Is the substrate-pH on target?

Plants are very sensitive to substrate-pH, which refers to the acidity or basicity of the growing mix. Low substrate-pH level may lead to micronutrient toxicities, whereas deficiencies in micronutrients are common at high pH. Here is the way to check if substrate-pH is on the target:

Are you picking the right pH test method?

Consistency starts with having a single, trained person performing the test.

  • The Plug Squeeze method for small plugs and liners.
  • The Pour-Through method is ideal for large pots or with slow release fertilizers in the mix.
  • The Saturated Paste Extract methodis commonly used by laboratories.
  • The 1 soil:2 water method is used by many growers.

Performing the right pH test

Target ranges

With all these methods, the target ranges are the same and a range from 5.6 to 6.4 is adequate. However, the ideal range is higher for iron-efficient crops such as geranium and marigold (target 6.0 to 6.6), and lower for iron-inefficient plants such as petunia and pansy (target 5.4 to 6.2).

Do crops appear healthy? 

Do not over-react, and use test results along with your experience.

Is your pH meter reliable? 

There are hundreds of pH meters available on the market, and some are not adequate for greenhouse production. Calibration procedures should be easy, quick and user friendly with 1 or 2 point calibration. A good meter should have automatic temperature compensation. If the meter will not calibrate, or the pH value slowly drifts up or down while reading, it may be time for a new sensor or meter.

Reliable pH meter

Is your pH meter calibrated correctly? 

pH values are only as good as the last calibration. Standard calibration solutions of 7.0 (and sometimes 4.0) are needed to calibrate the pH meter every day the meter is used. Standard calibration solutions cannot be recycled back into the fresh solution container after use. Just throw it away! 

Is the pH probe/electrode stored correctly?  

This is the most common reason pH meters deteriorate. Store pH meter, electrode and standard solutions at room temperature (not in the greenhouse). The pH probe can be stored in pH 7.0 standard calibration solution or according to the manual. Probes cannot be stored in distilled water or deionized water. If maintained properly, the probe can last for over a year.

Are the soil or solution samples taken correctly? 

Samples need to be randomly taken from different pots or trays distributed through the whole crop, but from the same cultivar and crop age. Take at least 5 samples and then combine the individual sample into one sample for pH testing. If a nutrient deficiency or toxicity symptom is suspected, it may be necessary to take samples from normally-growing plants separately from plants showing symptoms. 

Soil and solution samples taken correctly

For more information: Contact authors Jinsheng Huang and Paul Fisher of University of Florida IFAS Extension, and Dr. Bill Argo of Blackmore Co. Thanks to our Floriculture Research Alliance at University of Florida sponsors including A.M.A. Plastics, Blackmore Co., Everris, Fine Americas, Greencare Fertilizers, Klasmann-Deilmann, Pindstrup, Premier Tech Horticulture, Quality Analytical Laboratories, Sun Gro Horticulture, and leading young plant growers. The University of Florida does not endorse any product, and our research focuses on quality testing on these and competing products to assist grower success. June 30 2015.