How do you measure the pH of soil?

How do you measure the pH of soil?

Measuring soil pH is one of our most popular applications for our pH meters. Whether it be for agriculture to maximise nutrient availability or for testing for soils during earth moving or construction, measuring soil pH is important. In this month’s article we are going to go through a couple of different procedures you can use to measure soil pH in your location.

Method 1 - Procedure for direct ground measurement 

Can be used with pH meters with a spear tipped electrode suitable for direct measurement. Faster, but can be less accurate than method 2.

1) Dig, discarding 5 cm of topsoil 
2) Perforate the soil (with auger or shovel) to a depth of about 20 cm or more 
3) If the soil is dry, moisten it with a small amount of soil preparation solution or if this is unavailable distilled water 
4) Wash the electrode with tap water (not distilled) 
5) Insert the electrode pushing it slightly into the soil to ensure proper contact
6) Observe the measurement 
7) Wash the electrode with tap water (not distilled) and (using a finger) gently remove any soil remaining on the electrode (avoid using a rag or cloth) 
8) Repeat the procedure in different locations in the field 
9) Consider the average of the measured data

Direct soil measurements can be more variable in their readings as you are taking a measurement from a small sample area which may be anomalous, so for the most repeatable and accurate results it is advisable to measure the pH of a soil/water solution. 

By creating a soil/water mix you will dissolve more of the salts that can affect soil pH so it will give you a better indication on the soil pH levels. A suitable soil/water mix is created by using a sample of soil and soil preparation solution HI 7051 (or for some requirements you may substitute this with deionised water). It may also be better to use this procedure if you have to test a stony field in which you risk damaging the electrode if you were using method 1.

Method 2 - Can be used with all pH meters with a glass electrode. This is more accurate than method 1, and a lower risk of damaging the electrode. 

Procedure for the measurement of soil solution (1:5)

A) Sampling 
1) Extracting Soil Sample. Take 1 sample per 1000 m2 (0.25 acre) of homogeneous area. Even for small areas, 2 samples are recommended (the more the samples, the better the end-results, because the result is more representative). 
2) Avoid extracting samples from soil presenting obvious anomalies and consider them separately. 
3) Sample quantity: Take the same quantity of soil for each sample. For example, use bags with similar dimensions (1 bag per sample). 
4) Depth of extraction: General: dig and discard 5 cm (2") of topsoil. Herbaceous crops: from 20 to 40 cm of depth (8" to 16"). Orchards: from 20 to 60 cm of depth (8" to 24'’). 
5) Spread the soil samples on the pages of a newspaper and let the soil dry in a shady place or put it in an oven at 40°C. 
6) Crumble the dried soil and mix all the samples together to obtain a homogeneous mixture, discarding stones and vegetable residues. 
7) From this mixture, take the soil sample for analysis.

B) Soil solution preparation and measurement 
1) Sift the soil at 2 mm. 
2) Weigh 10 g of soil and put it in 50 ml of soil preparation solution HI 7051 (use a suitable beaker) or 20 g of soil per 100 ml of soil preparation solution HI 7051. 
3) Mix for 30 seconds. 
4) Wait for about 5 minutes.
5) Mix again and measure the pH of the solution

If you would like to discuss measuring the pH of soil or the pH of anything please feel free to contact one of our friendly Scientists via email or phone on 1300 737 871.

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