How To Measure Soil PH Value With PH Tester
Soil over-acidity and over-alkali are important factors limiting plant growth and quality. Most plants can not grow in soil that is too acidic or too alkaline. Therefore, it is vital to know the pH and humidity of the soil.
What is soil pH
Soil pH refers to the acidity and alkalinity of the soil. The pH value ranges from 0 to 14. According to the pH value from small to large, it is acidic, neutral, and alkaline.
A pH 7 indicates that the soil is neutral, less than 7 is acidic, and greater than 7 is alkaline. Soil pH also has a significant influence on the availability of nutrients.
For example, the availability of phosphorus in neutral soil is large; the availability of trace elements (manganese, copper, zinc, etc.) in alkaline soil is poor.
Plants can grow normally within a wide range, but each species has its own suitable pH.
For example, the suitable range of soil pH for apples is 5.3~8.2, and the optimum range is 5.4~6.8. When the pH value of the soil exceeds the optimum range, as the pH value increases or decreases, the growth of plants is hindered. Most plants can not grow at pH >9.0 or <2.5.
Five levels of Soil pH
- Strongly acidic soil (pH less than 5),
- Acidic Soil (pH5.0-6.5),
- Neutral Soil (pH6.5-7.5),
- Alkaline Soil (pH7.5-8.5),
- Strong alkaline soil (pH greater than 8.5).
Why Test Soil pH?
The chemical definition of pH is the negative logarithm of the H+ ion activity in solution. Soil pH value is an indicator of soil pH and is one of the critical factors affecting soil’s basic properties and fertility.
It directly affects the transformation and availability of soil nutrients, thereby affecting plant growth and development. Soil pH is easy to measure and is often used as an essential reference for soil classification, utilization, management, and improvement.
At the same time, in soil physical and chemical analysis, soil pH value is closely related to the analysis methods and results of many projects and is the basis for evaluating the results of other projects.
Soil pH is divided into water immersion pH and salt immersion pH.
Water immersion pH is the pH measured by extracting the soil with distilled water and represents the active acidity (alkalinity) of the soil.
Salt immersion pH is the pH value measured by extracting the soil with a certain salt solution, which generally reflects the potential acidity of the soil.
The solution is usually 1 mol-1 KCl solution or 0.5 mol-1 CaCl2 solutions. When the soil is extracted, K+ or Ca2+ will exchange with Al3+ and H+ adsorbed on the colloid surface, making their part exchanged into the solution. Therefore, salt immersion’s pH is lower than water immersion’s.
How many methods test soil pH?
There are colorimetric, potentiometric methods and soil pH testers for the determination of soil pH.
The pH error of potentiometric measurement is about 0.02 units, which has become a standard indoor measurement method.
The commonly used method for rapid on-site measurement is the mixed indicator colorimetric method, which has a poor accuracy of pH and a pH error of about 0.5.
What is a pH Soil Tester?
A soil pH meter is an instrument used to measure soil pH. It can be directly inserted into the soil to determine the pH of the soil. Compared with the traditional method of measuring soil pH, the operation is simple and can reflect the actual situation of the soil.
The soil pH tester consists of an ammeter with digital indication, a metal sensor, and a digital switch device. It is the system with a metal sensor as the core uses the current generated by the redox reaction in the chemical reaction when the metal sensor contacts the soil. The magnitude of the current value drives the battery data of different pH values and humidity values corresponding to the ammeter.
How to use soil pH tester
1)When the Soil is dry or fertilized too much, and the soil pH cannot be measured, it should be watered at the measuring point first and then measured after 28 minutes.
2)Before using the tester, the metal absorbing plate part must be completely wiped clean with an emery cloth to avoid affecting the measured value.
If the tester is first used, there is protective oil on the surface of the metal plate, which must be inserted into the soil several times, and the protective oil layer must be polished before use.
3)When measuring the pH value, insert it directly into the soil of the test point, and the surface of the metal plate must be completely buried.
It will take about 10 minutes to get the correct value. Excessive soil density, moisture, and fertilizer can affect the measurements, so it takes several measurements at different locations to get an average.
5) After 10 minutes, the pH of the tester is very stable. At this point, press the white button below, and the humidity is displayed immediately.
Soil pH Meter tips:
1) The metal probe of the instrument should not be inserted into the soil for too long to avoid oxidation damage and damage to the probe surface. After the measurement, the soil particles on the surface of the metal probe must be wiped clean with a cleaning cloth in time.
2) Confirm that the metal probe should be cleaned and kept dry before storing the instrument.
3) Keep the probe away from other metal substances;
4) This instrument is only used for wet soil measurement. Do not insert probes directly into aqueous solutions.
