In the short term you could try Epsom salts, which is magnesium sulfate. There are longer term solutions but they depend on the level of other nutrients in the soil. For example Sul-Po-Mag can be used if potassium isn't already high, or dolomitic limestone if the pH isn't already high. Depending why your soil is somewhat alkaline you may not want to add more lime with the latter. If you're in the USA you could call your local Extension office and ask what they recommend as they know the soils in your area. It probably would be a good idea to get a lab soil test to find out for sure what the levels of various nutrients are.
