According to the North Carolina Department of Agriculture and Consumer Services, there are 13 mineral nutrients that plants need from the soil. These are divided into six macronutrients required in large amounts and seven micronutrients, only required in trace amounts. The macronutrients are further subdivided into primary and secondary nutrients.
The primary nutrients are nitrogen, phosphorous and potassium. As indicated by their name, these are the most important nutrients for plant growth. Plants use large quantities of these nutrients, so fertilization is often required to ensure they are present in sufficient quantities.
Nitrogen is required for cell metabolism and for manufacturing proteins and enzymes. It gives chlorophyll its green pigment and encourages rapid plant growth. Nitrogen deficiency can cause older plants near the bottom of plants to yellow. The rest of the plant may take on a light green hue.
Phosphorous is also necessary for photosynthesis. It encourages the plant cells to divide and is important in the formation of flowers and seeds. It also encourages roots to grow. Phosphorus deficiency can be detected by burnt looking leaf tips. If the condition persists, older leaves will darken, "turning a dark green or reddish purple," according to The University of Arizona Cooperative Extension.
Potassium helps produce high-quality fruit and allows crops to resist plant diseases. It is involved in root growth and photosynthesis, as well. Potassium deficiency is very rare, since even desert waters and soils have enough potassium to nourish plants. When potassium deficiency does occur, it can cause the leaves to wilt or scorch.
The secondary nutrients are calcium, magnesium and sulfur. According to the North Carolina Department of Agriculture and Consumer Services, soils usually contain enough of these nutrients to supply plants without any additional fertilization. If they are not present in sufficient quantities, however, they can cause a variety of health problems for plants. Insufficient calcium can cause poor root and tip growth in plants and, in severe cases, can cause root rot and poor-quality new leaves. Magnesium deficiency can harm the production of the green pigment necessary for photosynthesis and cause yellowing or reddening of older leaves. Sulfur is necessary for producing amino acids, enzymes and vitamins. Sulfur deficiency causes young leaves to have a pale coloring, thin, spindly plants and whithering leaves.
The micronutrients are boron, copper, chloride iron, manganese, molybdenum and zinc. Although they are necessary for plants, they are generally required in such small quantities that they don't have to be supplemented. Recycling leaves, grass clippings and other organic matter into the soil provides plants with the essential micronutrients.