Welcome Grandpappy to Got I gather you are doing a pond I personally would nt throw any of the educated soil back into the pond in case any stones are amongst it which would puncture your liner. You can get pond baskets to plant your plants in filling the top of the plant in basket with small gravel prevening the soil spilling into your pond from the soil in basket.

Agree, don't put any soil into it. As Thrupennybit says, pond plants are planted in basket type pots with growing medium contained inside those. Just fill it up with water now, then buy the plants you want, the proper growing medium for ponds, and the basket pots and add those later.

welcome from me too. the rigid liner is usually fibre glass. certainly pack soil in tightly between the liner and the hole you dug. this will help give stability to the sides.

Usually I put the plants for the pond from the purchased pot into an 'aquatics' pot then fill with gravel to anchor it down. water lily rhizomes are tied into the basket using fishing line as it is transparent and doesn't rot, then fill the basket/pot with gravel. Wash /rinse any gravel/stones you use before putting in the pond. It is usually filthy. There is enough nutrients in the plants compost so you don't need to add more.

if you intend to have a few fish they will provide all the nutrients the plants will need.

Before you go any further make sure....
-That there are no sharp rocks or pointed stones at the bottom of the hole to puncture or crack the bottom of the liner when there are several hundred pounds of water are pressing on it.
-place a carpet liner on the bottom before you place in your ridged liner, again to prevent cracks or punctures as the pond settles into any unexposed sharp objects over the years.
-fill the pond slowly 1/8 at a time backfilling the area between the liner and the hole and tamping down and around using a piece of 2x2 lumber blunted on the working end by sanding. This is what you use your excavated dirt for as well as using it for....Leveling...
-Yes, here is the hardest part to rigid liners, at every stage from initial placement to gradually filling in stages to backfilling in stages you must continually check that all parts of the top rim are continuously level. You can do this by getting a board that will sit across the diameter of the pond and placing a 3 ft level on the spanning board and continually check that the level is true all around the clock as you set the pond in and fill with water and back fill. Do this over a period of a week to allow for settling. This is an aggravating task but it will result in a water level that parallels the top of the pond rim and not an unsightly water level that is low on one side and high on the other.
-do not use rocks to cover over spaces between the pondliner and your hole. If you don't close that space by backfilling with soil, water will accumulate there and when the temp goes below freezing the accumulated water will freeze, expand and crack your liner for good.
-REMOVE! any dirt that falls into the pond before you start filling with water and during your backfill be very neat and (by working slowly) that a minimal amount of dirt inadvertently falls into the pond. Otherwise you will have a mud puddle and not a pond.
Remember- no dirt in the pond! You may use your rocks to set upon the rim once your pond has settled in for a more natural look as well as some plantings around the outside of it.
For now do not place anything in the pond, it is too late in the year for that. During this time the most important life form will be born in your pond, the Biofilm. Go on the web to get a good explanation of this marvelous living thing. Next spring, go to the pond store or garden center and purchase only two water lily plants that come preboxed in net bags. That will be all that you need for next year. They don't look like much but they grow fast and will flower as things heat up. If this is going to be a static pond with no pump, every month overflow the pond by 1/4 to rid it of hydrogen sulfide gas(that rotton egg smell that goes with stagnant water.
With no water pump it would be best if you have no fish. There will be no oxygen for them and oxygenating pond plants will not be of any use to them. They will just suffer greatly. Don't worry though frogs will find your pond or you can purchase a few green frogs from a biological supply house. No bullfrogs if you want to keep your neighbors happy. In the spring your water will look like green pea soup and filamentous algae will form. No algicides for as your pond plants spout up they will remove excess nutrients and the water will clear up somewhat. Remember that your pond will transform into a living thing sustaining all kinds of life forms which you can see and millions you will not able to see without magnification. It is not meant to be as clean as your bathtub. But be patient will this. Go slowly, the following year you can place a water iris in a basket as mentioned previously. As for mosquitoes there is a larvacide that goes by the name of "mosquito dunks" that kills only mosquito lavrve and is harmless to anything else, plant or animal. With the mosquito borne Zika virus this is a must for your pond and everyone else's. Have Fun!
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Wow Loosestrife! That was almost a 'pamphlet' :) I learned a lot, I did mine all wrong because I put loads of oxygenating and 'shelf' plants in it straight away. I'm now wondering if that's why I had such a problem with algae (sorted it now) We can't buy frogs in the UK as far as I know, but if anyone knows better I would be chuffed as I cant seem to get them in mine, and I am very impatient! But you 'can' buy pond snails which eat algae so are very helpful in your pond :)

In buying pond snails I would keep them in a bucket of water for a few days. The reason for this is to check for leaches which will detach looking for other meals.

...another mistake then. Not to worry!