Soil Testingwith pH and EC Pens
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Terms & Conditions of Use
How to use this equipment and interpret the results, without breaking the bank.
A number of inexpensive pen-like instruments are available that can be used in the greenhouse to determine growth medium pH and soluble salts. The cost of the individual pH and electrical conductivity (EC) "pens" ranges from $50-80, and a combination pH/EC pen costs about $120. Proper use of these instruments provides accurate pH and EC measurements, which a grower can use to make immediate crop management decisions or to seek further in-depth testing by a commercial soil test lab.
- Doug Cox
A number of inexpensive pen-like instruments are available that can be used in the greenhouse to determine growth medium pH and soluble salts. The cost of the individual pH and electrical conductivity (EC) "pens" ranges from $50-80, and a combination pH/EC pen costs about $120. Pens are available from most greenhouse, agricultural and forestry supply houses. Proper use of these instruments provides accurate pH and EC measurements, which a grower can use to make immediate crop management decisions or to seek further in-depth testing by a commercial soil test lab.
Supplies
In addition to the pens, you will need the following items to take care of the pens and do pH and EC (soluble salts) tests. Many of the items can be obtained for free or inexpensively from home or the grocery store. Of course, more expensive, professional supplies can be purchased from a science lab supply company, but the expensive accessories won't necessarily make the tests turn out any better. Here is the basic list of what you'll need:
* pH 4 and 7 buffer solutions to calibrate the pH pen
* EC standard solution to calibrate EC pen
* Plastic funnel (5- to 6-inch top diameter)
* Distilled water
* Basket-style coffee filters
* 16-oz. plastic cups or clean 15-oz. glass jars
* 3-oz. waxed paper cups
* Plastic colander (6-inch dia. min.)
* Shallow baking pan or 8- to 10-inch diameter plant saucers
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 pens and of course, the instructions accompanying the pens you buy should be followed carefully. On some you push a button, on others you turn a small screw to set 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 3-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.
Calibration is a must to get any useful information from pens and meters. How often calibration needs to be done depends on the type Á of meter and frequency of use. 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. Most pens come with a couple of small packets of buffer or standard solution to get started, so you will need to purchase more for future use very soon.
Direct Measurements
The pens can be used to directly measure pH or EC of some types of samples with no special sample preparations.
pH pens. They 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, then allow a stable reading to develop. 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.
EC pens. These can be used to measure the EC of fertilizer solutions and to check the operation of fertilizer injectors. The EC pen is used in the same way as the pH pen. To check the operation of an injector, you will need to: 1) Determine the EC of water without fertilizer; 2) check the fertilizer solution after the injector has run for several minutes; 3) subtract the EC of the water from the fertilizer solution EC; and 4) compare the results to the table on the fertilizer bag or product literature. This will tell you whether or not you are getting the ppm you think you are.
Always rinse the sensors of the pH and EC pens carefully with distilled water between measurements to avoid contaminating the next sample with residue from the previous sample or buffer.
Media Testing
In simple terms, greenhouse growth media is tested for pH and EC by extracting the sample with water and making the pH and EC measurements on the extracts. Some testing Á meters claim you can stick the sensor probes directly into the growth medium and take readings, but this approach has not gained wide acceptance in the field of professional soil testing.
The two most common testing systems for on-site measurements of pH and EC are the traditional 1:2 dilution method, outlined here, and the relatively new Leachate PourThru method. Both methods are easy to do using the pens and can be used to help diagnose problems and track pH and EC changes as a crop grows.
1:2 Dilution Method
Sample the growth medium. Take samples from the root zone or use all of the material in several pots of plants representative of the crop or problem under diagnosis. Never sample from the surface alone because nutrients and soluble salts accumulate here and do not represent the fertility status of the root zone. Sampling is a good time to inspect the plant's roots, a small or diseased root system can often be the best explanation of apparent fertility problems.
