Just like people, plants need nutrition. In addition to needing light, oxygen, and water, plants need chemical nutrients. A healthy soil yields healthy plants. The soil is the permanent base of the garden, while the plants are temporary. Plants obtain carbon (C), oxygen (O) and hydrogen (H) from air and water, and other chemical elements from the soil.
Plants need different amounts of seventeen essential elements to grow. Enriching the soil, not just fertilizing the plants, provides a pathway to successful gardening. Soils in California usually contain an adequate supply of most of the essential elements that plants require. Only three elements nitrogen, phosphorus and potassium are usually low enough to limit plant growth unless fertilizer containing these elements is applied.
Macronutrients
Native soils are rarely fertile enough to supply adequate quantities of all the elements that plants need. The chemical elements that plants use in relatively large quantities are called macronutrients. They are divided into two groups with three primary and three secondary macronutrients. The primary macronutrients are nitrogen (N), phosphorus (P) and potassium (K). These nutrients, and their percentages of each contained in fertilizer, are always listed on fertilizer packages in alphabetical order. For example, a fertilizer that is labeled 10-15-5 contains 10% nitrogen (N), 15% phosphorus (P) and 5% potassium (K). The remaining 70% consists of other elements and fillers.
Nitrogen: Plants need nitrogen to grow healthy green leaves. Green leafy plants, such as lettuce and sweet corn, need a lot of nitrogen. Most soils do not contain enough nitrogen for optimum plant growth.
Phosphorus: Plants need phosphorus, to store energy, for root growth and flowering. San Diego soil is usually low in phosphorus. Annual vegetables and flowers usually need a fertilizer with phosphorus to grow well.
Potassium (potash): Plants need potassium to strengthen cells and resist disease. It helps plants absorb nitrogen and phosphorus. It also prevents leaves from yellowing around the edges and encourages flowering. San Diego soils vary in the amount of potassium they contain.
Vegetables and other shallow rooted plants usually grow better with additional phosphorus and sometimes potassium. These three elements, and any other elements that a soil may lack, can be supplied with an appropriate fertilizer and compost.
Fertilizers may be either organic (natural) or inorganic (chemical). Organic fertilizers release nutrients gradually as they decompose. Chemical fertilizers in a soluble form, release nutrients immediately for plants to use unless they are formulated as a slow release product. Compost and similar organic amendments contain a low concentration of many nutrients, but usually do not provide enough nitrogen and phosphorus for optimum plant growth.
There are also three secondary macronutrients: Calcium (Ca), Magnesium (Mg), and Sulfur(S). Most California soils have an adequate supply of these macronutrients. Go to the detailed Plant Macronutrients table for information on plant symptoms of deficiency and symptoms of excess.
Micronutrients
Essential elements that plants need in relatively small amounts are called micronutrients. The micronutrients are boron, chlorine, copper, iron, manganese, molybdenum, nickel and zinc. Adding composted plant foliage and other organic matter to soil is an excellent way to provide micronutrients. Go to the detailed Plant Micronutrients table for information on plant symptoms of deficiency and symptoms of excess.