My first setback

It was bound to happen. THe first problem arose this week. I had filled two SWAMP boxes with potting soil from Cedar Grove which is a mix of sand and recycled yard waste. My wife planted lettuce, arugula and spinich seeds. I set the boxes inside in a south window of an unheated house and covered the soil with a thin black plastic mulch cover. After two days I peeked. The soil was overgrown with a quarter inch layer of fungus which looks like the fungus I used to see on starting plants which is associated with damping off disease. I expect to see that here also. I expect that the seeds either won't sprout or will die as soon as they sprout. Chalk one up to experience. That's why we're doing this. I will restart those plants in sterile potting soil.