How to measure soil pH with potentiometric methods?
Sample preparation and measurement
Weigh two parts of 10 grams of air-dried soil that has passed through a 1mm sieve, and put them in a 50ml beaker.
One part is added with CO2-free distilled water, and the other part is added with 25ml of 1molL-1KCl solution (at this time, the soil-water ratio is 1:2.5, and soil with organic matter is 1:5), stirred on a stirrer for 30 minutes, and after standing for 30 minutes, measure the supernatant suspension of the sample with a pH meter.
Reagent preparation:
1molL-1KCl solution: Weigh 74.6 grams of KCl and dissolve it in 400ml of distilled water, adjust the pH to 5.5-6.0 with 10% KOH or KCl solution, and then dilute to 1 liter.
Precautions
1)The electrode plug and socket of the instrument must be kept clean and dry. When not in use, the short-circuit plug or the electrode plug should be plugged in to prevent dust and moisture from infiltrating and reducing the instrument’s input impedance and affecting the measurement’s accuracy.
2) For different samples, a suitable pH electrode should be selected.
3) When the sample is measured, the electrode’s lead wire must be kept still and not touched by hand. Otherwise, the measurement will be unstable.
4) Deionized water must be used to prepare the standard solution, and its conductivity should be less than 2 μS/cm, and it is best to use it by boiling. If the quality of pure water cannot be guaranteed, purchase the corresponding standard buffer solution.
5) To ensure the accuracy and reliability of the standard buffer solution, the alkaline solution should be packed in a polyethylene bottle and sealed tightly. A standard buffer should be stored in a refrigerator (low temperature 5-10 ℃). If turbidity, mildew, or precipitation are found, do not continue to use it.
6) Do not use standard buffers beyond the shelf life, and do not pour the used standard buffers back into the standard solution storage bottle.
7) When calibrating, use a standard buffer as close as possible to the pH value of the sample for calibration, and the temperature of the sample should be as consistent as possible with the temperature of the calibration solution.
Clean the electrode with deionized water after each measurement; never wipe the electrode with paper.
Why soil measurement data is easy to drift when measuring pH value
1) soil-water ratio: Generally, the soil suspension is thinner, the measured pH value is higher, and the dilution of alkaline soil is greater. Therefore, when measuring soil pH samples, it is recommended to use soil-water ratios of 1:5 and 1:1 for acid soil measurement and 1:1 or 1:2.5 soil-water ratio for alkaline soil measurement.
2) soil samples: Different soil samples have different components, which are composed of pure water, organic compounds, inorganic substances, heavy metals, etc., and have high requirements on the pH meter, so We should choose the one with high input impedance (≥3*1012Ω). pH meter.
3) the electrode: The soil sample to be tested is a suspension with complex composition. When choosing an electrode, it is best to select a solid polymer reference electrolyte and a special electrode for soil pH measurement (FZ-331) with low input impedance of the membrane.
If the solid soil is measured, the special electrode for soil pH measurement (FZ-551) is selected, which solves the problem of the clogging of the electrode liquid junction by the particles of the suspension and the problem of reducing the high impedance of the measurement part.
4) standard buffer selection: The standard buffer pH value ±3pH from the sample should be selected to calibrate the electrode.
How to Improve soil pH
Improving soil pH can promote plant growth, and adding ammonium and urea-based fertilizers, sulfur, and organic matter to soil can lower soil pH.
Organic matter —— After being decomposed by microorganisms, organic acids can be produced.
Ammonium fertilizers – usually used as nitrogen fertilizers. The acidifying effect of diammonium hydrogen phosphate, monoammonium hydrogen phosphate, and ammonium sulfate is stronger than that of urea nitrate and ammonium nitrate. Calcium nitrate and potassium nitrate will raise the pH and should be applied sparingly if the soil pH is already high.
Sulfur—Soil bacteria synthesize sulfur into sulfuric acid
Aluminum Sulfate and Ferrous Sulfate —— lower pH through chemical reaction
Conclusion:
Most plants grow on soils with relatively balanced pH. Too high pH can easily affect significant plant growth and quality factors and even make it difficult to survive.
Soil PH value is one of the essential basic properties of soil and is an indicator of soil formation and maturity. In soil, it participates in many chemical reactions and affects many soil properties, biological activity, synthesis, and decomposition of organic matter. The ability of soil to retain nutrients and the migration of elements during soil development are all related to soil pH.
All plants have a suitable pH range. Therefore, it is vital to know the PH value of plant soil. A soil PH tester is also one of the indispensable tools in agricultural production.
