The 1:2 substrate:water method
Goal: Use the 1 substrate:2 water method to test substrate pH and electrical conductivity (EC) for container-grown plants. In some countries, a different ratio of substrate to water is used, for example 1:1.5 or 1:5, with similar methods but different target EC ranges.
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
- pH meter
- an EC meter
- 2 beakers for sampling and measuring pH and EC,
- a gallon of distilled water and
- the target potted plants.
Step 2. Sample collection
- Take samples from 10 or more pots within the group of plants to be sampled.
- Combine the substrate samples and thoroughly mix.
- Collect a small amount of substrate (about a tablespoon) from the bottom 2/3 of the container. The bottom 2/3 is typically where the roots are located in the pot. For plugs or bedding plant flats, use the substrate from the whole cell as a sample.
- Remove the big roots from the sample.
Step 3. Prepare sample solutions with one volume of substrate and two volume of distilled water
- Measure out a known volume of substrate from the combined sample into a beaker or cup. Usually 60 to 120 milliliters (2-4 ounces) is sufficient.
- The substrate should be compressed in the beaker so that it is close to or slightly more compressed than when it was in the pot. This can be done by tapping the beaker firmly on a table three times.
- In a separate container (not the container already containing substrate), measure a volume of distilled or deionized water equal to twice the volume of the substrate sample.
- Mix the water into the substrate sample and stir lightly.
- Allow the solution to equilibrate (30 to 60 minutes) before measuring pH and EC.
Step 4. Measure pH and EC
Measure both pH and EC directly in the slurry using calibrated meters.
How to interpret it
The 1:2 substrate testing method is a popular method used by growers to test pH and EC in container substrate. It is rapid, reliable, and has well-established guidelines.
To ensure the highest quality data, the testing procedures need to be consistent. Consistency starts with having a single, trained person performing the test.
- Measuring water into the same beaker that already contains the substrate can mean an incorrect amount of water is applied, resulting a false EC reading.
- Too much or too little compression will affect the results by changing the amount of substrate in the sample.
- The presence of fertilizer prills will provide an artificially high EC reading.
- When using slow-release fertilizer, consider the pour-through substrate testing method, or remove slow-release fertilizer prills before adding water to the sample.
Acceptable pH range
- 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 0.25 (more fertilizer may be required)
- Low fertility: 0.25 to 0.75 (young plants or salt-sensitive crops)
- Moderate fertility: 0.75 to 1.25 (too high for young plants. Suitable for most established plants)
- High fertility: greater than 1.25 (may need to leach or reduce fertilizer)
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.