https://www.reference.com/home-garden/soil-fertility-fd6838fde6db1eea#
I am worried about a layer of soil spread over your 1.5 feet of compost. This is kind of upside down.
A normal soil has horizons; the very first one is mostly pure organic matter in some stage of decomposition, leaves, twigs, rotting logs. The next horizon is topsoil. Mostly a few inches deep. Then there are the alphabet labelled layers down to bed rock.
When there is decomposed organic matter sitting on top of the top soil, the organisms in the soil wake up, come out of dormancy, climb up to the surface to eat this decomposed organic matter, whew, I am going to call this DOM for short. After eating, the micro and macro organisms then go back down into the soil and poop it out. This mixes the DOM into the top soil better than we humans can do.
The majority of soil life only lives in soil that has air. Compacted soil or water logged soil starts using a different process that is anaerobic. Most plants have evolved to thrive in aerobic soil ecologic systems. DOM keeps pore spaces open and able to be filled with air. As important to clayey soils as it is to sandy soils.
Subsoils are compacted, lack air. Depending on the soil type, soil life is limited as well as DOM. What little 'fertility' or chemistry plants have to have (to photosynthesize and make their own food/carbohydrates) that might have made it past the hungry decomposer community and leaching would actually wind up in the subsoils, not the topsoil. Kind of interesting.
If there is little to no decomposed organic matter at the surface, life in the soil goes to sleep, goes dormant, dies until there is decomposed organic matter available at the surface. Once there is decomposed organic matter, DOM, the 'food' all the other non decomposing organisms eat...the life revives, reproduces and the soil is better for plants.
If you go and get soil out of some virgin forest or even from a rain forest in the tropics, that soil is going to have very little if any 'fertility' or chemicals that plants need to grow, reproduce make food for us humans and food animals. Most of all the chemicals plants need to uptake for photosynthesis to take place is locked in the live biomass. Really helps curb population growth of plants. Nature adds just enough to the soil of an established ecosystem through decomposition of organics to allow perhaps ONE new plant to be added or allowed into the ecosystem. Nature isn't into mono crops and efficiency of bushels per acre. There is never the correct amount of fertility in any soil unless we humans try to figure it out and add it to the soil in just the correct amounts; concerning fertilizer my dumb ditty is : Less is best, More is death and None is dumb. Fertilizer is in no way plant food, plant nutrients...just chemistry and we have to put our lab coats on to understand how this works. Just a little too much and it is death for plants. When a gardener has become experienced, they are able to SEE the symptoms of excess of a chemical, lack of a chemical and when everything is just right. I fertilize usually only when I see a deficit. I check the pH of the soil in pots and all garden soils.
Everything we humans touch is artificial. The closer we can stay towards using indigenous soils, indigenous plants, learning how to manage those resources correctly the better our crops will grow. Potting soil is sterilized and contains amazingly very little soil if any soil at all. Sounds a bit like your soil; you made a pot or maybe a 'swimming pool' (this is what I've imagined) and filled it with unsterilized, possibly raw or hot organic matter. Too much nitrogen. That will kill trees, certainly. Drainage is suspect. Swimming pool needs drainage.
It would be nice to have a picture of the bottom of this...swimming pool. Do you remember what that soil was like? What are your plans? A landscape or vegetable garden? Adding balanced fertilizer is a must. Compost, unless hot doesn't add but miniscule bits of Nitrogen.
When something is being decomposed the decomposers are an entire community whose job it is to decompose EVERYTHING that dies, immediately. They are in the soil, on our skin and in the air. They are everywhere and they use Nitrogen for energy to do their work. They have first priority. The decomposing community have dibs on any Nitrogen available before plants get the left overs.
As long as we still have stores and the internet, I will always buy a balanced fertilizer from the store. Cheaper than soil tests, I have a very good idea what is in my soil. I have added decomposed and aged horse manure as a winter cover. The soil organisms do the mixing all winter long. I make sure to use a lower percentage of Nitrogen in the fertilizer for the next season's crops.
An initial professional soil test is gold. Whatever one adds to the soil from that point on will be calculated based on that initial test; either added to the numbers for your 'fertilizer' program, subtracted (adding non decomposed organic matter such as chips or straw is a subtraction from any balanced fertilizers added). A second soil test a year or two later will quadruple the benefits you get doing the soil tests.
That will tell you whether or not your fertilizer program needs changing (build up of too much phosphorus for example, no nitrogen or potassium left over for the next crop).
I am worried that your compost with a foot of soil over it will become a stinky swimming pool of stagnant mud or it could be the reverse. Your compost could cause a 'perched water table'. An upside down perched water table. Whatever type of soil spread over the compost would not be able to drain well and that would keep that layer of soil wet and mushy. The fine pore spaces of the new soil over the large fluffy pore spaces of the compost would cause any water drainage in that soil to stop at that barrier until the entire layer of new soil is saturated. Then the water will begin to drain into the compost. Filling the swimming pool? Need to SEE what you've got and first thing I'll be looking for is negative drainage away from this 'swimming pool' how to accentuate that drainage and make it legal?
I apologize for length of this 'non' answer. I am thinking through the different scenarios your 'boo boo' made. Seriously interesting. I've never seen this problem in real life. It is amazing though because people really need to do the 'right' thing, get lots of 'wrong' advice and few give up before they remove 1 1/2 feet of 'top soil' and fill with compost. You've done this and my brain is working over time. I would love to hear the Master Gardeners at the Co-op trying to figure an answer. You have to let us know what they say. I'd like to save you any more major work and dough. Do you have a friend with a backhoe familiar with a backhoe insured to use a backhoe? Have you used a rototiller? (Mixing). Also will need (sounds odd by now but) a compactor. I've no idea the size of this project but at this depth and percentage of DOM, you might need a road compactor.
