.1038/s41477-018-0108-y. Published 19 February 2018.
• Lewis, A. L., Sarkar, B., Wade, P., Kemp, S. J., Hodson, M. E., Taylor, L. L., Yeong, K. L., Davies, K., Nelson, P. N., Bird, M. I., Kantola, I. B., Masters, M. D., DeLucia, E., Leake, J. R., Banwart, S. A. & Beerling, D. J. (2021) Effects of mineralogy, chemistry and physical properties of basalts on carbon capture potential and plant-nutrient element release via enhanced weathering. Applied Geochemistry, 132. https://doi.org/10.1016/j.apgeochem.2021.105023. Published 21 June 2021.
• Green, H., Larsen, P., Liu, Y. & Nelson, P.N. (2024) Carbon dioxide removal via weathering of sugarcane mill ash under different soil conditions. Applied Geochemistry 165, 105940. https://doi.org/10.1016/j.apgeochem.2024.105940 Published 24 February 2024.
• Holden, F.J., Davies, K., Bird, M.I., Hume, R., Green, H., Beerling D.J. & Nelson, P.N. (2024). In-field carbon dioxide removal via weathering of crushed basalt applied to acidic tropical agricultural soil. Science of the Total Environment. 955,176568. https://doi.org/10.1016/j.scitotenv.2024.176568   Published 9 October 2024.
• Nelson, P. & Buss, W. (2024) Spreading crushed rock over farmland can remove CO₂ if we do it right. The Conversation https://theconversation.com/spreading-crushed-rock-over-farmland-can-remove-co-from-the-atmosphere-if-we-do-it-right-240303 Published 28 October 2024.