Lime application and soil acidity – the economics of current rules of thumb

Take home messages

• Use LimeAssist calculator to analyse the economic benefits of lime application, help calculate lime rate requirements and estimate the time for re-application.
• Current ‘rules of thumb’ widely used for lime recommendations are likely to cause subsoil acidity deep in the soil profile that can be costly to treat.
• Improvements in treatment and management of soil acidity can come from monitoring your own soil pH changes over time to determine acidification rates and responses to lime rather than reliance on rule of thumb applications.

Background

While soil scientists recommend treatment of soil acidity with lime to maintain the soil asset, agronomists aim to remove a range of constraints to crop production, and economists advise growers to make investment decisions based on maximising economic returns. In addition, growers can also have different goals depending on their stage of life, such as winding up or winding down operations. All of this can make advice giving and decision making complex.

However, a commonality between all these sources of advice is that agriculture causes acidity and sooner or later, liming will be necessary to correct that acidity. The difficult decision is when to prioritise liming over other inputs, especially when grower’s funds are limited. Advice is easy to give when funds are not limited, but this is rarely the case. Most often lime is competing with fertiliser and other inputs.

Advisors grapple with: do I address fertiliser constraints or apply lime? With some advisors, lime generally takes a back seat to fertiliser inputs and advice is often: apply lime when you can afford it. However, a neutral soil pH is essential for good root growth and effective uptake of fertilisers. Ignoring the need for lime will only decrease the effectiveness of fertilisers and other inputs.

Fertiliser usually provides a quick visual response, but the benefits of lime may not always be seen in the first year or not be large enough to be noticeable. In most cases, liming is about maintenance of soil pH, stopping yield losses and having options to sow many different crop species, rather than just acid tolerant ones. Growers can be disappointed if benefits are not seen immediately and think that the lime hasn’t worked. Lime application is considered expensive to apply (freight being the major cost) and it should be considered a capital cost, and so, returns are spread over a long period of time which makes calculation more difficult.

This paper discusses economics of liming and a new tool called LimeAssist to help producers and advisors make lime decisions based on calculation of net present value (NPV) using a discount flow model.

Impacts of soil acidity and lime

Despite investigation into alternative soil acidity amendments, lime remains the most effective ameliorant to increase soil pH (Li 2021).Advisors may need to justify to growers that acidity is limiting their crop production and that the benefits of liming outweigh the costs. This is especially true now as soil pH (CaCl2) targets have changed from 5.0/5.2 at 0-10cm to above 5.5 (about 5.8 measured in water) to avoid subsurface or subsoil acidity developing.

There are commonly known benefits to correcting soil pH with lime applications such as the increase in root growth and uptake of nutrients and water, reduction in aluminium and manganese toxicity, increase in availability of nutrients (especially phosphorus), and maintenance of soil biological functions. However, there are some less commonly promoted benefits. Condon et al. (2021) reports that soil acidity stress increases susceptibility to biological and environmental stresses like waterlogging, herbicide injury and disease.

Yield benefits from liming have been measured in a range of crops through paddock trials in the high rainfall cropping zone (HRZ), where the yield differences from limed and untreated plots using a randomised and replicated design. The mean results are graphed against known soil pH and mean responses are shown in Figure 1. The response curves are for surface application of lime in minimum till systems, ameliorating acidity in the top 10cm.

Figure 1. Mean relative yield by crop type as measured in SFS trials in HRZ.

At present SFS does not have a large dataset on the benefits incorporation of lime to develop a separate response curve. However, trials in other areas show that the incorporation of lime generally increases the speed of the yield increase (in eight trials versus four trials) with an average yield increase of 4% compared to surface lime application in the first three years. In ideal conditions, lime might move approximately 2cm per year, and will take five years to reach 10cm. Incorporating lime will amend the top 10cm, and therefore, yield increases are likely occur for up five years until surface lime catches up.

SFS trials have indicated that subsoil acidity at 10-20cm can also reduce yields by 3 to 4% (unpublished trials). These results are similar to those recorded at other regional locations as part of the subsoil acidity project (Li 2021).

Historical trends for lime applications

In the HRZ, the rule of thumb for lime application has been to apply 2.5t/ha about every 10 years. This rule of thumb was developed in the Acid Soil Action program in the late 1990s, about 20 years ago. Researchers who were involved at the time, indicated that rate and frequency was decided more on what they thought growers would apply rather than what was necessarily needed. While rules of thumb are good communication aids, the danger is they can become outdated as farming practices change or become inaccurate as advances in scientific understanding of different scenarios improves - both apply with soil acidity and liming. Common rules of thumb are shown in Table 1 along with their relevance today.

Table 1: Common liming rules of thumb developed several decades ago and their relevance today.

Ideally advisors/growers should look to develop their own rules of thumb for acidification rates and lime rates for their typical soil types and farming systems. Growers can do this by monitoring changes in pH over time following liming (acidification rate) and calculate how much lime (t/ha) is needed to change soil pH by 1 unit, 2 years after liming (soil pH buffering capacity) or have it measured by a laboratory. Alternatively, the LimeAssist online tool can help estimate lime requirements.

LimeAssist - new online tool

LimeAssist is a new lime decision tool designed to help advisors and growers calculate the economic returns from lime applications. It uses a simple discount cash flow model, which is the best analysis to use when evaluating lime benefits and costs over time. An Excel version of the economic calculator was first developed by Kerry Stott, an economist at Agriculture Victoria, which was then converted into an online version by the Centre for Excellence in Research Digital Innovation (CeRDI) team at Federation University.

