Here’s a reminder from the Vermont Vegetable and Berry Growers Association:
USE THE RIGHT TEST FOR YOUR GREENHOUSE SOILS
Remember to test your potting soil and/or compost-amended greenhouse soils. Do not use a standard field soil test; use the Saturated Media Extract test, also called Greenhouse Media test. This test includes soluble salts, available nitrate and ammonium, as well as pH and water-extractable nutrient levels. It is relatively inexpensive compared to the cost of any reduction in crop growth. Make sure your mix or soil has been moist and warm (room temperature) for at least a week before testing.
The cost is $22 at UMaine and now includes organic matter; at UMass it is $15 and you should ask for the organic matter test, too, and add $5. Put my e-mail at the bottom of the submission form if you want me to get a copy of the results so I can provide recommendations.