Diagnosing soil acidity

Page last updated: Monday, 29 October 2018 - 9:07am

Please note: This content may be out of date and is currently under review.

Commercial soil sampling kits

Prepaid commercial soil sampling kits are available and may be the most convenient method for the do-it-yourself approach as they include full instructions, sample bags, postage and laboratory testing of the samples. These kits are primarily aimed at topsoil sampling and testing for fertiliser recommendations, but the subsurface 10-20 and 20-30cm layers may be sampled for pH at the same time (a suitable sampling tube, or commercial soil sampling contractor able to accurately sample the subsurface, will need to be used).

Field pH testing

Laboratory testing of pH provides the most accurate measurement of soil pH. Field testing with a hand-held pH probe may provide an indication of areas that need accurate soil sampling and testing. Hand-held pH probes are available from scientific equipment suppliers and come with instructions.

When field testing soil pH, it is usually more convenient to use deionised or distilled water instead of 0.01M calcium chloride and so the results will need to be converted by subtracting 0.7. It is important to maintain the probe in good condition and calibrate with standard pH buffer solutions each day it is used.

Soil pH test kits that use indicator solutions and colour to estimate pH are inexpensive and easy to use. However the results are subjective and should be used with caution. The chemicals used with the kits are subject to deterioration.

Electro-magnetic (EM) mapping

Elecro-magnetic (EM) mapping shows soil type variability across the farm. While soil type does influence the pH of the soil and the acidification rate, EM mapping cannot take into account the farming and liming history of the soil. The pH will differ according to how long the soil has been farmed, what type of farming has been conducted and how much and how often lime has been applied. Consequently, the relationship between EM results and current soil pH is variable.

Monitoring soil pH

Monitoring changes in soil pH is an important part of soil acidity management. Re-sampling every three to four years will enable you to refine your liming program, to prevent good soil from becoming acidic and avoid liming unnecessarily on soil that doesn’t need it.

Because pH can vary within a paddock, tracking changes in the soil pH requires samples to be collected from the same location over time. Samples need to be properly geo-located, preferably by GPS, to allow comparable repeat sampling.

Sampling 25% of a farm each year enables a four-year rotation. This is an adequate time frame to detect changes and allow adjustment of liming practices.

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