How should I improve sandy soil?

Two weeks ago I've planted some seeds. It seemed like a perfect spot, on a clearing in a thicket near river. There's plenty of sunlight and concealment is absolutely perfect - you have to go through a lot of bushes, including thorny rapsberry bushes, to get to it. The only problem is that the soil contains a lot of sand. I though that it wouldn't mather that much because my plants are supposed to grow like a weed, but now I've noticed that they're growing slower than expected. I've accepted the fact that these plants won't amount to much so I'm going to plant a new batch. But how should I improve the soil? I was thinking of going to the forest and digging up a sack or two of the rich forest soil, but that seems like a lot of work, especialy since I can't take car before nightfall (my Dad works all day). My second idea was to take some of the compost from my back yard and mix it with the sandy soil on my growing spot. Would this be good enough? It's not really proper compost, it's just a pile in my orchard where we throw the grass after mowing our lawn. the grass after mowing our lawn. We don't turn it or antything, but the grass at the bottom is over a year old and it should be well decomposed by now. So, would this work or do I need to bring new earth to cover the poor one found at my spot? And is it too late for planting?