Revisiting soil tests to lift their value

Key points

  • Independent soil test analysis service looks to cut crop inputs waste
  • Presentation and use of soil tests said to be little altered since the 1970s
  • The service provides spatial information showing input needs of whole farms using Google Earth maps 

While many growers have a large number of soil test results, one nutrient management specialist suggests they could be much better used to guide expenditure on crop inputs

An innovative way of using soil test results is helping to improve decisions about lime, nutrient and gypsum rates to maximise the return on every dollar invested in these inputs. 

Photo of ute and man

Many growers have a supply of soil testing results from across their farms. Nutrient management specialist Wayne Pluske of Equii suggests growers could improve how they use the results to make better decisions about costly inputs.

PHOTO: Precision Soiltech

The independent service, developed by Western Australian-based nutrient management specialist Wayne Pluske of Equii, consolidates, interprets and maps a property’s full collection of soil test results, both historic and new.

It aims to realise the full potential of soil tests for cost-effective input decisions by modifying what inputs are bought and where they are applied, whether precision agriculture has been adopted or not.

Mr Pluske says the normal presentation of soil tests has not altered much since the 1970s despite massive improvements in every other aspect of farming. 

“Soil testing has not kept pace with mapping and variable-rate technologies, which is a travesty because soil testing should be the integrator for the use of such technologies to increase profitability,” he says.

“It’s ridiculous that decisions about our most expensive inputs are based on a 1970s approach.

Mr Pluske says consultants and growers asked him to set up an analysis service because they wanted more information from their tests.  

He says many growers are saying the fertiliser recommendations they are given are unchanged even when their soil test results are changing: “They’re thinking their soil test interpretations might be more focused on selling them fertiliser than on better use of the nutrients in those fertilisers. They want separation of product and advice.”

Technical farm consultant Geoff Fosbery of ConsultAg says many growers believe input recommendations are biased toward sales, even though a lot of the response curve data is collated from sources such as the Department of Agriculture and Food, WA, and CSIRO.

“When the [input] recommendation is given [to growers], that’s when question marks come up,” he says. “It’s who’s interpreting the results and providing the recommendation to fit the farm business that can lead to poor advice.”

Mr Pluske says many growers have a large collection of soil tests that have been delivered on an annual basis, discussed fleetingly and rarely revisited. That data is valuable but many people do not have the time, desire or skills to bring it all together, he says.

“And they want advice that is independent of sales when they sit down to make decisions.”

How it works

Mr Pluske says the new service starts with consolidation and verification of historical soil tests, including geo-locating sample sites.  

Map showing subsurface pH results and lime applications

All new-season and historical topsoil and subsurface pH results and past lime applications have been used to estimate the quantity of fresh lime required to prevent acidity from limiting production for each site.

“Often the sample site called ‘A’ in one year is in a different part of the paddock to site ‘A’ in another year. If we are not confident of a sample site we either exclude that test result or, if decisions are still being made on a whole-paddock basis, we allocate results to a paddock rather than a site.”  

Mr Pluske says all data management and mapping is done using Microsoft Excel and Google Earth, which means the grower does not have to rely on Equii forever, or pay for new software.

Map showing estimate of revenue loss

The map shows estimates of revenue loss because of topsoil and/or subsurface acidity based on new-season and historical soil tests, intended crops and likely yields. Revenue losses from acidity can be compared with losses from other limitations such as phosphorus, potassium and sulfur to determine which inputs will be most cost-effective.

He says it is important to realise that soil tests can be used to make decisions beyond the year they are collected because results for pH, phosphorus buffering index (PBI) and organic carbon do not change dramatically from one year to the next. 

Also, test results can be adjusted according to lime and phosphorus inputs, meaning older tests can be used for many seasons. This means a more complete picture of soil levels and input requirements across the whole farm can be built up. 

Mr Pluske says with repeat samplings from geo-located sample sites it is possible to see trends in results over time to gauge the effects of previous management. “And mapping of results (from the geo-location) is a game-changer because ‘a picture tells a thousand words’. 

“When you see what inputs are required and where they’re required on an aerial image you can clearly see which inputs are most important for the whole business,” he says. 

“Often this shows the largest gains can be achieved by altering what products are purchased and how they are used across the farm. Further gains can then be realised by varying rates within paddocks.”

He says presenting information in this format has been the missing link in precision agriculture because many growers have varied fertiliser and lime rates without objectively determining what is the best input to vary, how much to vary it and for what likely financial benefit.

Mr Fosbery has wanted a service with this type of mapping ability for more than 10 years.

“Looking at a table of figures can be very confusing, whereas if you have a picture and a ‘traffic light system’ in front of you … you know what’s not a worry, what’s something to watch out for and where you have to address something,” he says.

Limiting inputs

Mr Pluske is interpreting soil tests to identify the most limiting inputs so scarce dollars are invested where they are likely to generate the highest return. 

Opportunity costs of not applying phosphorus, potassium or lime are estimated to gauge the downside of swinging dollars from one input to another.

He estimates soil ‘buffers’ of nutrients because if a grower is considering applying a maintenance rate of a nutrient, he says it is unlikely every site across the farm has an equal need for it. 

“Often estimates suggest sites already contain soil phosphorus equivalent to 100 to 200 kilograms per hectare of fresh monoammonium phosphate (MAP), before any more phosphorus is applied,” he says. “This means applying phosphorus to these sites is less urgent than for other sites which have inadequate or lower amounts of soil phosphorus.”

He says many growers like to apply some ‘insurance’ phosphorus, in case their situation is different to all the trials on which soil test interpretations are based.

Mr Pluske says soil test results and input recommendations are presented in a way that puts growers in control of decisions that make a real difference to their bottom line. 

“There’s no problems with the lab analysis results, we are presenting them better and this has growers interested in soil testing again because they can use results to make huge improvements,” he says.

More information:

Wayne Pluske,
0418 726 121,

This article is based on a talk Wayne Pluske gave to growers and advisers at the GRDC-supported SPAA Precision Agriculture Expo held in Moora, Western Australia, earlier this year.


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