If they included governmental owners, it'd dwarf these private holdings. For example, the largest landowner is really the United States Federal government: 640 million acres.
I am not sure what the exact break down of useful vs non-useful is, but I know most of the land in my state that is owned by the government could be used for other things.
Keep in mind to we are not just talking Bureau of Land Management, but also Park Service, All Federal Agencies many of which have abandoned, or under used facilities in many towns and cities through out the US, Military who owns large sections of land that are non-active bases, etc etc etc
Several states have even attempted various law suits because the federal government is preventing economic development of the land
For federal land, significant projects are open for public comments, and they absolutely respond to the people who live in the area. I've seen numerous development projects proposed by the rich get shot down after the community got involved through the comment process.
Do we live in the same country? Do you actually want other people to tell you what you have to do with your private property? I suspect that if you were a land holder being told what you had to do with your land you would be rather miffed.
I just don't understand how a citizen of the US living under the US Constitution comes to the conclusion that they should have authority to tell other people what they can do with their property.
There are other ways to mitigate this problem than giving voters the authority to dictate how a property owner uses their land.
It seems obvious that ForHackernews wasn't talking about things like the chemicals you've described.
What exactly do you mean by this? Private land is for private use.
On the government side it amazes me we can't get / keep lead, mercury, and other toxins out of land and resources we collectively own. Dilution also looks like a very expensive and increasingly ineffective answer to the pollution problem.