Hi. My bonsai's soil has little white bug. What is it?

I'm going to guess it is a springtail

İs it harmfull? İf it is harmfull, what should I do? İs ciggarette ashes usefull?

Can I use apple cider vinegar and water to soil?? What is Chinese Elm optimal soil ph?

Ozbek wrote: Can I use apple cider vinegar and water to soil??

Why would you want to do that? If it is a springtail, you need to adjust your watering. Let the soil dry between waterings, don't let it dry out completely.

What is Chinese Elm optimal soil ph?

I really have no idea.

I want it because low ph levels kills the springtail (I learned it in some forums) If I can't control springtail, they will spread the entire house!! So I want to get rid of them immediately.

From the first link on a Google search:
"Springtails are tiny insects. Their size ranges from 0.25 to 6 mm. They get their name from a spring-loaded structure, called the furcula, located on the underside of their abdomen. When the insect is disturbed, the furcula is released causing the insect to be flung into the air. One jump can cover 10 centimeters.
Springtails normally live in damp soil. They eat mold and fungus. They are common in flowerbeds, under logs, paving stones and landscape timbers. Woodpiles are also a common place for springtails to hide. Springtails lose water through the surface of their body. If their environment becomes dry, they try to migrate to a wetter place. When the dampness is corrected, the springtails disappear very quickly. Eliminating dampness is very important in preventing or eliminating springtails.
Springtails are mostly a nuisance pest, doing little damage to plants. They will chew roots in the soil where they’re located, and can inhibit plant hardiness. But they seldom do major damage.
Springtails frequently occur in the soil of potted plants. Inspect the potting soil of every plant you intend to bring home by testing it with your thumbs and watching for springing. Lichen or other soil covers that are organic food sources for the pests should be examined carefully. Be especially cautious if plants show signs of excess moisture. If you suspect insect activity, quarantine house plants, preferably outside, to allow the soil to completely dry before bringing them in.
Letting soil dry completely as possible is the best way to deal with infested pots already in the home.
Sprinkle Safer Brand diatomaceous earth on potted soils to help them dry and to repel springtails."

Well, it seems the answers are a Google click away, aren't they?

Of course I search at Google. I have already seen this link too. But I couldn't find home made insecticide to use on springtails. Maybe it's related to my bad english.

You do not need sinsecticide, you need to care for the plant properly. Or just accept there are living things in soil. A few bugs won't hurt.