It is well-known that lime moves very slowly in the soil and, if subsoil acidity is an issue now, lime must be incorporated to fix it – it is too late to top-dress lime as a maintenance strategy.
But if topsoil and subsoil pH are okay now, long-term research is finding that being on the front foot and top-dressing lime now helps slow soil acidification through the profile.
Long-term lime experiments
In 1994, the Western Australian Department of Primary Industries and Regional Development established a lime trial at the Wongan Hills Research Station. The sandy soil had a topsoil pH CaCl2 of about 5.0, midsoil pH (10 to 30 centimetres) of about 4.2, and deep subsoil (below 30cm) of 4.8 or greater. Initial soil data was only collected to 40cm depth.
The trial initially consisted of five rates of top-dressed coastal lime sand. Trial plots were later split into various other lime rates to see the long-term impact a range of liming strategies would have on soil acidity. Some liming strategies had an additional 1.5 tonnes per hectare of lime in 1998, and 3t/ha of top-dressed lime in 2014.
This article reports the results from four of the five lime scenarios (Table 1), sampled in 2018 and compared with the initial soil pH measured in 1994:
- no lime applied over 24 years (black);
- a low lime rate strategy (red) with up to a total of 4.5t/ha over 24 years;
- moderate-rate liming strategy (orange) with up to a total of 6.5t/ha over 24 years; and
- higher-rate liming strategy (blue) with up to a total of 8.5t/ha over 24 years.
Table 1: Selected lime rates applied at Wongan Hills since 1994.
| 1994 | 1998 | 2014 | Total lime applied over 24 years (t/ha) | |
|---|---|---|---|---|
Nil | 0 | 0 | 0 | 0 |
| Low rate (t/ha) | 0 | 1.5 | 0 | 1.5 |
| 0 | 1.5 | 3 | 4.5 | |
| Moderate rate (t/ha) | 2 | 0 | 0 | 2 |
| 2 | 1.5 | 0 | 3.5 | |
| 2 | 1.5 | 3 | 6.5 | |
| High rate (t/ha) | 4 | 0 | 0 | 4 |
| 4 | 1.5 | 0 | 5.5 | |
| 4 | 1.5 | 3 | 8.5 |
Results
Low lime rates had little impact
By comparing the soil pH profiles over time from the liming strategies, we can see how higher lime rates prevented subsoil acidity from getting worse.
Plots that received one-off lime in 1994 and no further treatment (Figure 1) had an improved topsoil pH but little pH change from 10 to 30cm. The higher lime rate (4t/ha) maintained a higher pH in the 30 to 40cm depth.
Figure 1: Soil pH at sites limed in 1994 that received no further lime and were tested in 2018.
Figure 2a compares plots that received a further 1.5t/ha in 1998 (on top of the starting lime rates in 1994, see Table 1). The highest lime rate (5.5t/ha by this stage) improved soil pH down to 50cm.
The orange treatment (3.5t/ha) had improved pH but it was still less than 4.5 from about 10 to 30cm depth. The lowest lime rate, 1.5t/ha total, improved topsoil pH but soil pH at depth was declining.
The further 3t/ha of lime applied in 2014 raised topsoil pH above 6.0 in all three scenarios (Figure 2b).
However, only the highest lime rate strategy kept soil pH above 5.5 in the topsoil and 4.8 in the subsoil (except for 20 to 30cm). There is little difference in subsoil pH between the moderate (6.5t/ha) and low (4.5t/ha) lime rate scenarios.
Playing catch-up
This data shows that if the soil has already acidified, even applying lots of lime at the surface (red scenario, applying 3t/ha in 2014) will do little to treat subsurface pH.
Playing ‘catch-up’ is not an option and mixing the lime into the subsoil is the best option to treat subsoil acidity.
Figure 2: (a) Soil pH in 2018 after an additional 1.5t/ha in 1998; (b) Soil pH in 2018 after a further 3t/ha of lime applied in 2014. The total amounts of lime applied over 24 years are listed in the graph legend. The starting soil pH from 1994 is included for comparison.
Figure 3: Amount of residual lime in the topsoil where a cumulative of 8.5t/ha lime was applied over 24 years at Wongan Hills.
Preventing subsoil acidity
Repeated surface lime applications can prevent subsoil acidification via two mechanisms.
First, given enough time, lime moves slowly down the soil profile.
Alkali movement in the study caused only approximately 0.5 unit change in soil pH and only in the 10 to 20cm depths over the 24 years (noting the sites had been subjected to ongoing acidification from continuous cropping).
Second, lime can react with acidying protons (released from nitrogen fertiliser), preventing them from moving deeper into the profile and acidifying the subsoil, essentially counteracting acidity before it even starts.
Residual lime
Once pH rises above 6.8, lime dissolution slows, leaving undissolved lime stratified at the soil surface.
In the trials, the lime stratified from zero to 4cm, with most of the lime staying in zero to 2cm (Figure 3). This residual lime is not ‘wasted’.
It can react over time with acidifying protons to stop acidification, but it is not as effective or profitable as it could be if it were incorporated.
Summary
Over time and compared to the zero lime plots, repeated surface applications of lime at higher rates (total 8.5 kilograms/ha) over 24 years:
- lifted topsoil pH from a pre-trial pH of 5.0 to 6.4;
- significantly increased subsurface soil pH in the 10 to 20cm layer from 4.2 in 1994 to 5.3 in 2018; and
- increased soil pH in 20 to 30cm from 4.2 in 1994 to 4.5 in 2018, though this increase was not significant. pH at 20 to 30cm in the zero lime plots reduced to 3.9.
The highest lime rate maintained soil pH above 4.8 below 30cm depth. The zero lime plots acidified from pH 4.9 to 4.1 from 30 to 40cm depth.
Conclusions
If the subsoil is already acidic (less than 4.8), lime incorporation is necessary to treat the problem.
If the soil has not acidified, top-dressing lime can protect the subsoil from further acidification. Most of the improvement will happen in the top 20 to 25cm.
Soil acidification rates will vary at every site based on soil type, starting pH, cropping rotation, fertiliser use, and so on. Testing soil pH in at least 10cm increments to more than 30cm is necessary to know whether incorporation is needed or not.
References
Azam, G and Gazey, C 2021. ‘Slow movement of alkali from surface-applied lime warrants the introduction of strategic tillage for rapid amelioration of subsurface acidity in south-western Australia’. Soil Research, 59, 97–106.
This article was produced as part of the GRDC ‘Maintain the longevity of soils constraints investments and increase grower adoption through extension – western region’ investment (PLT1909-001SAX). This project is extending practical findings to grain growers from the five-year Soil Constraints – West suite of projects, conducted by the Department of Primary Industries and Regional Development, with GRDC investment.