Try using a larger grade of perlite. Or substitute pumice or turface or other high-fired, pelletized clay products. You need something of sufficient pore size to bump up the drainage and provide aeration. Even adding very coarse sand into the mix will help. The compaction is due to the very similar particle size of the peat and soil - with insufficient quantities of perlite, either through initial application or because it is rinsed away, all you are left with is mud. And since heaven only knows exactly what "topsoil" is, you might try purchased humus, screened compost or bark fines in its place. And are you adding ground limestone to counter the pH effects of the peat?

You might find screening through some of the posts or archives of the Container Gardening forum helpful - lots of discussions on seed starting or container mixes and various recipes. Also there is always the option of going with a bulk commercial mix that is already blended.

You could also try adding some vermiculite to keep the soil moister and help with compacting. I would also add some fir bark. I used to make my own potting mix too, probably have some lung disease now from the peat moss dust- awful! Better to leave the mixing to the professionals- buy bulk. The time and manpower you spend in labor mixing your own blend is probably equal to or less than the cost of buying it.

My first impression: You have a heavy dirt mix. There are positives and negatives to that. The heaviness of the media is part of why so much floats to the surface.

My second impression: You have an extraordinarily high ratio of perlite. Again, part of why so much floats to the surface.

How much mixed soil do you use in a season? And how committed are you to using your own blend?

What grade perlite do you use? I'll second the suggestion to find a heavier grade.

I'd question using vermiculite. Yes, it's a substitute for perlite, smaller size and darker color...so it's harder to see when it floats away, but's it just as light.

Are you by any chance able to locate bark fines? It'll help you do all kinds of things, pretty much all very positive.

A great substitute for perlite is grain hulls. I've usd hammermilled rice hulls and am quite satisfied. I've also heard of oat hulls being used, and the results were positive, but have not used them personally, but I would if I had a good local source. One tremendous advantage of grain hulls as opposed to perlite: No dust.

Thanks for your suggestions. What all 3 of you say makes sense - the perlite we use is very fine grade (it's horticultural grade - very dusty). And the top soil is heavy in sand. So, you are right Gardengal - the components all have similar size. I've been mixing soil almost every day since it's spring and lots to repot and seeds to sow. We use the equivalent of about half a cubic yard a week. My concern with substituting perlite, is that many of the plants remain in their same pots for years, and I know bark breaks down (although I prefer it for my own use). Many of these plants have historical value in our collection and are decades old. So I want to take care as I'm repotting them! Honeybunny - I agree about using commercial mixes - I think they're great. And Heptacodium, I will look into grain hulls - they sound ecologically friendly.