I heard that it's good to put metals like steel in the soil in which the plants grow in. Is that true? How does it help the plant? Does it act like a source of minerals?
Minerals are metals. But, I'm not sure they'd be in any useable form in something like steel. Where did you hear that?
The best approach is to have your soil tested, and then use minerals, fertilizers, etc., to adjust the balance of nutrients to what is appropriate for the plants you want to grow. The nutrients that one plant requires in high concentration may kill off another plant, so it's best to check on the requirements for the types of plants you're trying to grow. If you're growing multiple types of plants together (such as in landscaping), you should check that they are compatible for the same soil types, and that your soil meets those requirements. If you have knowledgeable employees at your local gardening center, they can help with that, as can your county agricultural extension office (if you're in the US). Both are also good resources for finding out what types of plants will grow easily in your climate, and which will require a lot of extra care.
If you are concerned about Mg soil concentrations, I would have it tested by a competent soils lab, as Moonbear also described. Deficiencies occur most commonly in sandy soils, where this macronutrient can be leached out. A typical symptom of Mg deficiency is interveinal chlorosis (loss of green color between leaf veins).What about adding Mg in vast quantities since it is a standard metal solid. However from http://www.agr.state.nc.us/cyber/kidswrld/plant/nutrient.htm [Broken] it seems that inserting metals in vast quantities is unnessary if the soil already contains these nutrients when bought.