Whether or not soil amelioration will give a good return on investment largely depends on the yield boost (in absolute, not percentage, terms) that follows. The simplest calculation to assess the likely economic benefit from ameliorating soil constraints, is:

Margin = (yield uplift x grain value) – cost

Grain value and amelioration costs are relatively easy to source or calculate. Yield uplift is trickier to gauge but it is worth calculating because it makes a big difference to the financial outcome. For example, using Table 1, if the cost of spading is $220 per hectare and the expected yield uplift is achieved, the cost of amelioration is recovered in year two with returns of approximately $200/ha in that year. If, however, the yield uplift is only half of what was expected, it will take three years to recover the cost of amelioration with less than half the return in year three.

Table 1: Two scenarios with returns on spading costing $220/ha.

More detailed calculations are outlined in Assessing the profitability of soil amelioration and Tackling amelioration on variable soil types. Confidence around soil amelioration profitability is often a deciding factor to begin ameliorating or not; it is worth putting some effort into gauging likely yield uplift.

Yield uplift and its resultant profitability is a common way to prioritise where to start soil amelioration on the farm. Many growers start with those paddocks more likely to pay off in the short term. Paddocks with a better return on investment (ROI) are typically those with inherently better soil types but where current productivity is limited by easily identified and rectified soil constraints. There may be paddocks where you are confident of a substantial yield uplift that guarantees good returns. Common examples include a 1.5 tonnes per hectare yield uplift in cereals from using a modified one-way plough costing $60 to $80/ha, or a 0.5t/ha increase in canola from better establishment from using a wetting agent costing about $20/ha.

More thinking is needed when the outcome is less certain. Liming and ripping an acid, compacted, coloured deep sand can lift yield and profit markedly, but it is unrealistic to expect the same yield uplift on an acid, pale sand that holds less water or on a non-acidic deep loam where roots are already able to get to depth.

How to gauge yield uplift

Yield uplift depends on the constraint, how severe it is and a realistic appraisal of how well you can ameliorate it. Yield uplift will be specific to your soil type, farm and amelioration capability. The first few attempts at amelioration can be of lower quality with a suboptimal yield uplift but tends to improve with experience.

On-farm experience and ability

Many growers have a sense or ‘gut feel’ when soils are not performing as well as they expect or as well as they used to. This is valuable information that should not be ignored just because it is subjective. It is a starting point to investigate further and look for more objective information on what the yields could be.

Also consider issues that may arise during or after amelioration such as bringing up stumps, rocks or sodic soil to the surface, stimulating weeds, or poor establishment. If you do not have the inclination or ability to deal with these issues the yield uplift will not be as good as expected. Post-amelioration management is particularly important as yield uplift is only relevant if the crop establishes well.

Guides and trial data

Research trial data and published guidelines, such as Table 2, can be a sound guide of potential yield uplift. Use them to check your thinking only. They are a guide rather than gospel and should not replace data you have from your own farm. The more severe a soil constraint is and the better the amelioration job, the bigger the yield uplift is likely to be. On the flipside, poor amelioration will result in suboptimal uplift and there will be no yield uplift at all if ameliorating misdiagnosed constraints.

Table 2: Cereal crop yield response to amelioration (t/ha) on water repellent soil.

Source: adapted from Table 5 in Davies et al. 2019.
* Very high response on an extremely repellent site.

Further examples of yield uplift are outlined on pages 33 to 34 of Tackling amelioration on variable soil types. Use the iLime app to check the ROI from liming, and ROSA (Ranking Options for Soil Amelioration) to estimate the yield potential after fixing both single and multiple constraints for the next 10 years. Contact DPIRD for a copy of ROSA.

Results at nearby farms on similar soil types

If other growers in your area have ameliorated similar soil types, their uplift numbers will be more relevant than general trial data. The further away and the more different the soil types that have been ameliorated, the less relevant the outcomes will be to you.

Yield maps

Compare high-yield/low-yield years in areas with soil constraints

Use a good year with high yields and no adverse effects on yield – such as frost or a dry spring – as an indicator of what areas can yield when everything goes well. Compare it with another year when moisture, especially in spring, was the main limitation. The good year shows what the country can do; the dry year shows how different areas stack up when the pressure is on. Data from the good year can guide what you might expect if the constraint is ameliorated.

Yield changes over time

Look at how yield has changed in paddocks over time. Areas where yields have declined or failed to increase at the same rate as other areas probably have yield uplift potential. Areas with higher yields show what you might expect if the constraint is ameliorated. For example, acidity is a silent killer. It may not be obviously reducing yields from one year to the next but, even with all the vagaries of seasonal variability, the impact is clear in the longer term (10 to 20 years).

Areas that used to be more productive are evidence that the soil is inherently capable of producing more than it is. This is proof yield uplift is possible, which in turn increases confidence and decreases the risk of ameliorating.

Test it out

The best way to assess the yield uplift potential from soil amelioration is to test it out on your farm. Some activities might appear profitable in research trial plots, on someone else’s farm or in someone else’s business, but the numbers might not translate to your farm, your capabilities and your business.

Borrow a machine, use a contractor, or if you already have the equipment do a small test area to see how it responds. This is particularly important in marginal profitability situations because profit comes from less cost – not wasting dollars on ineffective amelioration – as well as from more yield.

Risk vs return

Soil amelioration comes with risks that can reduce yield uplift. Expensive machinery, complex operations on variable soil types and expensive activities have a greater risk of mistakes than cheaper and easier options. For example, addressing non-wetting surface soil with wetters or cross-row sowing, as opposed to delving. Poor amelioration can cause yield penalties for many years, significantly reducing or eliminating the profitability of that amelioration activity. Paddocks where post-amelioration establishment and trafficability are likely to be poor can also dampen potential yield uplift.

However, good amelioration and post-amelioration management can mean a yield and profit boost for multiple years. Deep ripping in Nyabing saw an average net benefit of $106/ha over two years, spading led to profits of just under $200/ha in Brookton, while lime trials in the north-eastern wheatbelt had a cumulative net benefit of $800 to $1100/ha after seven seasons of wheat.

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 (DPIRD), with GRDC investment.

References

Davies S, Betti G, Edwards T, McDonald G, Hall D, Anderson G, Scanlan C, Reynolds C, Walker J, Poulish G, Ward P, Krishnamurthy P, Micin S, Kerr R, Roper M & Boyes T 2019, ‘Ten years of managing water repellent soils research in Western Australia – a review of current progress and future opportunities’, GRDC Update Papers (GRDC Codes DAW00244, DAW00242, DAW00204 and CSP00139).