Longevity of subsoil amelioration in a high rainfall zone: a five-year overview

Take home messages

• Subsoil amelioration can increase yields and improve soil structure in dense clay subsoils, but the effectiveness of different ameliorants and their longevity is poorly understood.
• Deep applied animal and plant matter treatments increased yields by 13-53% in the first three crops, with variable results in the fourth crop, and no response in the fifth crop, yet NDVI showed benefits to growth in all crops
• Application of manure and lucerne into the subsoil produced the highest and most consistent yield responses.
• The improvements in yield response appeared due to greater water storage and drainage.

Background

Australian cropping systems are often limited by poor soil physical and chemical properties associated with sodicity, acidity, and salinity. Sodicity is estimated to cost up to AUD $1.3 bn y-1 in lost wheat production alone (Orton et al. 2018). Sodosols are a type of texture contrast soil (i.e., heavy clay subsoils overlain by sandy topsoils), with a subsoil having an exchangeable sodium percentage (ESP) greater than 6% on the cation exchange capacity (CEC). The subsoils of sodosols are prone to dispersion and waterlogging, which causes plant nutrient deficiencies, and intensifies the effects of drought (Adcock et al. 2007). Sodosols cover 23% of the Tasmanian land area (Doyle and Habraken, 1993) and are a major agricultural soil due to their suitable position in the landscape for grazing and broadacre cropping (Cotching et al. 2001). However, intensive cropping activities can exacerbate subsoil constraints by reducing topsoil structure and organic carbon (OC) reserves (Cotching et al. 2001). As a result, topsoil bulk density is increased and macro-porosity, and water infiltration are reduced. These properties starve crops of oxygen, nutrients, and water (Zhou et al. 2007), and ultimately decrease grain yield. They are also difficult and costly to remediate once degraded, resulting in further susceptibility to waterlogging, and drought over time.

Subsoil amelioration (SSA) has demonstrated crop yield benefits of up to 63% in dense subsoils in high rainfall zones (HRZ) in southwest Victoria (Gill, 2008). The SSA process is achieved by slotting high rates of organic amendments, such as poultry manure litter, at depth into the clay subsoil. While productivity gains have been attributed to enhanced nutrition (Celestina et al. 2018; Uddin et al. 2022), significant improvements in soil structure through reduced bulk density and increased macro-porosity and saturated hydraulic conductivity have also been recorded (Gill et.al. 2009). Improvements in soil structure encourage deeper root growth in the clay horizon and increases in plant available water capacity have also been well documented (Gill et.al. 2009; Armstrong et al. 2017). However, it is unclear which amendments work best, how long nutritional benefits to crop growth persists, or when improvements to soil physical properties begin after initial treatment. Nor is it clear under which rainfall zones and soil types the most profitable responses to amelioration occur. These uncertainties may restrict farmers’ confidence for adoption of subsoil amelioration

practices. Therefore, this study aims to give an overview of the key agronomic effects of subsoil amelioration on a Sodosol in northern Tasmania. Specifically, this study reports on the yield, NDVI, and differences in soil water retention after five years of continuous cropping following a once-off application of several different forms of common farm available amendments (organic and in-organic).

Methods

Site, treatments, and experimental design

The trial site was established at Nile, northern Tasmania (41°39’32.3” S, 147°18'17.3" E) in 2018. The soil type is a Brown Sodosol with a clay loamy fine sand topsoil generally 20–28 cm deep overlying a bleached clayey fine sand A2 horizon of variable thickness (commonly 5–12 cm) which overlies a B horizon with heavy clay texture and high bulk density (Table 1). The topsoil was acidic and the subsoil was moderately sodic (13–22 % ESP). There were also high levels of exchangeable Mg of over 50% of the CEC in the subsoil (data not presented).

Table 1. Background soil properties of subsoil amelioration trial, Nile, 2018. Electrical conductivity (EC), organic C (OC), mineral N is sum of nitrate-N + ammonium-N; ECEC, effective cation exchange capacity (ECEC) is sum of exchangeable Ca, Mg, Na and K; exchangeable sodium % (ESP); bulk density (BD).

Climate data was recorded at the Australian Bureau of Meteorology weather station number 091311 at Launceston Airport (2004 - present), 14 km from the trial site. Mean maximum temperature ranged between 12 and 25°C and mean minimum temperature was between 2-11°C (2004-2023). Annual rainfall is on average 619mm (2004-2023) and ranged between 487mm (2019) and 804mm (2020) (Figure 1).

Figure 1. Variation in rainfall during the five growing seasons between January 2018 and December 2022.

Treatments consisted of 1) “Control” - no ripping/no amendment (standard commercial practice) and 2) “Deep rip” deep ripping with no amendment (secondary control). 3) “Deep manure” - poultry manure pellets (20 t/ha), 4) “Deep green chop” - lucerne pellets (20 t/ha), 5) “Deep straw and nutrients” - wheat straw pellets (20 t/ha) + NPKS (1.1 t/ha) (+ additional N in crop), 6) “Deep nutrients” - NPKS 22-8-5-8 +Zn, Cu granular (+ additional N in crop) (1.1 t/ha), 7) “Deep manure fresh” - fresh poultry manure (15 t/ha), 8) “Deep gypsum” - granular powder (5 t/ha), 9) “Surface animal manure” - poultry manure pellets (20 t/ha), and 10) “Surface green chop” - lucerne pellets (20 t/ha). Treatments were applied on 16th of April 2018. Treatments with “surface” in the name were applied by lifting the rip tynes and dropping the amendments on the surface, and treatments with “deep” were slotted at a depth of 30–35 cm forming a concentrated ‘sausage’ of material. The slotting operation used a custom-made subsoil manure ripper (see acknowledgements) with two rippers spaced at 80 cm. Plots were arranged in a randomized block design and were 2.5 m wide x 25 m long with four replicates.

