The saturated paste extract method

Goal: Use the saturated paste extract method (SPE) to test substrate pH and electrical conductivity (EC) for container-grown plants. This method is also called the saturated medium extract (SME).

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 do it

Step 1. Requirements

  • pH meter
  • EC meter
  • a beaker for measuring pH and EC
  • a spatula for mixing sample
  • coffee filters for extracting solutions
  • a gallon of distilled water
  • the target potted plants

Step 2. Sample collection

  • 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.

Step 3. Add distilled water into substrate samples to the saturation point

  • Add distilled water
    Place 4 to 8 oz (120 to 240 mL) of the collected substrate in a beaker. Note that when taking a sample to send in for laboratory analysis, two cups (400 mL) of substrate is generally required.
  • While constantly stirring the sample with a spatula or knife, slowly add distilled water until the sample has reached a consistent moisture level. The saturation point occurs when the sample becomes a thick slurry that behaves like a paste, the surface glistens with water, but there is no free water on the surface of the sample.
  • Allow the solution to equilibrate for 60 minutes.

Step 4. Measure pH and EC

  • Measure pH and EC
    Measure pH directly in the slurry using a calibrated pH meter.
  • Extract the solution from the substrate by squeezing the slurry through a paper towel or a coffee filter.
  • Measure the electrical conductivity (EC) of the extracted solution using a calibrated EC meter.
  • Compare the pH and EC measurements with the guidelines below.

How to interpret it

The Saturated Paste Extract method is commonly used by laboratories. The SPE can also be successfully conducted onsite by growers, so long as careful attention is paid to how the sample is prepared.

To ensure the highest quality data, the testing procedures need to be consistent.

  • Consistency requires a single, trained person performing the test.
  • The level of saturation may only have a slight effect on substrate-pH, but can have a more significant effect on substrate-EC (e.g. low water volume can lead to a falsely high EC reading).
  • 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.

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: 0.75 to 1.9 (typical for young plants or salt-sensitive crops)
  • Moderate fertility: 2.0 to 3.0 (too high for young plants. Suitable for most established plants)
  • High fertility: >3.0 (may need to leach or reduce fertilizer)

Target EC levels are lower for slow-growing plants, and crops grown with controlled-release fertilizers.  A range from 1.0 to 1.5 mS/cm is recommended for woody ornamentals by Mylavarapu, R. and T. Yeager. 2011. UF/IFAS Extension Nutrient Management Series: Container Media Nutrient Test Interpretation. EDIS Bulletin SL180, edis.ifas.ufl.edu.

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.