sowing seeds problems

hi everyone! i`m new today and i need some help please! every year when i sow any type of seed in trays in my greenhouse i lose a lot of my seeds through green algae type growth on top of my trays. it is an unheated greenhouse as i cant afford heat but theres plenty of air day times and i am careful with watering-even misting doesnt appear to help this situation. i was wondering if there was some chemical i could use when watering or include in the compost mix at sowing?? (i mix my own combination of compost according to what i am sowing)

I do a lot of hardy overwinter sown seeds and the green stuff has been a problem in the past. I now use grit on top of my compost and that seems to sort it. It probably makes the surface dry quicker but the seeds are still damp underneath. This is my theory. 

It usual a sign of too wet and too much compaction-perhaps the compost mix is too heavy?

Similar to nutcutlet I sprinkle vermiculite of the top rather than compost-seems to ease the problem.

As already said it's usually caused by too much wet. If your greenhouse is unheated it may be that there is too little evaporation for what feels like the right amount of watering. Making your compost more free-draining, using grit/vermiculite on top will help. Also suggest that you try watering from below - maybe use some capillary matting rather than misting.

Good luck

Rose5, Try making up a seed tray with half washed sand and grit and half compost, seedlings do not need much feed to start them off.
I actually use quarter compost and three quarters sand and fine grit then prick out as soon as two leaves are set into a half and half mix potting on later into two thirds compost one third grit.
You will find this drains better although I still put a covering of grit or vermiculite on top and as the seedlings grow they get more feed from a stronger mix of compost, the grit helps drainage..
Winter sown seeds do have problems if you do not have heat and in my part of the country I do not bother finding that early February gives me all the plants I need at the time I need them.

Frank.

I agree with Frank; I have been adding grit to compost for a couple of years and it makes a big difference.

thanks very much to you all i will certainly try watering from below and the grit on top andwill try more drainage to my mix-thanks to you all very much!

Vermiculite as a top covering rather than compost helps mine, also water from the base. It stops the seedlings rotting