For each pound of vegetables harvested, do you lose that many pounds of soil?

No, the plant is mostly water and most of the dry matter is carbon. Imagine drying the vegetables and burning them. The ash you will get is the phosphate, potassium etc. which has come from the soil.

However, cultivation of soil dramatically reduces the organic matter, by mineralising it and turning the soil carbon into CO2.

But it's worse if you cultivate a field every single year and manure it, but don't give it a break by letting the grass grow and allow animals to graze on it.

Worse still is cultivating a field every year without even feeding it any manure or compost, only synthetic mineral fertilisers. This will, pretty effectively, turn topsoil into subsoil (sand, clay, gravel etc.)

As I have said before, please don't make sweeping generalisations about agriculture. Vegetables are grown differently in different parts of the world. There are good ways to grow them and bad ones. There are ways they are grown widely now, ways they have been grown in the past and ways they potentially could be grown in the future. They are not the same. Just like meat, milk and eggs

Do not lose that many pounds of soil.

Mass of vegetables comes from water and fixed carbon in photosynthesis.

Neither. The plant gets its mass from the air. See my source.

E equals m c squared.

That's Einstein's famous equation. If you convert mass to energy, mass times the speed of light squared equals energy. The speed of light is huge, and the speed of light squared is extra huge. A tiny bit of mass gives an incredible amount of energy. That's why nuclear power (and nuclear bombs) require so little fuel. You're suggesting going in the other direction, turning energy to mass. It would take an amazing amount of energy to make a little bit of mass.

Many nuclear bombs have less than 10 pounds of material that undergoes the nuclear reaction to be turned into energy. In theory, it works in either direction. So, to get a10 pound plant by converting energy to mass, you would need more energy than a small nuclear bomb.

You would also need technology that we currently don't have, but that's a different issue.

No you don't loose the soil. You will loose a small amount of minerals but the rest is produced from water and the carbon dioxide in the air using photosynthesis. That's why we are all Carbon based life forms.

The only thing the soil loses is a few nutrients, namely nitrogen fertilizer. The plant's mass comes from H2O and CO2 NOT the soil

The water was taken from the rain, Carbon from atmospheric Carbon Dioxide. The rest from the soil.

The plant doesn't turn the sun's energy into mass, but it combines carbon from the carbon dioxide in the air with water.

nope. a lot of the mass will be water.