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Geo-referenced, catchment based soil pH monitoring in three agro-ecological regions of Western Australia.

Joel Andrew1, Chris Gazey2, Stephen Carr1, David York1, David Gartner2, Adam Clune2 and Breanne Best2

1 Precision SoilTech, www.precisionsoiltech.com.au Email joel@aglime.com.au
2
Department of Agriculture and Food, Western Australia. Email cgazey@agric.wa.gov.au

Abstract

Soil acidity is considered to be one of the main soil constraints to sustainable production in Western Australia. The ability to monitor regional scale soil acidity through temporal sampling of large numbers of geo-referenced sites was tested in different agro-ecological regions (Crop Variety Testing zones) of the WA wheatbelt. Over 700 geo-referenced sites in three regional catchments (Wongan Hills, Beacon and Wickepin) were sampled at least seven years after initial sampling. Soil samples were taken to 0.3 m at 0.1 m intervals at each site with each layer being analysed for pH (1:5 0.01M CaCl2). Acidification modelling estimated an average acid input of 2.04 kmol H+/ha.yr though corresponding soil pH decline was not observed. When initial (T0) and current (T1) pH was examined, positive average topsoil pH changes of approximately 0.3 units in all catchments were measured. Residual maximum likelihood (REML) analysis determined liming to be the only measured management practice influencing soil pH change with the impact of acidifying practices (i.e. fertiliser application, nitrate leaching and crop removal) likely to be masked by lime application. Additionally, no statistical difference was found to exist when resampled sites were compared to sites not previously sampled, indicating that historic soil fertility sites represent wider catchment soil pH and can be used to monitor soil pH change.

Key Words

Acidity, Monitoring, Soil pH, Lime, Avon River Basin

Introduction

Soil acidity has been shown to be a major constraint to agricultural production (Whitten et. al., 2000) and affects over one third of the Western Australian wheatbelt (Davies et al., 2006). Monitoring soil acidity was highlighted as a priority by the National Land and Water Resources Audit (2001) as a means to benchmark and measure progress towards soil condition targets. For example, the Avon Catchment Council (ACC) has set topsoil and shallow subsurface targets (pH 5.5 and 4.8 respectively) to be achieved by the year 2020 (ACC, 2005). Current soil acidity monitoring protocols endorsed by the NLWRA Audit Advisory Council (2007) recommend a five year period between sampling events (T0 and T1) when measuring soil pH change. Implications of this protocol are that a ‘time lag’ of at least five years will be created between establishing a monitoring programme (T0) and resampling (T1, T2...) to determine change in soil pH. Commercial soil sampling companies, such as Precision SoilTech, store geo-referenced location and pH data for growers after soil fertility sampling. Resampling geo-located sampling sites collected prior to 2003 will allow soil pH changes to be examined over the recommended five year time period, effectively removing the time lag effect.

This paper reports on a large scale soil acidity monitoring trial conducted in three regional catchments of Western Australia’s Avon River Basin. A commercial database of historical, geo-referenced, soil fertility test sites was analysed and sites selected for resampling to assess topsoil and shallow subsurface soil pH change. Additionally, the impacts of various land management practices on soil pH change are examined and progress towards ACC soil pH targets determined.

Methods

Database Analysis and Catchment Selection

Analysis of the Precision SoilTech (PST) commercial sampling database (excess of 100,000 geo-referenced sampling locations) provided the basis for this study. Spatial and attribute analysis of this database provided historic sampling location and soil pH data which was compared to current soil pH to determine soil pH change. Only sites which contained topsoil (0-10 cm) and shallow subsurface (10-20 cm) pH data and had been initially sampled prior to 2003 were included in this analysis. Sampling time T0 refers to data collected between 1998 and 2002 and sampling time T1 refers to current pH data collected in 2008. Sampling occurred in three Crop Variety Testing (CVT) zones which represent differences in annual rainfall and temperature (see Figure 1).

The number of sites within each sub-catchment in the CVT zones M2, M2 and L2 was calculated using Geomedia 6 GIS software (Intergraph) and catchments containing less than 150 sites were discarded. Soil Landscape Mapping Units (SLMU) (Schoknecht et. al., 2004) were used to determined major soil groups in each CVT zone and then compared to the soil groups within each sub-catchment represented by sampling sites (i.e. site located within SLMU polygon). The sub-catchment with the most soil groups in common with the CVT zone was selected. Catchments selected were Beacon (CVT zone L2; 61,000 Ha), Wongan Hills (CVT zone M2; 22,000 Ha), and Wickepin (CVT zone M4, 34,000 Ha) (Figure 1).

