@ Parmnparsley- Plants can suffer from deficiencies just like people.
Magnesium deficiencies are characterized by yellowing between the veins of the plants, and the withering of the leaves. When leaves wither from magnesium deficiency, they tend to curl up on top of themselves rather than under themselves.
The cause of magnesium deficiency in your garden is likely due to imbalances of other minerals or problems with your water's PH levels. Too much calcium or potassium will cause magnesium to be bound to the soil, not allowing the plants to assimilate the nutrient.
To fix the problem add a tablespoon of Epsom salt (Magnesium Sulfate) to a gallon of water and spray your plants with the solution. You want to facilitate foliar absorption if the PH or Nutrient balance in your soil is preventing your plants from absorbing certain nutrients.
You should also test the PH levels in your water and soil. Hard water and acid water will prevent root absorption. Add a teaspoon of wood ash (not cherry) to a gallon of water to increase alkalinity by 1 point. Wood ash also prevents slugs from eating your plants and will eventually neutralize the PH in your soil.