If soils have a pH below 7 then the soil is considered acidic, if it has a pH above 7, then it considered alkaline, if it has a pH of exactly 7, then it is neutral. However most soils have a pH in the range of pH 4 – 8, which means that most soils are slightly acidic.
Materials and Method.
For the laboratory method, 10 cm³ of air-dried soil was placed in a plastic bottle along with 25 cm³ of de-ionised water, following which the stopper was replaced. It was then placed on an orbital shaker for 15 minuets to ensure that the contents were thoroughly mixed. The electrodes of the meter were then placed into the solution, and the pH level was taken from the digital display.
For the BDH method, a stopper was first placed in the end of a BDH tube, air-dried soil was then added to a depth of 20 mm. A further 25 mm of barium sulphate was added above the soil in the BDH tube, de-ionised water was then added up to the first mark on the BDH tube. The tube was then shaken until all of the contents was wetted and thoroughly mixed after a stopper had been placed in the other end of the tube. If necessary remove the stopper and top-up the de-ionised water to the first mark. Indicator was then added to the second mark, the stopper replaced and then shaken well again. The suspension was allowed to settle for a few minuets before the clear solution at the top of the tube was compared to the reference card, with the nearest colour match being the pH reading.
Results
The results obtained from both methods are shown in the table below.
Chart 1.
Discussion
As can be seen from the chart above the differences between the two methods are fairly low, this indicates that both methods are relatively accurate. The electrochemical method is slightly more accurate however this level is rarely needed in the field and because this method also takes longer and needs to be undertaken in a laboratory is not usually used, unless necessary. The BDH method is usually accurate enough for most decisions to be made and because it can be undertaken relatively quickly and in the field, is the method most usually used. However at very low pH levels the BDH method becomes less accurate giving a lower reading than the soil actually is, however at more neutral levels is reasonably accurate.
The results also show that most soils are slightly acidic, with the exception of Hanslope which is alkaline, and Bengeo which is neutral. Danbury is the most acidic, followed by Hornbeam, Hamble and then Alluvium. Low pH on its own cannot damage plants; its effect is indirect affecting the availability of nutrients and their effects. While the nutrients, such as nitrogen, may still be in the soil they will become insoluble and unavailable for the plants to absorb. Other nutrients such as manganese can be released into the soil water at toxic rates below pH 5.5. At low pH levels, conditions also become unfavourable for certain micro-organisms, slowing decomposition or nitrogen fixation.
High soil pH levels lead to the low availability of many nutrients such as Phosphorus, Iron and Manganese. Soils with low pH are more common than soils with high pH, this is due to several reasons. When organic matter is mineralised nutrients are released along with hydrogen ions. These hydrogen ions lead to a pH drop and an acidic soil, hydrogen ions are also released when ammonium is converted into nitrate. The leeching of calcium, magnesium and potassium as well as heavy cropping that also removes these nutrients leads to a drop in soil pH.
Conclusion
This experiment shows that while laboratory methods for testing pH are more accurate, in most circumstances the BDH method of testing is more suitable. This method provides a quick, reasonably accurate way of finding the soil pH level, allowing decisions to be made on the nutrient and soil structure status.
References
Bohn, H. L.; McNeal, B. L. and O’Connor, G. A., Soil Chemistry, New York, Wiley and Sons.