Sample Collection

Soil samples were collected using the Precision SoilTech sampling machine which consists of a utility mounted vacuum system to lift samples from the soil profile. All samples were collected using the same sampling method of bulking 10 cores over a 3 m x 10 m area at each sampling location. Topsoil and shallow subsurface was collected at sampling times T0 and T1. Subsoil (20-30 cm) samples were collected and analysed at sampling time T1 which can be used in future monitoring (data not presented). Each location was recorded using a Rinex Saturn H Box guidance computer in datum GDA94.

All sites in the Wongan Hills catchment were initially sampled in January 1999 with resampling occurring during the summer of 2006/07. Sampling sites in the Beacon and Wickepin catchments were separated into two groups to determine if growers from the PST database had different average soil pH than other growers in the surrounding catchment. These groups were termed; ‘Resample Group’ – sites retrieved from the PST database and previously sampled between 1998 and 2002, and ‘Control Group’ – sites that had not been previously sampled. Control Group sites were selected using SLMU’s and aimed to sample soil groups represented by the Resample Group. A total of 719 Resample group sites were resampled to determine T1 data; 287 sites at Wongan Hills, 240 sites at Wickepin and 192 sites at Beacon. A further 336 Control Sites were sampled in the Beacon and Wickepin catchments (138 and 198 sites respectively).

Paddock history for the ten years preceding the study (1998 to 2007) was collected at the time of sampling through a grower survey. Cropping (rotation and yield), fertiliser (product and application rate) and liming (source and application rate) data were recorded. Effects of land management were assessed. First, residual maximum likelihood (REML) analysis was carried out using Genstat 10th edition (VSN International, Oxford) to assess which management practices influenced soil pH change, and second, the Lime and Nutrient Calculator (DAFWA, 1999) was used to estimate the rates of acidification.

Figure 1. Map of the south-west of Western Australia indicating the Crop Variety Testing (CVT) areas and the location of catchments from which samples were collected for the Soil Acidity Monitoring trial. Rainfall regions are represented by H (High -450 to 750 mm), M (Medium -325 to 450 mm), and L (Low -<325 mm). Zones are signified by 1 (North), 2 (North central), 3 (Central), 4 (South central), 5 (South), 5E (South east) and 5W (South west).

Results & Discussion

Soil pH change

Overall increases in average topsoil and shallow subsurface soil pH were observed in the Beacon, Wongan Hills and Wickepin catchments (Table 1). Significant average topsoil pH increases of approximately 0.3 pH units were observed in all catchments since initial sampling. Linear mixed modelling of the factors soil type, nitrogen application, lime application and cropping yield indicated that lime application was the only factor to significantly influence soil pH change with no difference measured in amount of lime applied between catchments. Modelled rates of acidification indicate that pH decrease induced by nitrate leaching (1.17 kmol H+/ha.yr), application of nitrogenous fertilisers (0.38 kmol H+/ha.yr) and crop removal (0.49 kmol H+/ha.yr) would have been observed if lime had not been applied. In the three catchments surveyed, lime application has not been sufficient to increase topsoil pH to levels above the targets set by the ACC (i.e. pH 5.5). Hence, liming should continue until topsoil pH is increased to above pH 5.5 and then be application rates reduced to a rate that will maintain soil pH, as modelled acidification rates show soil pH will decline if liming is not continued.

Shallow subsurface soil pH increases varied across CVT zones with the largest change of 0.4 pH units observed at Wongan Hills, a slight pH increase measured at Wickepin (0.21 units) and no change measured at Beacon. Lower annual rainfall in the L2 CVT zone may explain the lack of downward movement of lime in the Beacon catchment.

Table 1. Initial (T0) and current (T1) average topsoil and shallow subsurface soil pH from Beacon, Wongan Hills and Wickepin regions of the Avon River Basin.

 

Topsoil (0–10 cm) Avg. pH

 

Shallow subsurface (10–20 cm) Avg. pH

Location

T0

T1

T1-T0

T0

T1

T1-T0

Beacon

4.97

5.25

0.28

 

4.94

4.97

0.03

Wongan Hills

4.69

5.02

0.33

 

4.49

4.87

0.38

Wickepin

4.78

5.10

0.32

 

4.74

4.95

0.21

               

LSD (0.05)

0.10

0.10

n.s.

 

0.15

n.s.

