Potassium (K)
Potassium is soluble and leaches unless held for plant use by soil organic matter. This nutrient stimulates the vigor and general health of the plant and promotes root growth. For that reason, root crops, such as beets and carrots, need more potassium than other crops.
Plants deficient in potassium develop gray or buff-colored areas near the margins of older leaves. In time, the entire leaf margin becomes scorched, and light yellow areas may develop throughout the leaf.
Calcium (Ca)
Calcium is used to make strong cell walls. This nutrient is seldom deficient, because garden lime contains calcium, and it is also added along with the phosphorus in fertilizer, in the form of dicalcium phosphate. Under water stress conditions, however, some plants may have difficulty absorbing and transporting it to all parts, resulting in blossom end rot in tomatoes and peppers, cavity spot in parsnips, cabbage tipburn, black heart in celery, and internal browning in Brussels sprouts.
Magnesium (Mg)
Magnesium is used in the production of chlorophyll. It is added to the soil in the form of dolomitic lime or epsom salts. Although magnesium is soluble, it is held in place for plant use by soil organic matter and clay particles. A deficiency might occur in acid, sandy soils or if too much lime or potassium is added to the soil. These elements interfere with the plant’s ability to absorb soil magnesium.
Magnesium deficiency causes yellowing between the veins of older leaves. This discoloration then moves to younger leaves, and the older leaves fall prematurely.
Sulfur (S)
Sulfur deficiency is rare, thanks to air pollution. Rainfall provides between 1 and 100 pounds per acre of this nutrient every year. Plants need this nutrient, in about the same amount as phosphorus, to make certain proteins. These compounds give radishes and the crucifers their pungency and odor.
Micronutrients
Micronutrients are seldom limited in most gardens if you take care of the soil. Deficiencies may occur in soils very low or very high in organic matter, coarse-textured or strongly weathered soils, or soils with very high pH. Of these, pH is the most important factor. Manganese, iron, boron, copper, and zinc all become less available as the pH increases, so keep the soil pH between 6 and 6.5.
Boron is the micronutrient most often deficient. Beets, turnips, celery, broccoli, and cauliflower are most susceptible to boron deficiency, which usually shows up as a blackened sunken spot with internal or external dead or water-soaked areas. Fruits are rough and spotted, and flowering is reduced. Stems of some plants, particularly the crucifers and celery, are brittle, hollow, or cracked.
With micronutrients, there is a thin line between too little and too much. Excessive amounts of some, especially boron and manganese, can damage sensitive crops, such as cucumbers. Do not add micronutrients to your garden unless you have had a deficiency positively identified by a specialist. Then, carefully, follow the label directions and recommended rates of application.
