The need for accuracy in the measure of soil pH
Soil pH can be measured directly, using a specialist and very expensive soil pH meter or alternatively and much more cheaply by using a standard bench type or hand held meter.
Precision is not important here! Such is the variability in pH encountered in the field that whilst the meter needs to be accurate (as in from a reputable manufacturer) it does not need to be precise and thus only needs to be accurate to one decimal place and with a resolution of no more than +/- 0.2.
Thus the quality of the pH meter is not critical and the type used to monitor pool and aquarium waters is ideal; however it should relatively accurate (+/- 0.1), come from a reputable manufacturer (preferably with a calibration certificate), and be in good working order.
A pH meter, and in particular the probe is a delicate instrument that is easily damaged. It must be calibrated and stored correctly before and after every use! Look after your instrument and it will give you good results, don’t and you may as well use your finger!
The measure of soil pH
To measure the soil pH put a 10g air dried sample into a clean and suitable container (glass, ceramic or plastic). Add to this 20ml (20g) of de-ionised or distilled water. It is worth noting that the water does not need to be of a very high analytical quality, it need only be free of ions that will affect our result and so the water used and sold by garages to top up car batteries is suitable.
Mix the soil and the water with a non-metallic stirring rod and leave for 30 minutes to equilibrate. Calibrate the pH meter according to the manufacturer’s instructions and using fresh buffer solutions.
Make sure you wash the probe with de-ionised water and tissue dry the meter after calibration and between readings.
Put the meter into the soil solution and gentle stir the probe or swill the container to bring the solution and probe into full contact. Allow the reading to stabilise, this can take some time and then read and record the value.
Repeat the process for each sample but remember to clean the probe between samples. It may also be prudent to periodically check the calibration and to definitely check it at the end. It is easy with many hand held pH meters to unknowingly move a calibration dial during operation and if not identified will lead to incorrect pH values being recorded. Once recorded the pH, its mean and standard deviation, can be calculated. Again, as is the standard rule of thumb, the standard deviation must be under 10%.
We can now use the pH reading and the textural classification to estimate the soils ‘buffer capacity and the corresponding lime requirements. The buffer capacity refers to the amount of Ca required to raise the soil pH by 1 point. In sandy soils this capacity is low and so a smaller amount of Calcium is required to raise the pH than in a heavy clay soil. Experience in the UK has led to the development of simple charts for determining buffer capacities from textural classification and from the pH reading the lime requirements are calculated. How well these charts migrate to soils outside the UK is unknown however whilst time consuming the practical measure of the soils buffer capacity is a relatively easy procedure.
The measure of soil buffer capacity
Soil buffer capacity is measured through the construction of buffer curves to determine the precise effect of Calcium additions on soil [solution] pH. A series of soil samples are each treated with increasing amounts of calcium and are then allowed to equilibrate over a period of a week(s) after which the pH is taken and the results plotted to produce a curve of the soils pH response, it’s buffer curve’. This is then used to calculate the precise lime requirements to adjust the pH to the desired value, its buffer capacity, which correlates to the clay content and CEC.
Relatively easy to measure the method requires only plastic bags, a rapid lime source (usually quick lime Ca(OH)2) and scales +/- 0.01. Alternatively finely ground (powdered) calcium carbonate (CaCO3) can be used but the reaction is much slower and the soil must be left for at least 2/3 weeks to equilibrate.
First one needs to approximate the soils buffer capacity and for this purpose the chart below for UK soils can be used. Then working from the soils measured pH value treatments are constructed in increments to produce a buffer curve response up to pH7. As the target pH for the heaviest treatment is neutral then if the measured soil pH is above pH 7 then the soil is already basic, and unless it is below 6.2 it’s measure is not only difficult but serves little purpose as it only reveals the upper values of the curve.
The number of treatments therefore depends on the difference between actual soil pH and the target pH. Ideally at least 3 replicates of five treatments should be constructed to produce a reasonable curve and each successive treatment should be constructed to produce an estimated increase of 0.2pH over the previous treatment.
Each treatment or bag should contain100g-500g of air dry soil to which a Calcium source in increasing increments has been added and thoroughly mixed before being wetted up to 40% of its water holding capacity*.
Left to incubate for one to four weeks depending on Calcium source the pH of the treatments is then measured and plotted to give a buffer curve from which the buffer capacity and the effects a given quantity of lime will have on soil pH can be calculated.
It is important to emphasise that the soil and calcium are mixed dry and thoroughly before any water is added. Powders only mix well when they are dry, a principal well understood and exploited by good cooks! If you fail to mix properly then it will take many, many weeks to equilibrate and your measurements will be wrong: a bit like biting into a cake filled with pockets of dry flour.
*If the soils water holding capacity is unknown 40% is the point at which water just yields from a piece of soil squeezed between finger and thumb. Recommended treatment quantities are given in the table below.
APPROX. BUFFER TREATMENTS (to raise 500g soil by 1.5pH)
g CaCO3 500g soil
pH raise / lht sand (S) / med sand (M) / loam (L) / heavy (H) / clay (C) 0 + (start) nil nil nil nil nil
0.2 0.08 0.24 0.28 0.32 0.4
0.4 0.16 0.48 0.56 0.64 0.8
0.6 0.24 0.72 0.84 0.96 1.2
0.8 0.32 0.96 1.12 1.28 1.6
1.0 0.40 1.20 1.40 1.60 2.0
1.2 0.48 1.44 1.68 1.92 2.4
1.4 0.56 1.68 1.96 2.24 2.8
1.6 0.64 1.92 2.24 2.56 3.2