Probing new ways to map pH

Image of a pH testing unit

Planfarm’s mobile soil-pH-testing unit that is being used this year to further assess the use of instantaneous pH probes in WA following GRDC-funded research last season.

PHOTO: Planfarm

Mapping soil pH and only applying lime to areas that need it is crucial to improving liming efficiency.

This is especially pertinent in eastern areas of the northern and central agricultural regions, where lime transport costs are high and it is expensive and time consuming to soil test at the density required.

In these areas, the correlation between electromagnetic and gamma surveys and soil pH can also be hit and miss for some soil types and soil moisture contents, making this type of mapping problematic.

Recognising these challenges, the GRDC’s Geraldton Port Zone Regional Cropping Solutions Network (RCSN) group funded a pioneering Western Australian study in 2014 into the accuracy and practicalities of using instantaneous pH probes to measure soil pH.

Carried out by Planfarm consultant Richard Quinlan, the study found effective measurements could be made by the probes when soil moisture content reached two per cent or higher.

Richard says to produce effective pH or lime application maps from the probe data, paddocks need to be sampled about every two to three hectares (optimum sampling density will depend on soil variation within the paddock).

He says a limitation of the instantaneous probes – which are widely used in parts of eastern Australia – is that sample depth is only 0 to 10 centimetres.

“Much of the decision-making regarding lime quantities depends on soil pH levels at 10 to 20cm and 20 to 30cm, so just sampling the 0 to 10cm doesn’t tell you enough,” he says.

Planfarm has this year incorporated instantaneous probes on a prototype mobile pH-testing unit that can insert the probe to a depth of 30cm, making it more useful for WA’s soil acidity profile.

For the 2014 RCSN project, Mr Quinlan compared two types of probes to measure soil pH – the Veris pH detector (containing an antimony probe that is inserted directly into the soil) and the traditional glass pH probe.

The antimony probe had good correlation with results from the glass probe when the soil was tested in the standard (1:5 calcium chloride (CaCl)) solution.

“When the antimony probe is inserted directly into the soil (direct soil) in the field, the correlation with the standard test method is lower but accurate enough for growers to be able to apply lime variably across the paddock,” Mr Quinlan says.

“When testing using the direct soil method, there is still a need for the antimony probe to be scaled using the traditional 1:5 test method. 

“It is estimated from this project that one soil test every 10 samples would be required.”

Like traditional soil tests, Mr Quinlan says using the probes is only measuring the pH of a particular soil in a particular spot in the paddock.

He says if the paddock is highly variable, more samples are needed.

“We did find that the instantaneous probes were effective on sands and loams, but a longer response time of about 30 seconds was needed on yellow sands – compared with about 15 seconds on loamy soils,” he says.

“The response time is the time the probe took to settle on a measurement after being inserted directly into the soil.”

On all soil types, moisture content had a significant effect on pH reading and soil probe conductivity increased markedly when soil moisture reached two per cent. This increase in conductivity correlated with an increase in test accuracy.

Mr Quinlan says non-wetting soils are likely to affect direct soil probe pH measurements and probes would need to be inserted towards the lower end of the 0 to 10cm layer to improve the accuracy of results.

He says in the 2014 trials conducted through this project, variable lime application – based on a map from instantaneous probes at a density of 2.6ha per sample in a paddock – was 18 per cent cheaper than using a blanket lime-application strategy.

“More lime was applied where it was needed, which could also reduce the requirement for future lime needs in that paddock,” he says.

“The results are applicable to both new and mature liming programs, where significant differences in soil pH might still exist even after several years of blanket lime application due to variations in soil buffering capacity – or the ability of soil to resist changes in pH.”

Mr Quinlan says the key message from the 2014 RCSN project is that instantaneous soil probes inserted directly into the soil are capable of accurately measuring soil pH and are cheaper than taking samples and measuring them in the traditional method. However, the maps will not be as accurate as using traditional, standard (1:5 CaCl) soil tests.

“What you will get is a cost-effective map that indicates areas of low and high pH in the paddock and a guide as to where to best apply your lime,” he says.

More information:

Richard Quinlan, Planfarm,
08 9964 1170,;

Julianne Hill, RCSN,
0447 261 607,;

Chris Gazey, DAFWA, 0429 107 976,

GRDC Project Code PLN00010

Region West