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PH & EC Meters

How to Use pH & EC Meters (Pens) to monitor Crop Nutrition

Storage of Pens
pH pens need to be stored with storage solution, buffer solution or water in the cap. The sensor needs to remain hydrated or it will fail to operate. Some pH sensors can be rehydrated by soaking in buffer solution for a few hours. EC pens can be stored dry with no solution.

Calibration of Pens
All of the commonly available pens and more expensive types of pH and EC meters work on the same basic principles and if they are calibrated properly should give the same readings. All pens and meters must be calibrated or standardized to give consistent and dependable readings each time they are used. If a pen or meter has no means of being calibrated it is probably not going to give good results.

Calibration procedures differ somewhat depending on the brand of pen. On some you push a button; on others you turn a small screw to set the display at the proper value. For pH pens, calibration is done with the pH 7 buffer solution or by a “two point” calibration using the pH 4 and pH 7 buffers. Calibration involves placing the pH pen in a 2 oz. cup of buffer solution, allowing a stable reading to develop, and then, if necessary, adjusting the displayed value to the pH of the buffer. The EC standard solution is used in the same way to calibrate the EC pen. After calibration the used solutions should be discarded. Also some pens need to be soaked before use. Just follow the instructions that come with the pen.

Some EC meters give readings in TDS. These meters have a “conversion factor”. The conversion factor is the ratio at which the pen converts EC (EC is measured in millisiemens per centimetre (mS/cm) or microsiemens per centimetre (uS/cm) .One millisiemen = 1000 microsiemens.) to TDS ( measured in parts per million or ppm). This is why an Oakton meter will give a different read than a Hanna meter or a Blue Lab meter. An Oakton meter will convert EC at a conversion factor of 0.7 usually. While a Hanna meter might convert it at 0.5. This means an EC of 1.0 (mS/cm) will read 700 ppm on the Oakton meter and will read 500 ppm on the Hanna meter. If you were to set the conversion factor at 1.0, the meter would read 1000ppm at an EC of 1.0 (mS/cm). Some meters you can adjust the conversion factor and others you can’t. It is important to know the conversion factor in order to calibrate your meter. The reason it is important, is because most generic brand calibration solutions are for meters that have a conversion factor of 0.5. That means if the calibration solution is 1000ppm, it will have an EC of 2.0 (mS/cm). An easy way to avoid all the confusion is to measure in EC only.

Calibration is a must to get any useful information from pens and meters! How often it needs to be done depends on the type of meter and frequency of use. The readings of even the most expensive laboratory meters tend to “drift” over time and must be brought back to the proper reading fairly often. When you first start using the pens, plan on calibrating at the beginning of every testing session until you find out how much the readings drift between sessions.

Direct Measurements Using the Pens
On certain types of materials, pens can be used directly without any special sample preparations.
pH pens can be used directly to measure the pH of irrigation water, hydroponic solutions, and pesticide solutions. Calibrate the pen first according to the instructions, rinse off the buffer, and then place the pen directly into a sample large enough to completely immerse the sensor. Agitate the pen slightly in the sample to dislodge any air bubbles and then allow a stable reading to develop.
After taking reading, rinse the sensor again and place cap with storage solution back on the pen.
Checking the pH of irrigation water is useful, but remember that an alkalinity test is needed to get a complete picture on how water pH might be affecting your crop.

The EC pens can be used to measure the strength of fertilizer solutions. The EC pen is used in the same way as the pH pen. To determine the EC of nutrient solution. Subtract the EC of the water from the nutrient solution EC. Compare the results to the table on the fertilizer bag/bottle or product literature. This will tell you whether or not you are getting the EC/ppm you think you are.

Testing Growth Media
Growth media can be tested by extracting the sample with distilled water and measuring the pH and EC of the filtered extract. Some low-priced testing meters claim that you can stick the sensor probes directly into the growth medium and take readings. This approach has no basis in scientific soil testing and it is not recommended for professional growers.

Growth medium samples
Take samples from the root zone or use all of the material in a container. Never sample from the surface because nutrients and soluble salts are always highest here and do not represent the fertility status of the root zone. Sampling is a good time to inspect the roots – a small or diseased root system can often be the best explanation of apparent fertility problems.

Extraction

The extraction procedure described here is known as the “1:2 dilution method.”

  1. Mix growing media subsamples (samples from multiple locations) thoroughly to prepare a representative sample.
  2. Using the standard measure, obtain 1 level volume of media, compressing it slightly. Be sure to always use a consistent pressure when packing the measure. Try to duplicate the same degree of media compaction as in the crop.
  3. Empty the measured media into the large container.
  4. Add 1 measure of deionized water to the large container and mix with the soil.
  5. Let the mixture stand for at least 15 minutes. This is now a 1:1 soil/water mixture.
  6. If the media is largely organic, take a pH reading by immersing the electrode directly into the slurry and record the reading. For mineral or sandy media, read the pH after sieving to avoid damaging the glass electrode. After removing the probe from the media rinse it according to manufacturer’s instructions.
  7. Add another volume of water, stir, and wait 5 minutes. You now have a 1:2 soil/water mixture.
  8. Sieve enough of the slurry into a clean beaker to immerse the EC probe and record the reading.

