Bringing hiking shoes with dirt/soil to the US?

Something to understand here:

Yes, our agricultural inspection people tend to care about mud on shoes–they care about anything that can carry pests or disease that we don’t have here (Which is why the lists can seem pretty nonsensical at times–they don’t care about the items, they care about hitchhikers.) However, they aren’t interested in playing gotcha unless you fail to declare something. If you have any question about whether they care about something you declare it and see what they say–there is no penalty (other than loss of the item–note that in this case the forbidden item would be the dirt, not the shoes) for bringing forbidden materials. The only time they’ll actually penalize you (I’ve seen it happen) is if you don’t declare the material.

Yes. It’s a problem.
Yes, you need to declare it.
No, it’s not a ‘big deal’ but you’re going to have to stop at the USDA booth and they’ll take your shoes and disinfect them with some nasty spray. Having them clean as possible will make it easy. But if they’re caked in dirt they’re going to make you wait while they do it.
Source: Been there, done that. Came back from England during Mad Cow with a backpack. And other trips.

U.S. CBP does care about soil on your shoes, but as long as you’ve made a reasonable effort to clean them, then it won’t be a problem, especially if the soil was not from a farm or some such thing.

As gerrit mentioned, two of the questions on the customs declaration form that you fill out when entering the United States are:

I am bringing (…) (d) soil or have been on a farm/ranch/pasture

I have (We have) been in close proximity of livestock (such as touching or handling)

When I actually had visited a farm in Thailand and so answered yes to the first of those questions, the immigration officer asked about it. When I told him I had been to the farm in Thailand, he had me go to the secondary agricultural inspection.

At agriculture inspection, they asked what sort of animals I had been around and when I told them that it was elephants rather than livestock, they weren’t very concerned with that. However, they did still ask whether I had mud and such on the shoes or clothes that I was wearing while there. In my case, it had been dry while I was there and the ground was mostly covered in grass, so I (truthfully) told them that my shoes probably had some light dust, but no caked on mud. They said that wasn’t a problem at all and didn’t even bother physically looking at anything in my bag.

If it had been rainy while I was there and my shoes had actually been muddy, I expect they’d have had me clean them as gerrit experienced, especially if I had been around cows or other such livestock rather than just elephants.

Given that they weren’t concerned at all about the light dust and such, I don’t think you’ll have any problem at all if you’ve thoroughly cleaned your shoes but were just unable to get them completely clean. Since you haven’t even been on farms, it sounds like you should truthfully be able to answer ‘no’ to the inspection questions, in which case they’re unlikely to ask about, look at, or care about your shoes at all.

Just declare the shoes as soiled and let customs officials handle the rest.

I live in Australia, we’re an island so these things are strictly controlled. If you’ve cleaned them well they generally let you through, otherwise they will clean them for you. I actually don’t mind the cleaning service, it takes them about 2-3mins compared to my 10min effort.

Cleaning them thoroughly should be good enough.

Here is what happened when we tried to get in with dirty boots in March 2016.

At Charlotte International Airport, we were spotted and stopped by agricultural inspectors while waiting to collect our luggage. They took us into a separate room and provided us with shoe cleaning stuff. We were made to clean our shoes thoroughly and only let out after the inspectors were satisfied. They were friendly, polite, and entirely reasonable, but we did nearly miss our connecting flight.

Last time I brought hiking boots into the US, the customs form asked two separate questions:

I am bringing (…) (d) soil or have been on a farm/ranch/pasture

I have (We have) been in close proximity of livestock (such as touching or handling)

Since we had been hiking in England, crossing common land grazed by cows, we answered "yes" to the first question. See this question for more information. This was a later trip (May 2018) than the shoe cleaning debacle, we had learned our lesson and nothing special happened despite answering yes.