Alternative pH testing method shows promise
Author: Paul Kelly | Date: 13 May 2015
By GRDC western regional panel member Paul Kelly.
A study into the use of instantaneous pH probes inserted directly into moist soil to measure pH has found they are capable of making accurate measurements and are cheaper than traditionally used soil tests when many samples are required across a paddock.The research found the probes were a cost effective way of indicating areas of low and high pH and to help guide where to best apply lime in paddocks.
The work was conducted in Western Australia’s Northern Agricultural Region by Planfarm agronomist Richard Quinlan and funded by the Grains Research and Development Corporation (GRDC) through the Regional Cropping Solutions Network (RCSN) in the Geraldton port zone.
Soil pH is typically measured in WA by taking a soil sample and sending it to a laboratory where the soil is mixed in a solution of 1:5 CaCl2 and measured with a glass probe.Whilst this industry standard is an accurate method, it is relatively slow and expensive and therefore limits the number of samples that are typically taken across a paddock.
But instantaneous pH probes provide the potential to relatively quickly and cheaply measure soil pH at many sites across a paddock, allowing pH and lime application maps to be drawn.
Mapping soil pH and applying lime only to parts of the parts of the paddock where it is needed, rather than blanket rate applications, is essential for improved liming efficiency and cost-effectiveness.
In 2014, a trial conducted as part of the project by Planfarm showed variable rate application of lime, based on a map generated through sampling with instantaneous probes at a density of 2.6 hectares per sample in a paddock, was 18 per cent cheaper than using a blanket lime application strategy.
Mr Quinlan found the instantaneous probes, which are widely used by grain growers in parts of eastern Australia, could make adequate measurements when soil moisture content was at least 2 per cent.
This increase in conductivity correlated with an increase in test accuracy.
To produce effective pH or lime application maps from the probe data, paddocks needed to be sampled about every two to three hectares.
However, Mr Quinlan said the optimum sampling density would depend on soil variation within a paddock.
The work found that the probes were effective on some sands and loam soils, generating results in about 15 seconds.
But a longer response time (the time taken for a probe to settle on a measurement) of about 30 seconds was needed on yellow sands.
Mr Quinlan stresses that, like traditional soil tests, the probes only measure the pH of a particular soil in a particular spot in the paddock, and in paddocks with highly variable pH, more samples are required.
A limitation of the instantaneous probes is that their sample depth is only 0-10cm.
Much of the decision making process regarding the quantity of lime that needs to be applied to paddocks depends on soil pH levels at 10-20cm and 20-30cm.
Sampling just the 0-10cm layer doesn’t provide enough information.
Planfarm has subsequently 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 pH profile.
The GRDC RCSN project complements an ongoing, significant investment by the GRDC into helping WA growers better manage soil acidity.
Soil acidity research led by the Department of Agriculture and Food (DAFWA) is part of the collaborative research effort ‘Soils Constraints West’ which aims to develop and deliver solutions for a range of soil constraints limiting productive grain cropping in WA.
Soils Constraints West represents more than $33 million of new research aimed at addressing non-wetting soils, subsoil constraints, soil compaction and soil acidity over five years.
The GRDC encourages growers to adopt long-term liming strategies to address soil acidity, which costs WA agriculture more than $500 million per year in lost productivity.
Richard Quinlan, Planfarm
08 9964 1170
Paul Kelly, GRDC western panel
0427 275 022
Natalie Lee, Cox Inall Communications
08 9864 2034, 0427 189 827
GRDC Project Code PLN00010