December 2001

Comparing Substrate Tests By Todd J. Cavins, Brian E. Whipker and William C. Fonteno

Substrate testing is a great tool to help ensure a healthy, profitable crop and to identify improper nutritional trends before problems occur. For most situations, monitoring pH and EC (electrical conductivity) in the greenhouse is sufficient to help keep plants nutritionally on target. Both PourThru and 1:2VS (one part substrate to two parts water volumetric slurry) are great tools for testing these parameters in the greenhouse. Occasionally, a complete substrate analysis is necessary. When a substrate sample is sent to a commercial lab or university for complete nutritional analysis, an SME (Saturated Media Extract) is commonly performed.

With all of these options, you might be wondering, Can I send my PourThru and 1:2VS samples to a lab for complete analysis? How do these three substrate test values compare? The first step in answering your questions is to look at how each test is performed.

• Irrigate. Irrigate the crop one hour before testing, making sure the substrate is saturated. If the water supplied by your automatic irrigation system varies, then water the pots/flats by hand. If using constant liquid feed, irrigate with fertilizer solution as usual. If using periodic feeding: (A) irrigate with clear water and test a day or two before fertilizing and (B) test on the same day in the fertilizing cycle.

• Place a saucer under the container. After the container has drained for 30-60 minutes, place plastic saucers under the containers to be sampled. If you are testing seedlings in bedding plant flats, place one cell pack in each saucer.

• Add water. Pour enough distilled water on the surface of the substrate to get 50 mL (1.7 oz) of leachate in the saucer. The amount of water needed will vary with container size, crop, irrigation system and environmental conditions.

• Collect and test the leachate for pH and EC. Make sure you get about 50 mL (1.7 oz) of leachate each time you test. Leachate volumes over 70 mL (based on 6-inch pot extractions) will begin to dilute the sample and give a non-representative EC reading.

• Collect root substrate. Remove the plant from the container and collect substrate from the middle 1/3 of the rootball. Place 1 cup of root substrate into a clean container. If controlled-release fertilizer is present, remove the capsules and ensure no capsules are damaged (damaged capsules will release fertilizer and provide nonrepresentative EC values).

• Add water. Add a volume of distilled ç water equal to two times the volume of the root substrate (two cups water).

• Mix. Thoroughly mix substrate and water. Allow the mixture to set for a minimum of 20 minutes, but not more than one hour.

• Test the samples for pH and EC. Test the slurry 20 minutes to one hour after mixing. Take care not to damage the pH or EC probes on substrate components when inserting them into the slurry.

•ÊCollect root substrate. Remove the plant from the container and collect substrate from the middle 1/3 of the rootball. Place two cups of the root substrate from multiple plants into a clean container that can be transported to the lab without spilling. If necessary, remove slow-release fertilizer capsules (ruptured capsules will release fertilizer and provide nonrepresentative EC values).

• Add water. Mix in a volume of distilled water that is just enough to cause the substrate and the water mixture to “glisten.”

• Allow mixture to equilibrate. The amount of time varies; 20 minutes to one hour is average.

• Extract substrate solution. Three options are available. The first is a vacuum system; the second is gravity filtration; the third is cheesecloth. Note: Some labs test the pH of the substrate at different stages of the test. Testing after vacuum extraction can result in values approximately 1/2 a pH unit lower than pH measures taken in the substrate-water mixture previous to vacuuming or pH values obtained by gravity filtration or cheesecloth extraction.

The major difference between each of the tests is the dilution factor. PourThru is an undiluted substrate solution sample (the substrate solution is simply displaced by the distilled water). With SME, the substrate is diluted just to the point of saturation (approximately one part substrate to one part water), and 1:2VS is super-saturated with twice the volume of water as substrate. The amount of dilution is important because as the amount of water added increases, the EC and nutrient concentration values become lower.

Since PourThru extractions are not diluted, they can be sent to labs for complete analysis. Combine the leachate from five samples and send it to a lab in a clean, sealed container. Keep in mind that the lab may not provide recommended nutrient concentration values. However, conversion provided in this article will allow you to compare your PourThru values to standard SME values (See Figure 1).

The SME extraction is lab oriented and most commonly performed when substrate samples are sent to commercial or university ç labs for complete analysis. When sending in a substrate sample for complete analysis, collect about two cups of substrate from various plants. Historically, SME values have been the standard pH, EC and nutrient values used for horticultural crops. Values reported on fact sheets and in magazine articles are generally SME values unless otherwise noted. Figures 2, 3 an d 4 provide examples of how SME and PourThru pH, EC and nutrient values compare for several crops.

Substrate samples collected by the 1:2VS method are generally not analyzed for nutrient levels by commercial or university labs. The 1:2VS method produces samples that are so dilute that nutrient values may not be representative of the actual nutrient availability. However, 1:2VS is still a good method for on-site pH and EC testing. When comparing 1:2VS to PourThru and SME, remember that pHs are very similar. However, ECs obtained with 1:2VS should be much lower than PourThru or SME values (See Figure 2).

pH. PourThru, SME and 1:2VS pH values are nearly identical. There is slight variation, but the differences are not large enough to warrant various pH ranges for each testing method. Remember, as previously noted, that the same method can produce slightly different values (See SME protocol).

EC. There are large differences in EC values among the three tests. EC can be a good indicator of nutrient status. Keep in mind that EC measures the total salts (nutrient and non-nutrients) in the solution and does not indicate which nutrients are present or in what concentration.

An EC meter works by measuring the ease of which an electrical current can travel through a solution. The higher the nutrient concentration, ç

the easier an electrical current can move through a solution; thus, producing higher EC. The less dilute a sample is, the higher the nutrient concentration. Therefore, if PourThru, 1:2VS and SME were performed on the same crop, PourThru EC values would be highest followed by SME and then 1:2VS (See Figure 2-4).

The same relationship observed with the three testing methods and EC values also occurs with the individual nutrient values (See Figure 2-4). A sample comparison of PourThru and SME values for an actively growing poinsettia is provided (See Figure 2). In addition, PourThru and SME comparisons for geraniums and pansy are also provided (See Figures 3 & 4).

The bottom line is, substrate tests are helpful only if performed. Although complete nutrient analysis is necessary from time to time, on-site routine pH and EC substrate testing is an insurance policy that can eliminate up to 90 percent of nutritional problems found in today's production systems. The conversions provided allow you to determine the proper pH, EC or nutrient values even if the recommended values are based on different substrate testing methods. For further information regarding PourThru and other substrate testing methods as well as corrective procedures and recommendations, visit www.pourthruinfo.com, www.floricultureinfo.com or www.substrateslab.com. The PourThru Nutritional Mon-

itoring Manual can be purchased for $15.00 plus postage by contacting the North Carolina Commercial Flower Growers' Association at (919) 334-0093.