Interpretation of Results:
See Table 2 for EC values

Pour-through Method
An alternative to the 1:2 dilution method is the Pour-through Method, a procedure by which the runoff water is analyzed for pH and EC.
Pour-through extraction is a two step process. First, the media is progressively wetted until just saturated and left to stand for about two hours (or the extract can be collected 2-4 hours after irrigation). Then, a volume of water sufficient to produce about 100 ml of leachate (depending on container size) is drenched onto the surface. Care must be taken so that the water does not channel down the sides of the container. If applied carefully, the water does not immediately mix with the container solution, but evenly displaces it, driving it down into the lower root zone where some of it drains from the pot and is captured for testing.

Interpretation of Results:
See Table 2 for EC values

Saturated Media Extraction
Another alternative to the 1:2 dilution method is the Squeeze method. Many peat based media can be direct squeezed at
the saturated stage with good results. Wear rubber gloves when squeezing a sample. This prevents any salts on your skin from interfering with the accuracy of the EC reading. Some good sieving materials include fine mesh nylon stockings, heavy duty cellulose
cleaning cloths, and filter bags for making jellies. If you use an improvised filter, you must take care to thoroughly rinse and wring it out before and between readings. The mesh size should stop all but the finest particles from passing through. If desired, you can filter the extract further through a coffee filter or other funnel type filter.

  1. Fill the sample container about 2/3 full with media from a blended representative sample.
  2. Add distilled or deionized water slowly, while mixing with a spoon or spatula.
  3. The sample is the correct consistency when the surface just glistens but there is no free water (puddles) on the surface. A small portion of the sample should remain more or less solid without dripping but should slide easily from the spoon.
  4. Wait at least 15 minutes. Stir and add more water if required.
  5. Read and record the pH directly from the saturated sample if the media is organic or peat based. For mineral or sandy media, record the pH after extraction to avoid damaging the electrode.
  6. Extract the sample using the squeeze method.
  7. Read and record the EC.

Interpretation of Results:
See Table 2 for EC values

Interpreting Test Data
pH – soil acidity. Most greenhouse crops can grow satisfactorily over a fairly wide pH range. However, optimum pH ranges have been established for some crops and soilless media and mixes containing field soil. The optimum pH range for different media is shown in Table 1. The difference in optimum pH between the types of growing media is related to pH effects on nutrient availability in each type of media.

Table 1.
Optimum pH values
Growth Medium PH
Hydroponic / Aeroponic 5.5-6.0
Peat Based Mixes / Soil 6.2-6.5
Coco 5.8-6.0
Rockwool 5.6-6.0

What action to take on pH depends on the specific requirement of the plants being grown and knowledge of the factors which interact to affect the pH of media. Limestone (rate, type, neutralizing power, and particle size), irrigation water pH and alkalinity, acid/basic nature of fertilizer, and effects of mix components are the major influences on pH. Also, some plants are known to change the pH of the growth medium; geranium is the best example – the activity of its roots can significantly lower pH.

Soluble salts
Measuring soluble salts provides a general indication of nutrient deficiency or excess. Excess soluble salts is very common and generally results from too much fertilizer in relation to the plant’s needs, but inadequate watering and leaching, or poor drainage, are other causes. Sometimes high soluble salt levels occur when root function is impaired by disease or physical damage. Again, always check the condition of the root system when sampling soil for testing.

Seedlings, young transplants, and plants growing in media containing 20% or more soil are less tolerant of excess soluble salts. Soluble salts above the normal range for prolonged periods may cause root injury, leaf chlorosis, marginal burn, and sometimes, wilting.

Soluble salts below the normal range may indicate the need for increased fertilization.

Optimizing Soil Test Results
If used correctly, the three methods of growth media testing outlined here give valuable and useful results for crop production.

Table 2.
Soluble salts levels determined by different methods of soilless media analysis.
Measured in EC ( mS/cm )
1:2 SME Pour Through/Runoff Indication
0-0.3 0-0.8 0-1.0 Very low (Good for seedlings/propagation)
0.3-0.8 0.8-2.0 1.0-2.6 Low (Good for seedlings and salt sensitive plants)
0.8-1.3 2.0-3.5 2.6-4.6 Normal ( Upper range for salt sensitive plants)
1.3-1.8 3.5-5.0 4.6-6.5 High (Growth will slow dramatically)
1.8-2.3 5.0-6.0 6.6-7.8 Very high (Salt injury due to reduced water uptake)
>2.3 >6.0 >7.8 Extreme ( Severe injury, maybe even death)

Record Keeping
Growing media should be tested for salts and pH on a routine basis. It’s important to keep records so that you can chart pH and EC levels over time. Graphically charting your pH and EC values will provide you with a trend of timely information on whether the pH and EC are rising, falling, or staying steady. This is at least as important as the actual reading. It will enable you to make informed decisions about fertilizer concentrations, watering frequencies, and leach rates. Very often, growers who use routine media testing find they can produce superior crops with less fertilizer and lower leaching rates, thereby reducing waste and the possibility of environmental contamination.

References:
Dr. Douglas Cox, Plant and Soil Sciences, University of Massachusetts