Relaxed Farming - our farm in your classroom

As the days get longer and the nights less cold we are starting to plant more and more in the polytunnel. Our latest batch of plantings is a variety of seeds which we are sowing in pots/trays using fine bought-in compost without any big lumps or weed seeds. As the vegetable seeds are quite small, planting them in the main poly tunnel beds would mean they would have to compete with weed seeds, force their way through large lumps of compost, and be more at risk of being eaten by mice (who LOVE the pea, bean and coriander seeds) and other pests...
After almost filling the containers we firmed the compost down by gently pressing and then we planted the seeds - the golden rule is the seed is buried to about the depth of its size - the peas and beans in the photo have been pushed down, courgettes etc are lying on top ready to be buried.
Our second golden rule is to plant three seeds for each one we want - one for the weather, one for the pests and one for us! (Also see 14th March). However, some seeds are quite expensive and so these are carefully planted one in a pot and fingers crossed.
Our third golden rule is to label the pots, because no matter how good our memory is we are bound to forget which went where - we also make a record of when the seeds were planted.
And then the wait and the watering begins. Different seeds germinate at different rates - our carrot seeds planted on the 14th March are already over 3cm high, whilst the pepper seeds are yet to show any sign of growth - our parsnips also planted on the 13th March are not showing, but the leaf salad is pushing through the compost already!
The polytunnel can get very hot and watering at least once a day is needed for seeds in pots - why do the pots dry out so quickly when the onions and potatoes in the ground are fine?