When preparing pot's or trays of compost for sowing seed's lightly firm the compost using the bottom of the seed tray or pot. Water is drawn up through the compost by capillary action and without firming air pockets occur and the water columns for capillary rise are broken.
Do not firm loam based compost's root's must be well aerated and have an open structure for optimum plant growth. This compost come's in four grades 1 seed , 2 potting for seedling's, 3 potting on the plant's, 4 growing plant's such as tomatoe's to maturity. Loam based compost's give a long lasting supply of food's.
Soil less compost's are based on peat, plant food's are added to it if plant's are left in there pot's for more than two month's they need regular feeding with a fertiliser. The surface can dry out it look pale instead of dark brown when this happens plunge the pot half way up the pot in water leave it until you see the surface getting wet air bubbles cease to rise from the compost.
By sowing seed's in a propagator it promote the ideal conditions for seed's to germinate constant warmth and humidity.They help in producing crop's which can be harvested day's or week's earlier while similar produce in the shop's is still expensive.
Some large propagators have warming cables above and below soil level. As ventilation is adjustable plant's can remain until well established.