The Pour‐Through method to test substrate‐pH and electrical conductivity (EC)

Goal: Use the pour-through method to test substrate pH and electrical conductivity (EC) for container-grown plants. This method is also called the leachate method. See the video in Back Pocket Grower for the Florida Nursery Best Management Practices method of pour-through testing of outdoor container plants.

Why is it important?

Plants are very sensitive to substrate-pH and EC. Low substrate-pH level may lead to micronutrient toxicities, whereas deficiencies in micronutrients are common at high pH. Plants will become deficient in nutrients if substrate-EC remains low. High EC can lead to salt burn of roots, root diseases and stunted growth.

How to measure it

Step 1. Requirements

You will require: a pH meter for pH measurement, an EC meter for testing EC, a plastic saucer for collecting soil solutions, a beaker for measuring pH and EC, a gallon of distilled water and the target potted plants.

Step 2. Irrigate the plants

Irrigate the plants

Irrigate the crop, close to container capacity with minimal leaching using the fertilizer or clear water solution that you are applying for your crop one hour before testing. Make sure the substrate is thoroughly wet, but drained at the time of testing.

Step 3. Sample collection

Pour through

Randomly select 5 or more pots. Place each pot into a plastic saucer. Pour distilled water onto the growing substrate surface to displace leachate from the bottom of the pot. Only around 30 mL (1 oz) of leachate is needed to measure pH and EC. To collect this amount of leachate typically requires that about 75 mL (2.5 oz) of water needs to be applied for a 15-cm (6-in) diameter pot. Applying excess water will dilute the leachate sample, resulting in an artificially low EC reading. 

Step 4. Measure pH and EC

Measure

Measure the pH and EC separately in each leachate samples using calibrated pH and EC meters. Calculate the average pH and EC for the entire crop.

How to interpret it

The Pour-Through Method is especially useful for substrate containing resin- or plastic–coated controlled–release fertilizers that can easily break when using the 1 soil :2 water (1:2) or saturated paste extract (SPE) method.

It works well with large pots where it is hard to remove the plant. It is also a non-destructive sampling method.

Results tend to be more variable than the 1:2 or saturated paste extract method so it is very important to be consistent in the sampling time after irrigations, and the amount of dilution of samples.

Containers smaller than 7.5-cm (3-inch) diameter pots, such as plugs or liners, are not usually measured with a pour-through method because it is difficult to obtain the leachate sample. Instead, substrate in small cells are usually tested using the 1:2, saturated paste extract or plug press method.

Compare the pH and EC measurements with the guidelines below:

Acceptable pH range for greenhouse floriculture plants

  • Iron-inefficient 'Petunia' group: 5.4 to 6.2
  • General group: 5.6 to 6.4
  • Iron-efficient 'Geranium' group: 6.0 to 6.6 

EC value (mS/cm) for greenhouse floriculture plants

  • Very low fertility: 0 to 0.7 (more fertilizer may be required)
  • Low fertility: 0.8 to 1.5 (young plants or salt-sensitive crops)
  • Moderate fertility: 1.6 to 3.5 (suitable for many established plants)
  • High fertility: more than 3.5 (may need to leach or reduce fertilizer concentration)

For floriculture crops, North Carolina State University recommends higher ranges than those listed above. Target EC ranges are lower than those listed above for plants grown with controlled release fertilizers (CRF), or slow-growing crops.  The Florida Nursery Best Management Practices guideline is 0.8 to 1.5 mS/cm for plants grown with either liquid fertilizer only, or liquid and CRF; or 0.5 to 1.0 mS/cm for CRF only.

For more information: Contact authors Jinsheng Huang and Paul Fisher of University of Florida IFAS Extension Thanks to our Floriculture Research Alliance at University of Florida sponsors including A.M.A. Plastics, Blackmore Co., Everris, Fafard et Frères Ltd (Canada), Fine Americas, Greencare Fertilizers, Pindstrup, PremierTech Horticulture, Quality Analytical Laboratories, Sun Gro Horticulture, and leading young plant growers. August 23, 2014.