It is best to air-dry the sample at room temperature or below 80° F on a greenhouse bench. Spread the sample out in a baking pan or plastic saucer and remove any large pieces of root and other debris. Unless the sample is very wet, it should be dry enough in 24 hours to test. Screen the dry sample using the colander or similar sieve. The dried and screened sample is ready to test.
Extract and analyze. The extraction procedure for the 1:2 dilution method is as follows: 1) Combine one volume of air-dried growth medium with two volumes of distilled water. Using the items listed earlier, fill one 3-oz. cup with growth medium and two 3-oz. cups with distilled water and mix them together in a 16-oz. cup; 2) mix the sample and distilled water thoroughly by swirling the cup and then allow it to stand for 30 minutes; 3) after 30 minutes pour the mixture into the funnel supporting the coffee filter. (I find two filters put together work best.) Catch the filtered extract in another 16-oz. cup. The objective is to separate the liquid extract from the solids, which are discarded. The extract is now ready to test for pH and EC; 4) properly calibrate pens; 5) pour enough extract into a clean 3-oz. cup so that the sensor of the pen will be completely immersed in the extract. The pen should be swirled in the extract to dislodge any air bubbles and then leave the pen still until a stable (unchanging) reading appears. The stable pH or EC reading is your result; and 6) compare your EC reading to Figure 3, left. This EC figure is specific to the 1:2 dilution method. Do not use any other figure unless it states "1:2 method." The pH tables in this fact sheet or pH information from other sources can be used to interpret data.
Leachate PourThru Method
An alternative to the 1:2 dilution method is the Leachate PourThru method, a procedure by which the pot leachates are analyzed for pH and EC directly using the pens. Details of this procedure can be found at North Carolina State University's Web site at
www.floricultureinfo.com (look for the topic PourThru sampling). A big advantage of this method is that the growth medium is not sampled, so the plants are not disturbed and no time is spent preparing the growth medium for testing. Leachate PourThru is a great way to graphically track changes in pH and EC over time, but the testing protocol must be followed carefully to get the best results -- it is not as simple as just testing whatever comes out of the pot. If a grower plans to do only limited testing, say a couple of times during the growing season, the 1:2 method may be more straightforward.
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 soilless media and for mixes containing 20 percent or more field soil is 5.5-6.0 and 6.2-6.5 respectively. The difference in optimum pH between the two types of growing media is related to pH effects on nutrient availability in each type.
Both low and high pH problems are common in the greenhouse industry today. At low pH, calcium and magnesium may be deficient. Low pH is also part of the cause of molybdenum deficiency in poinsettias. On the other hand, iron and manganese become more available and may reach phytotoxic levels when pH is low (less than 5.
. Excess iron and/or manganese can be toxic to geraniums, marigolds and many other bedding plants. High pH leads to iron deficiency and chlorosis of crops such as petunia and calibrachoa. There are reasons why various pH levels have different requirements for a number of bedding plants (see Figure 2, page 54).
What action to take on pH depends on the specific requirement of the plants being grown and a knowledge of the factors, which interact to affect the pH of greenhouse media. Limestone (rate, type, neutralizing power, 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 lower pH very quickly in a short period of time.
Soluble salts. EC is a measurement of the soluble salts level in the growth medium and it provides a general indication of nutrient deficiency or excess. High EC 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 EC 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 percent or more field soil are less tolerant of high EC. 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.
Only minimum investments in money and time are necessary to start your own basic soil-testing program. Learning how to operate and care for the pH and EC pens and carefully following one method of analysis can help you get quick answers on the pH and fertility status of your crops.
Doug Cox is associate professor and extension specialist in the Plant and Soil Sciences Department at the University of Massachusetts, Amherst, Mass. He can be reached by phone at (413) 545-5214 or by E-mail at
dcox@pssci.umass.edu.
Source: Greenhouse Product News December 2003 Volume: 13 Number: 12
Copyright © 2008 Scranton Gillette Communications