Trial management and measurements

Field trial history and management is summaries in Table 2. Dry matter cuts were taken at growth stage (GS) 65 (dry matter, total N and tiller density) and at maturity (dry matter, grain, total N and tiller density). Normalized difference vegetation index (NDVI) measurements were collected at GS50 and 65 in all years, additional measures before and after this period were acquired in the last three crop seasons. A KEW plot harvester was used to harvest the whole plots for grain yields. After 5 years, soil cores (72mm internal diameter x 61mm height) were collected from the A2 horizon (because this was immediately above the amendment band). The soil water retention function was determined using the KuPF apparatus (UGT, Germany; ICT International, Australia) between saturation and -80 kPa, supplemented with ‘dry end’ retention data determined by pressure chamber data at -1500 kPa. Differences between treatment effects were analysed by ANOVA using R version 4.2.1 (2022/6/23). The least significant difference (l.s.d.) was calculated at P =0.05 for testing differences between treatments.

*Not reported

Table 2. Summary of crop history for the 5-year Nile subsoil amelioration trial. The first crop was planted in May 2018, however, this failed due to very strong competition from group B resistant ryegrass exacerbated by a very wet July.

Results and discussion

Treatment effects on yield

All manure and deep applied nutrient treatments generally increased yields compared to the control, but this decreased over time, whereas overall yield was strongly related to seasonal effects. Initially, there was a very strong crop yield response to surface manures (surface green chop and surface manure) compared to the control, resulting in a 61% increase in the first crop, 20% increase in the second crop, no significant response in crop three, and an 11% increase in crop four on average. The response to deep manures (deep green chop, deep manure, and deep manure fresh) was similar, but less marked in the first crop and more sustained with a 53% yield increase in crop one, a 27% increase in crop two, and 13% increases in crops three and four on average. The deep nutrients treatments (deep nutrients and deep straw and nutrients) were slightly less effective compared to the deep manures with a 51, 25, and 10% average increase compared to the control with no significant effects in crop four or five.

Figure 2. Different treatment effects on grain yield from a mixed cropping system after surface and subsoil amelioration treatments were applied in early 2018. Significant differences (P<0.05) to the control treatment are indicated by different letters (n=4).

Treatment effects on NDVI and nitrogen balance

The ten treatments were aggregated into four groups following similarities in yield response (Figure 2). Overall, NDVI followed similar patterns to grain yield when including the resistant ryegrass before it was slashed, but significant effects were observed until the fifth crop (Figure 3). Thus, indicating remaining N in the subsoil from deep placed manure and nutrient treatments. Further research is required to determine the longevity of the N remaining in the deep bands.

Figure 3. Treatment summary of NDVI results relative to control group at GS50-65. Treatments (n=4) were averaged as follows: Control (control, deep rip, and deep gypsum), Surface manure (surface green chop and surface manure), Deep manure (deep green chop, deep manure, and deep manure fresh), Deep nutrients (deep nutrients and deep nutrients + straw).

Treatment effects on soil physical properties – soil water retention

Soil physical properties were assessed in terms of water holding capacity at various points along the soil water retention curve in the A2 horizon (Figure 4). Overall, no significant differences were observed except for the deep green chop treatment (i.e., deep applied lucerne pellets), where the drainage (i.e., drainable porosity), and field capacity (i.e., readily available + poorly available water) all significantly increased (P<0.05). This is attributed to an increase in the number of macro- and meso-pores (i.e., 75-5000µm and 30-75µm diameter). However, there was no change to permanent wilting point (i.e., unavailable water storage). Therefore, this increase in macropores will likely improve drainage and oxygenation but not infiltration.

Figure 4. Comparison of average drainable porosity (white), readily available water -10 to -50 kPa (light grey), poorly available water -50 to -1500 kPa (dark grey), unavailable soil water (permanent wilting point) >1500 kPa (black) between treatments: measured as a proportion of the total soil water holding capacity for the A2 horizon. Error bars represent ±1 standard deviation and * indicates significant difference to the control (n=4).

Conclusion

Overall, subsoil amelioration with animal, plant matter and (NPKS) fertilisers significantly improved grain yields on a duplex waterlogged prone soil for four out of the first five years in a mixed cropping system in the HRZ of Northern Tasmania. Surface applied animal and plant manures also improved yields initially, but effects were inconsistent after two years. NDVI data demonstrated that crops continued to respond positively to deep applied nutrients (organic or mineral) after five years. Yield benefits were primarily attributed to nutritional benefits from treatments containing additional nutrients; however, improvements to soil water retention were also observed in the deep green chop treatment containing lucerne pellets. The most effective treatments for highest and most consistent yield benefits were deep manure pellets, deep lucerne pellets and deep manure fresh. Further research is required to determine return on investment, soil N recovery and the longevity of the N remaining in the banded layers.

Acknowledgements

The research undertaken as part of this project (Project DAV000149) is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support. We thank the late Michael Chilvers, Fiona Chilvers, and Rob Bradley for assistance with land on which the trial was conducted, and Bill Field and Bruce Dolbey for technical assistance. John McPhee, the Tasmanian Institute of Agriculture and Tasmanian Agricultural Productivity Group are acknowledged for assistance and loan of the subsoiler, built through a CFOC Innovation Grant (INNOV-145).

Useful resources

Subsoil amelioration - update on current research

References

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Contact details

Ms Bianca Das
Tasmanian Institute of Agriculture, Building S, Level 2
Old School Road
Newnham, 7248
Tasmania, Australia
Phone: 03 6226 2910
bianca.das@utas.edu.au
@btdasnz