Hi James,

Thank you for your question. 

Magnesium is included in the soil test because it is a critical nutrient for plant growth, acting as the fundamental component of chlorophyll and powering photosynthesis: https://extension.umn.edu/micro-and-secondary-macronutrients/magnesium-crop-production

Relatively high magnesium shouldn't be a direct hindrance your gardening success, but at very high levels, excess magnesium can limit a plant's ability to take up calcium, phosphorus, potassium, iron, and manganese. Reducing this number (as well as the phosphorus, which is a concern for runoff to our waters) is largely be a matter of not adding to what is already there, i.e., not adding fertilizers rich in magnesium/phosphorus or compost. This will allow plants to naturally take up the nutrients over the course of a few season.

You could use a cover crop or nitrogen fertilizer to ensure that nitrogen is available for plant growth.

For a bit about the possibility of compost as a source of excess magnesium and phosphorus, see: https://extension.umn.edu/nutrient-management-specialty-crops/correct-too-much-compost-and-manure

Hope this helps!