0.10

Impact of liming on soil pH change

High rates of lime application were shown to significantly increase topsoil and shallow subsurface pH when compared to WA state average (≤1t/ha) and no lime (0t/ha) applications in the Wongan Hills catchment. Lime applied in the two seasons prior to the study explained the lack of topsoil pH decrease and slight soil pH increase in the shallow subsurface where 0t/ha lime had been applied (Table 2). Soil pH increases similar to those observed where ≥2t/ha of lime had been applied (0.8 and 0.6 pH units in the topsoil and shallow subsurface respectively) are needed to achieve ACC soil pH targets in a seven year period. Therefore, initial lime applications should be at rates between 2-4 t/ha to achieve topsoil pH targets and then be reduced to maintenance rates.

Table 2. Influence of lime application rates on topsoil and shallow subsurface pH change at Wongan Hills. *Note: Lime application rates are shown as 100% NV.

Lime Application*

( t/ha )

Topsoil (0–10 cm)

 

Shallow subsurface (10–20 cm)

T0 pH Avg.

T1 pH Avg.

Avg. pH change

 

T0 pH Avg.

T1 pH Avg.

Avg. pH change

0

4.71

4.75

0.04

 

4.63

4.78

0.15

≤ 1

4.67

4.97

0.30

 

4.42

4.82

0.40

2 – 4

4.82

5.65

0.83

 

4.53

5.13

0.60

               

LSD (0.05)

-

0.16

0.18

 

-

0.16

0.15

Historical sites as benchmark for soil pH

No significant difference was shown to exist in current topsoil pH between Resample and Control groups in the Beacon and Wickepin catchments indicating that historic sampling data can be used as T0 data for soil pH monitoring. No difference in shallow subsurface pH was found between the two groups in the Wickepin catchment. However, differences were observed in the Beacon catchment with higher average soil pH being observed in the Control group (Table 3). This difference in shallow subsurface requires further investigation though is thought to be caused by sampling dissimilar soil types due to low SLMU resolution.

Table 3: Comparison of mean topsoil and shallow subsurface pH between Resample and Control groups.

Sampling Group

Topsoil (0–10 cm) Avg. pH

 

Shallow subsurface (10–20 cm) Avg. pH

Beacon

Wickepin

 

Beacon

Wickepin

Resample (T1)

5.25

5.10

 

4.97

4.95

Control (T1)

5.13

5.09

 

5.29

5.09

           

LSD (0.05)

n.s.

n.s.

 

0.14

n.s.

Conclusion

Using historical soil pH data as a benchmark to which current soil pH data was compared has allowed changes in topsoil and shallow subsurface pH to be observed in the Beacon, Wongan Hills and Wickepin catchments of the Avon River Basin. The application of lime was shown to increase topsoil pH in each catchment by an average of 0.3 pH units though soil pH levels still remain below the target pH levels set by the regional NRM organisation. Management practice of liming and rates at which lime was applied were shown to be the only factors to positively increase soil pH.

This work has demonstrated that historical data, similar to that used in this study, has great potential to be utilised by regional monitoring programmes as a means of measuring soil condition change without time lag effects. Availability and accessibility of such data should be investigated prior to the establishment of monitoring projects.

Acknowledgements

This project is funded by the Avon Catchment Council with investment from the Western Australian and Australian Governments through the National Action Plan for Salinity and Water Quality.

References

Anderson, W. and Garlinge, J. (2000). The Wheat Book: Principles and Practice. The Department of Agriculture, Western Australia. Perth, Western Australia.

Avon Catchment Council (2006). Avon Natural Resource Management Strategy. Northam, Western Australia

Davies, S., Gazey, C., Bowden, B., van Gool, D., Gartner, D., Liaghati, T. and Gilkes, B. (2006). Acidification of Western Australia’s agricultural soils and their management. In Proceedings 13th Australian Society of Agronomy conference, Perth, Western Australia, September 2006.

Department of Agriculture and Food Western Australia (1999). Lime and Nutrient Calculator. Perth, Western Australia

Intergraph (2007). Geomedia version 6. http://www.intergraph.com.au/

National Land and Water Resources Audit (2001) Agriculture in Australia: A summary of the National Land and Water Resources Audits Australian Agriculture Assessment 2001. Department of Environment. Commonwealth Government of Australia, Canberra

Schoknecht, N., Tille, P. and Purdie, B. (2004). Soil-landscape mapping in south-western Australia. Overview of methodology and outputs. Resource Management Technical Report 280. Department of Agriculture and Food Western Australia, Perth, Western Australia.

VSN International (2007) Genstat, 10th edition. Lawes Agricultural Trust. http://www.vsni.co.uk

Whitten, M., Wong, M. and Rate, A. (2000). Amelioration of subsurface acidity in the south-west of Western Australia: downward movement and mass balance of surface-incorporated lime after 2–15 years. Australian Journal of Soil Research. 38:711-728.

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