I strongly suggest that you research what other container nurseries use for a growing medium. You'll find that your mix would be highly undesirable. River bank soil and top soil would turn into something pretty dense and non porous, especially within the confines of a container.

The vast majority of nursery medium products are bark based...conifer bark, to be precise. Such mixes are highly porous, long lasting, not likely to support root diseases, etc. I'd look for something with conifer bark fines as the primary ingredient, peat moss and perlite as minor components. Bark should be anywhere from 80 to 100 percent for best results.

Most nurseries around here use peat moss as a substrate. I know that it's best medium for growers (hight water retention, no weeds, lightweight) but in my opinion, it's not so good for customers. When customer plants peat moss grown tree and doesn't take proper care of it, there is high chance that this tree will die. When peat moss dries up, it shrinks and roots can't get water from nearby soil. We acquired quite a lot of loyal customers because we used to grow in top soil (they probably had bad experience with peat moss grown trees). We intend to use peat moss as a component for aeration of substrate. What I have in mind is compost like this: 50 percent river bank soil, 20 percent top soil, 15 percent compost, 15 percent peat moss + osmocote exact. I reckon peat moss is great for 1-2 years old trees, but we are going to grow trees in containers for 5-6 years. Roots in peat moss tends to be thin, while in top soil they tend to be thick with a lot of adventitious roots on them. I'm talking of my experience (greenie nursery man xD) so take it lightly. I think it might be worth a try and if anything goes bad we still can use peat moss as substrate. By the way, that river bank soil is sandy and very rich in nutrients with almost no clay whatsoever and our top soil is very low in clay too. We shall see...

oh and peat moss gets covered by marchantia polymorpha soon after potting. At least that's what I saw in many local gardening centres. It doesn't like top soil substrate for some reason though.

I don't think peat moss makes a good potting medium, either. That's why I suggested the conifer bark. I've never seen an outdoor container nursery use peat moss (or anything like what you are suggesting, either). Even back in the 80s, when I was operating whole sale nursery, our mix was composed of 80% bark. Today, more nurseries are using closer to 100%.

I doubt you will get much support for your proposed 'recipe' from many other professionals -) FWIW, I confirm everything Rhizo states - neither topsoil, rivebank soil, compost OR peat moss are suitable container potting ingredients. They are all too dense and without adequate pore space and aeration......and peat moss will provide nothing that alleviates this.

I have worked at both a large wholesale grower and several retail nurseries, including one with a large growing operation. Even for small plugs or annual starts, they used a bark-based medium and the wholesale grower wouldn't consider anything else.

Liverworts are omnipresent in this business :-) And they will grow just as easily on topsoil as on anything else....come collect a few from my own garden! They will colonize any type of soil surface with a high moisture content and high nutrient levels but are less inclined to populate coarse, well-draining media such as those with high levels of bark, pumice or perlite. Do not overdo fertilization. Treating with Mogaton will help as will topdressing the containers with agricultural charcoal.

Seems like I will have to reconsider my plans. I hoped I wouldn't have to increase price of our arborvitaes, but price of 3 gallon container + price of coniferous bark substrate might change it. There is a lot of garden centres/nurseries around us, and so far we were able to compete with them price-wise. Should we have to buy containers/substrate, we wouldn't have such competitive prices anymore and we could possibly lose a lot of customers. I think, safest option for us is to keep 75 percent of arborvitae grown in soil and try to grow some of them in 3 litre containers and see how it goes. Thank you both for answers, I'm gonna have a closer look at this coniferous bark substrate.

Go ahead and try it. I've been using field soil in my pots for 8 years now. My soil is about 30% sand, 30% silt, and the rest clay, so it is a heavy dense soil. Water lightly.
Watch your pH. I've done some side by side work comparing colorado spruce, lodgepole pine, dogwood, silverberry, choke cherry plugs planted into soil vs plugs planted in a bark based mix. Only the silverberry noticed.

Details: 410 plugs for conifers, 412 plugs for deciduous, planted into Beaver Plastic styroblock 15 x 1000 ml blocks. (15 1 liter cavities per block)

If you are really worried about drainage, put 1.5 inches of wood chips covered with newspaper in the bottom. The paper will keep the dirt from filling in the pore space in the chips before it consolidates.

Soil has advantages over bark. You don't end up with that abrupt transition between the root ball and the surrounding soil.

For some plants it's too heavy.

I'm experimenting with compost based mixes since I can get it from the city inexpensively.

To hedge your bets, you may want to try raised beds. Till your field, then run a bed shaper over it. then plant your liners in the raised bed. Some bed shapers will also apply plastic mulch at the same time.

Of all the variables when I was running nursery stock, the one I'd be least tempted to monkey with was the potting medium. It'll make ya or break ya. Ditto on recommending bark based. Then add .....from a reputable source.

So far it seems to work better than I thought. About 8 cubic meters of river bank soil has been used mixed with compost and top soil (about 3-1-1 ratio). We decided to experiment a bit so my father only use this substrate for potting, while I use riverbanksoil/compost/topsoil mixed with commercial moss/bark compost (2-1 ratio). After 6 weeks everything grows like mad and I have to admit that adding moss/bark substrate makes it a lot easier to move around and yet it doesn't tip over even in strong winds. Otherwise I see no difference between them. We have been granted permission to take as much river bank soil as we want (flood prevention), but we are forbidden to sell it which we never intended to do anyway. So far, so good.

year has passed and I'd like to say this: "Substrate... I'll make ya or break ya" has burned into my head and it shall never be forgotten. Not that trees couldn't be grown in river bank soil or topsoil. They could. But boy has my life changed over the last year as I started to grow in peat moss with clay particles and wetting agent incorporated. No more weeding, no need to be precise on watering, everything grows same or better than before. As I said year ago, all nurseries around here grow trees in pure peat moss with particle size of 20-40mm. There's a reason for it. Price. Peat moss is a lot cheaper in Europe than composted pine bark. In fact, pine bark substrate is about twice as expensive per cubic metre. If proper care is taken, you can even get rid of marchantia polymorpha almost completely. It requires application of Mogeton before end of april so no spores will be blown around.

Mogeton is not registered for use in the US. How did you account for pH? Peat moss tends to be so highly acidic as to be an inappropriate media for any but the most strict acid lovers.

Peat moss we use is packaged in 4 cubic metres bales, dolomite is used to keep PH at 5,5 - 6,5, it has AquaSorb wetting agent incorporated so it doesnt become hydrophobic when it dries up (it's a german made mix and to be honest I'm very surprised by effectivity of this wetting agent. It has clay particles in it for ability to hold on fertilizer and make it a bit heavier. It's too bad you can't get Mogeton, because it works like a treat althought it's a bit pricy (9 Euros for small package)