clipped from: www.easybalconygardening.com   
Humidity
Correct humidity is essential to disease prevention and is crucial to the health of your plants. If the relative humidity (RH) drops below 30%, the growing process of your greenhouse plants will either be curtailed or halt completely. If the humidity goes above 80%, you will encourage mold not just on your plants but the greenhouse itself. That condition is an invitation to disease and also to the rotting of any wooden structures.
Fertilizer
If you are using the premixed type of fertilizer and you are not a proponent of industrial wastes in the blend you are using, make sure that you read the label. That label will also include the percentage of each essential nutrient your greenhouse plants require. Think organic whenever possible.