Soil Testingwith pH and EC Pens
A number ofinexpensive pen-like instruments are available that can be used in thegreenhouse to determine growth medium pH and soluble salts. The cost of theindividual pH and electrical conductivity (EC) “pens” ranges from$50-80, and a combination pH/EC pen costs about $120. Pens are available frommost greenhouse, agricultural and forestry supply houses. Proper use of theseinstruments provides accurate pH and EC measurements, which a grower can use tomake immediate crop management decisions or to seek further in-depth testing bya commercial soil test lab.
In addition tothe pens, you will need the following items to take care of the pens and do pHand EC (soluble salts) tests. Many of the items can be obtained for free orinexpensively from home or the grocery store. Of course, more expensive,professional supplies can be purchased from a science lab supply company, butthe expensive accessories won’t necessarily make the tests turn out any better.Here is the basic list of what you’ll need:
* pH4 and 7 buffer solutions to calibrate the pH pen
* ECstandard solution to calibrate EC pen
* Plasticfunnel (5- to 6-inch top diameter)
* Basket-stylecoffee filters
* 16-oz.plastic cups or clean 15-oz. glass jars
* 3-oz.waxed paper cups
* Plasticcolander (6-inch dia. min.)
* Shallowbaking pan or 8- to 10-inch diameter plant saucers
Calibration of Pens
All of thecommonly available pens and more expensive types of pH and EC meters work onthe same basic principles, and if they are calibrated properly, should give thesame readings. All pens and meters must be calibrated or standardized to giveconsistent and dependable readings each time they are used. If a pen or meterhas no means of being calibrated, it is probably not going to give goodresults.
Calibrationprocedures differ somewhat depending on the brand of pens and of course, theinstructions accompanying the pens you buy should be followed carefully. Onsome you push a button, on others you turn a small screw to set display at theproper value. For pH pens, calibration is done with the pH 7 buffer solution orby 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 thedisplayed value to the pH of the buffer. The EC standard solution is used inthe same way to calibrate the EC pen. After calibration the used solutionsshould be discarded.
Calibrationis a must to get any useful information from pens and meters. How oftencalibration needs to be done depends on the type Á of meter andfrequency of use. Readings of even the most expensive laboratory meters tend to”drift” over time and must be brought back to the proper readingfairly often. When you first start using the pens, plan on calibrating at thebeginning of every testing session until you find out how much the readingsdrift between sessions. Most pens come with a couple of small packets of bufferor standard solution to get started, so you will need to purchase more forfuture use very soon.
The pens canbe used to directly measure pH or EC of some types of samples with no specialsample preparations.
Always rinsethe sensors of the pH and EC pens carefully with distilled water betweenmeasurements to avoid contaminating the next sample with residue from theprevious sample or buffer.
In simpleterms, greenhouse growth media is tested for pH and EC by extracting the samplewith water and making the pH and EC measurements on the extracts. Some testingÁ meters claim you can stick the sensor probes directly into the growthmedium and take readings, but this approach has not gained wide acceptance inthe field of professional soil testing.
The two mostcommon testing systems for on-site measurements of pH and EC are thetraditional 1:2 dilution method, outlined here, and the relatively new LeachatePourThru method. Both methods are easy to do using the pens and can be used tohelp diagnose problems and track pH and EC changes as a crop grows.
1:2 Dilution Method
Sample thegrowth medium. Takesamples from the root zone or use all of the material in several pots of plantsrepresentative of the crop or problem under diagnosis. Never sample from thesurface alone because nutrients and soluble salts accumulate here and do notrepresent the fertility status of the root zone. Sampling is a good time toinspect the plant’s roots, a small or diseased root system can often be thebest explanation of apparent fertility problems.
It is best toair-dry the sample at room temperature or below 80° F on a greenhousebench. Spread the sample out in a baking pan or plastic saucer and remove anylarge pieces of root and other debris. Unless the sample is very wet, it shouldbe dry enough in 24 hours to test. Screen the dry sample using the colander orsimilar sieve. The dried and screened sample is ready to test.
Extractand analyze. Theextraction procedure for the 1:2 dilution method is as follows: 1) Combine onevolume of air-dried growth medium with two volumes of distilled water. Usingthe 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 thesample and distilled water thoroughly by swirling the cup and then allow it tostand for 30 minutes; 3) after 30 minutes pour the mixture into the funnelsupporting the coffee filter. (I find two filters put together work best.) Catchthe filtered extract in another 16-oz. cup. The objective is to separate theliquid extract from the solids, which are discarded. The extract is now readyto test for pH and EC; 4) properly calibrate pens; 5) pour enough extract intoa clean 3-oz. cup so that the sensor of the pen will be completely immersed inthe extract. The pen should be swirled in the extract to dislodge any airbubbles and then leave the pen still until a stable (unchanging) readingappears. The stable pH or EC reading is your result; and 6) compare your ECreading to Figure 3, left. This EC figure is specific to the 1:2 dilutionmethod. 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 beused to interpret data.
Leachate PourThru Method
Analternative to the 1:2 dilution method is the Leachate PourThru method, aprocedure by which the pot leachates are analyzed for pH and EC directly using thepens. Details of this procedure can be found at North Carolina StateUniversity’s Web site at www.floricultureinfo.com (look for the topic PourThrusampling). A big advantage of this method is that the growth medium is notsampled, so the plants are not disturbed and no time is spent preparing thegrowth medium for testing. Leachate PourThru is a great way to graphicallytrack changes in pH and EC over time, but the testing protocol must be followedcarefully to get the best results — it is not as simple as just testingwhatever 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 morestraightforward.
Interpreting Test Data
pH — soilacidity. Mostgreenhouse crops can grow satisfactorily over a fairly wide pH range. However,optimum pH ranges have been established for some crops and soilless media andmixes containing field soil. The optimum pH range for soilless media and formixes containing 20 percent or more field soil is 5.5-6.0 and 6.2-6.5respectively. The difference in optimum pH between the two types of growingmedia is related to pH effects on nutrient availability in each type.
Both low andhigh pH problems are common in the greenhouse industry today.
What actionto take on pH depends on the specific requirement of the plants being grown anda knowledge of the factors, which interact to affect the pH of greenhousemedia. Limestone (rate, type, neutralizing power, particle size), irrigationwater pH and alkalinity, acid/basic nature of fertilizer, and effects of mixcomponents are the major influences on pH. Also, some plants are known tochange the pH of the growth medium; geranium is the best example. The activityof its roots can lower pH very quickly in a short period of time.
Solublesalts. EC is ameasurement of the soluble salts level in the growth medium and it provides ageneral indication of nutrient deficiency or excess. High EC is very common andgenerally 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 orphysical damage. Again, always check the condition of the root system whensampling soil for testing. Seedlings, young transplants and plants growing inmedia containing 20 percent or more field soil are less tolerant of high EC.Soluble salts above the normal range for prolonged periods may cause rootinjury, leaf chlorosis, marginal burn and sometimes, wilting. Soluble saltsbelow the normal range may indicate the need for increased fertilization.
Only minimuminvestments in money and time are necessary to start your own basic soil-testingprogram. Learning how to operate and care for the pH and EC pens and carefullyfollowing one method of analysis can help you get quick answers on the pH andfertility status of your crops.