Unsuccessful Greenhouse Culture

I have purchased all my herbs for my greenhousefrom Richters in the >last two years (the total of my limited experience) and met with great success my first year. However, last summer all my herbs (a great variety) seemed to have stunted growth if any and many simply did not survive. I believe that they were well watered which leads me to believe I might have a problem with my soil. I have read through your database of Q & A and understand that I should enrich the soil (5-6 inches deep only) with a compost or well rotted manure but perhaps it needs to be completly replenished with new soil. I did use (on your reccomendation) a fish fertilizer last year as I did in the first. I am looking forward to making another attempt this year and would greatly appreciate your help.

A lot depends on the type of soil you have underneath the 5 to 6 inches (12 to 15 centimeters)of soil. Do you have boxes on top of benches filled with this soil or did you put a good mix on top of sand or clay or even a concrete floor?
Whatever your setup, the soil will have to be replenished every year. Micronutrients especially get depleted and their absence has a devastating effect on growth. Mix some well-rotted manure or compost into your soil which should replenish some of the micro-nutrients. This is probably all you need if you are working with boxes or beds on top of sandy soil. You might wish to have your augmented soil analysed for micronutrients in case the soil and plants and therefore your compost are deficient in some micronutrient(s). You would then have to add this nutrient -possibly from an inorganic source.
If you are working on top of a clay or concrete base the problem would be an excess of accumulated salts from lack of drainage. You would have to renew the entire soil every year.