It currently contains yield response curves to soil pH for wheat, canola, barley and pulses (faba bean and peas) (see Figure 1). In March 2022, acid sensitive and acid tolerant pasture response curves will be added. At this stage, the calculator only runs the economics of treating the surface soil (0-10cm) due to insufficient data to confidently calculate the yield penalty of deeper acidity. While other similar calculators exist in other states, this one is based on yield responses and acidification rates in the high rainfall zone of the southern cropping region, including southwest Victoria, Gippsland, SE of South Australia, Kangaroo Island and Tasmania.

LimeAssist is available at Lime Assist, and will be accessible through links in SFS, McKillop Farm Management and Kangaroo Island Productivity groups who have provided yield response data.

Using LimeAssist

The tool contains four steps to work through. Each step contains instructions and information buttons give explanations of what different input cells require and tips on use. The results can be saved, so that data are available for later use.

Step 1. Paddock information notes

The calculator caters for variable rate liming decisions, where one zone or multiple paddock zones can be selected. An estimation of soil pH buffer capacity is made based on organic carbon and clay content of soil and is used to calculate the pure lime rate required to achieve the desired pH change (target pH — current pH). The equation used to do this was developed by Aitken et al. (1990) and utilised by National Heritage Trust Australia (2001) in the National Land and Water Audit 2001 report. However, the equation used overestimates the lime requirement for hydrosols or ferrosols and so, results in these soil types should be treated with caution. Soil texture can be used to infer clay percentage which utilises relationships measured by Rab (2019). By inserting a farm delivery address, lime delivery costs can be calculated.

Step 2. Calculate lime costs notes

A lime supplier map helps identify nearby or convenient lime quarries and shows their measured moisture content and neutralising value. The contact details of the supplier are given, so the lime price can be obtained. A freight return distance cost calculation is provided at 10 cents/km/t. This distance and many of the default inputs can be overridden if required. The time frame of the analysis can be selected or left at the default period of 5 years. Likewise, discount and inflation rates can be overridden or left at the default settings of 7% and 2.2%, respectively. A summary of lime requirements is generated and shows the actual lime cost, delivered and spread.

Step 3. Calculate benefits notes

To calculate benefits, crop details need to be included (i.e. crop type, expected yield and price). If an acid tolerant crop was to be used other than wheat, such as lupins, oats or triticale, then select wheat. If another acid sensitive crop was used, then select barley or pulses (faba bean, field peas). Other than yield improvements, lime residual value (unused lime at the end of the analysis period) is calculated and valued. There is a lag time built into the lime responses so that benefits occur in the year following liming and peak in year two, where maximum pH change occurs before starting to fall as lime is used.

Step 4. Economic analysis notes

A graph of the cumulative net cash flow with inflation over the analysis period is generated and this shows the year where break even occurs. Two graphs show pH change over time with and without liming. These can be used to interpret where the pH falls below the trigger for re-liming, commonly 5.5. A summary of returns is given including the net present value and a modified internal rate of return. Selecting the full economic model provides further additional details on changes in yield and pH over time. More information on the discount flow analysis used or the soil technical relationships can be found in Stott et al. (2019) or in the calculator notes section.

Conclusions

Old ‘rules of thumb’ developed several decades ago but still widely used for lime recommendations may be inadequate and are likely to cause subsoil acidity deep in the soil profile. Improvements in treatment and management of soil acidity can come from monitoring your own soil pH changes over time to determine acidification rates and responses to lime rather than reliance on rule of thumb applications.

LimeAssist provides an easy way to calculate economic benefits of liming that can be used in helping to decide to lime or prioritise paddocks. The calculator does contain assumptions, and therefore decisions need to be made in conjunction with soil monitoring. The calculator makes it easier for an individual farm to develop its own rules of thumb, which should lead to improved soil acidity management.

Acknowledgements

The research undertaken as part of this project 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. The author also acknowledges the support of the Australian government through the National Landcare program.

References

Aitken RL, Moody PW, McKinley PG (1990) Lime requirement of acidic Queensland soils. I. Relationships between soil properties and pH buffer capacity. Australian Journal of Soil Research 28(5), 695-701.

Condon J, Burns H, Li G (2021) The extent, significance and amelioration of subsurface acidity in southern New South Wales, Australia. Soil Research 59(1), 1-11.

Li, G (2020) Final technical report: Innovative approaches to managing subsoil acidity in the southern grain region. NSW Department of Primary Industries.

Miller L (2016). Can incorporation of lime speed up yield response. In ‘Trial Results Victoria Edition 2016’. pp 88-92 Southern Farming Systems.

National Heritage Trust Australia (2001) National Land and Water resources Audit 2001 “Australian Agriculture assessment 2001, volume 1”. National Land and Water Resources Audit: Turner, Australian Capital Territory.

Rab A, 2019. Soil data for productivity improvement. Agriculture Victoria Research Milestone report 4: Final report (unpublished).

Scott BJ, Conyers MK, Poile GJ, Cullis BR (1999) Reacidification and reliming effects on soil properties and wheat yield. Australian Journal of Experimental Agriculture 39(7), 849-856.

Stott K, Crawford D, Norng S (2019) Quantifying the economic benefits of intensive point sampling and variable rate liming of 10 case-study paddocks in the high rainfall zone. Technical report: Agriculture Victoria, Melbourne.

Contact details

Lisa Miller
Southern Farming Systems
23 High Street, Inverleigh Vic. 3321
0488 600 226
lmiller@sfs.org.au