Wathingarra Future Carbon Project - 2023 Season Report

By Nathan Craig, WMG Chief Executive Officer

Key messages 

• Overall, Compost applied in 2021 significantly increased serradella establishment in 2023 compared to other amendments and amelioration methods.
• Dry matter production of serradella was low and similar between treatments due to below average seasonal rainfall.

Background

The soils of the West Midlands region can be characterised as deep sands that are often limited by nutrient leaching, soil water repellence, low nutrient levels, and compaction. This can be attributed to the low percentage of clay and low organic carbon levels, resulting in the poor holding capacity of nutrients, moisture, and soil structure. These soils typically hold 50 to 90 mm of moisture in the top metre and crops have historically been prone to haying off.

While soil amelioration practices have been adopted across the region to address the soil constraints such as acidity, compaction, and soil water repellence, increasing soil fertility through increasing soil carbon levels has been limited to the amount of organic matter returning to the soil in any given year. Increased soil carbon levels are correlated with improved soil fertility and soil functional resilience across all soil types in the world, but the challenge is how to effectively increase soil carbon levels on the sandy soils of the West Midlands region.

This trial is evaluating a range of organic amendment options available to farmers to increase soil organic matter. While most amendment or amelioration methods have been evaluated in isolation, this study will evaluate the stacking of many combinations of amendments and amelioration methods. It is expected that the synergistic benefits of a multiple amendments will lead to a long-term increase in soil carbon, health and crop grain yield.

Two novel amendments applied in this trial are frass and biochar. The use of black soldier fly (Hermetia illucens, BSF) to convert food waste into high-quality fat and protein meal leaves a by-product called ‘frass’. This by-product is made up of insect faeces, substrate residue and shed exoskeletons. Frass can have a rapid, positive impact on soil carbon and nitrogen (N) dynamics and microbial activity of the soil.

Biochar is a by?product generated from burning plant material under a limited oxygen supply. This process produces biochar, similar in appearance to charcoal, and has an estimated life in the soil of more than 1,000–2,000 years. Biochar has been previously evaluated for the sequestration of carbon and rehabilitation of degraded soils. The addition of biochar to soil can increase the stable organic carbon content of the soil and over time can increase the nutrient and water holding capacity of the soil. The combination of biochar and frass offer the opportunity to apply a shorter-term organic amendment (Frass) that reacts with the soil quickly along with biochar, which has longer term benefits and is generally more inert.

Methodology

The site was established at Wathingarra in 2021 on a deep brown sandy soil with high soil water repellence that is a typical cropping soil for the region. The site had previously completed its cropping phase of wheat-lupins-wheat for the 2018-20 period. The 2021 amendment treatments were applied using a Marshall spreader in non-replicated strips that were 12 metres wide and included compost, ironman gypsum (a mining by-product high clay content), and untreated control. At right angles to these amendments, four incorporation methods were applied following amendment application to incorporate the amendments and to reduce soil water repellence consisting of mouldboard ploughing or rotary spading to a depth of 25cm, shallow tillage with offset discs to 10cm depth, or an untreated control.

Each incorporation method contained two split plots, and one of these split plots had biochar (10 t/ha) and frass (4.4 t/ha) applied by hand spreading in 2022 and this was incorporated by the seeding process. There are 3 replications of each treatment in a split-plot randomised design. A simplified layout of the treatments is shown as Table 1.

The site was sown to wheat in 2021 and 2022 followed by Frano serradella in 2023 and 2024. Plant biomass was measured by cutting two 0.1m² quadrants randomly placed in each plot. They were combined into a composite sample for each plot and dried at 60 degrees Celsius for 72 hours. Samples were then weighed and multiplied by a conversion factor of 5 to get kg/ha. Plant establishment was measured by counting plants in two 0.1m² quadrants randomly placed in each plot and converted to plants per/m² by multiplying by a factor of 10.

The data was compiled in Microsoft Excel and analysed using the R statistical program (R Core Team, 2021) using the packages: NLME, plyr, dplyr, and tidyverse.

Results 

Results and Discussion

This study has found that serradella establishment was improved through the use of soil amendments and soil amelioration strategies that were implemented in 2021. The application of biochar/frass in the 2022 season appeared to have little effect on plant establishment in a year where low seasonal rainfall was received. There was a significant increase in plant count from the application of compost in the untreated control plots (Figure 1). Untreated control (no incorporation) Ironman Gypsum and Nil Amendment treatments recorded the lowest plant counts across all plots that were followed in the 2023 season. This is likely to be due to the dry seasonal conditions increasing the severity of high levels of sodium found in the Ironman Gypsum and soil water repellence for each treatment (respectively), however this was not directly confirmed in the 2023 year. An unexpected finding was that the application of compost significantly increased plant establishment irrespective of being incorporated by mouldboard plough, however, the mechanism for this was not identified in the 2023 year (Figure 2).

Biomass production of serradella at the Wathingarra site was not significantly impacted by the combination of multiple amendments and amelioration methods in the 2023 season (Figure 2). There was below average rainfall in 2023 where the Wathingarra site recorded a total of 330mm which limited biomass production and likely limited any evidence of treatment effects. The 2023 year was also an establishment year for the serradella crop which would have limited biomass production due to low plant numbers. Pasture growth and biomass production is dependent on plant density and allowing seed to set in the 2023 year will facilitate a thicker sward of serradella in the 2024 season. Grain yield was not measured for serradella in the 2023 year as this is a pasture legume that does not shed its seed easily.

Conclusion

The aim of this trial was to evaluate the stacking of multiple soil amendment and amelioration methods to improve soil health, crop growth, and grain yield. The second year of stacking three soil improvement methods showed no synergistic benefit to improving plant biomass production. Instead, there was confirmation of already proven methods of increasing soil productivity (i.e. mouldboard, compost) that increased plant establishment only.

This study will continue for the next four years to evaluate the long-term impact of the amendments on crop growth, grain yield, and soil health.

Acknowledgements

This site was supported by the Soil CRC and the Department of Primary Industries and Regional Development (DPIRD), and Future Green Solutions.