Many growers experiment with various ways of performing their own soil tests for pH and soluble salts on-site. Others send samples to different labs in search of timely and inexpensive media testing. Often growers end up with a confusing collection of tests done different ways with widely different numbers.
One report from the University of Massachusetts Extension’s Greenhouse Crops and Floriculture Program is meant to review current ways of testing media on-site or at a commercial lab and how they are differ.
Nutrient Extraction Techniques
The major difference between the current methods of greenhouse media testing is the way plant-available nutrients and soluble salts are extracted from the media samples for analysis. Differences between the actual methods of pH, soluble salts, and nutrient analysis and the laboratory equipment used to do the analyses are not as important as how the plant-available nutrients are extracted from the sample.
When most people think of soil testing, they might imagine some procedure which analyzes the solids in a mix for pH, nutrients, and soluble salts. However soil samples themselves are not actually analyzed during a soil test, but rather plant-available nutrients are pulled out or extracted from the sample using an extracting solution.
A massive amount of research has been done over the years to determine what the best chemical extracting solutions are for outdoor field soils used in farming. The situation is simple for soilless greenhouse media: plain water has proven to be a very effective extracting solution to determine fertility status. However, one of the main difference between testing methods is how and in what volume the water is used to produce an extract to test for pH, soluble salts, and nutrients.
For more information, check out the article from the University of Massachusetts.