The important thing is if the test is higher or lower than the one before. The trend in the results over time is more important than the exact value reported by the test. The bet way to look at these trends is in a graphical format. An example is given in figure 1. The test can be a complete media test that is conducted by a commercial lab.
Some growers take samples about once per month and send the samples to the lab for analysis.
This media test can only be done with crops that will be in the greenhouse for 3-4 months or longer. If they are there a shorter time, you won't have enough time to get any information on trends in the media test. Leachate analysis also called pour-through This is a media testing technique, but you leave the soil in place and just try to collect the leachate. You will need a pH meter and a conductivity meter to conduct this test. Also this test should be conducted weekly or every two weeks.