A material that nourishes growing plants. Fertilizers can be chemical or organic. Most chemical materials are derived from non-renewable resources such as coal and natural gas. They provide soluble nutrients that are readily taken up by plants but do not provide food for and sometimes repel soil micro-organisms and earthworms. Treating your soil with only chemical fertilizers can lead to a loss of organic matter, soil structure, and water-holding capacity, making your garden even more dependent on increasing inputs.
By contrast, organic fertilizers feed both plants and soils. For the organic gardener, fertilizer is used sparingly to bring out the best performance in plants. The soil in an organically managed garden is typically rich in biological activity and nutrients, creating the ideal conditions for plant growth.
Fertilizers have a standard code to show their relative makeup and to indicate their best use. Complete organic fertilizer has a rating of 4-4-4. These numbers indicate N (nitrogen), P (phosphorus) and K (potassium). The letters are consistent with the periodic table of elements, and always occur in this order: N-P-K. Nitrogen, phosphorus, and potassium are considered the “primary” nutrients, followed by the secondary group Ca (calcium), Mg (magnesium), and S (sulfur). Many plants absorb more calcium from the soil than they might phosphorus. These six elements are considered “macronutrients,” and must be available in some quanities for plants to grow. “Micronutrients” are also important to plant health, and they include B (boron), Cu (copper), Fe (iron), Mn (manganese), Mo (molybdenum), Zn (zinc), Cl (chlorine), and Co (cobalt).