There are sixteen elements essential for plant growth. About 90 percent of the plant’s dry weight is cellulose and sugars made from the carbon, hydrogen, and oxygen obtained from air and water, and much of the plant’s fresh weight is water used to transport nutrients, for chemical reactions, and for cooling plant tissues.
The plant gets the other thirteen elements from the soil. These can be divided into the major nutrients, or macronutrients, and minor nutrients, or micronutrients. The macronutrients are not more important, but they are used in greater amounts. Nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) are the macronutrients.
They are further subdivided into the primary (nitrogen, phosphorus, and potassium) and the secondary (calcium, magnesium, and sulfur) nutrients. The primary nutrients are the ones most often lacking in garden soils. The secondary nutrients are found in plants in smaller quantities than the primary nutrients but are just as essential. The micronutrients are manganese (Mn), iron (Fe), zinc (Zn), boron (B), molybdenum (Mo), copper (Cu), and chlorine (Cl).
Nitrogen (N)
Nitrogen is the nutrient most responsible for increases in plant growth and strongly promotes the growth of leaves, stems, and roots. Leafy crops, such as spinach, lettuce, and kale, need large amounts of this nutrient. Atmospheric nitrogen, which makes up about 80 percent of the air we breathe, cannot be used by plants directly, but must first be converted by microorganisms or fertilizers into nitrate and ammonium forms. The nitrogen compounds in organic fertilizers also must be broken down into these simpler forms before plants can use them.
The ammonium form is volatile but doesn’t leach readily. Bacteria convert it into the nonvolatile nitrate form, which leaches easily, especially in sandy soils. This bacterial conversion is rapid in warm soil. When soil is compacted and poorly aerated, bacteria convert nitrates back into atmospheric nitrogen, making it unavailable for plant use.
Bacteria use nitrogen to break down soil organic matter. The more organic matter you put into the garden, the more nitrogen will be used to help it decay, which means that what is perhaps the most useful plant nutrient is rapidly lost. In fact, about 50 percent of the nitrogen in manure volatilizes and is lost before plants can use it. And 30 to 60 percent of the nitrogen applied in commercial fertilizers is used by soil microorganisms to decompose soil organic matter. This means that only about half the nitrogen you apply to your garden is available for use by the plants.
Plants in the legume family actually add nitrogen to the soil. Nitrogen-fixing bacteria live in tiny nodules on the roots of these plants and convert atmospheric nitrogen into forms useful to the plant. Peas can actually fix about 72 pounds of nitrogen per acre, beans 40 pounds, soybeans 58 pounds, and peanuts 42 pounds. For this reason, these crops do not need to be heavily fertilized with nitrogen.
Some nitrogen is also added to the soil by other organisms, lightning, and rain, but the amounts are negligible.
Plants deficient in nitrogen are stunted with stiff, fibrous stems and small, yellowed leaves. The discoloration appears first on the older bottom leaves of the plant, then moves to progressively younger leaves.
Phosphorus (P)
Phosphorus promotes the growth and development of roots, flowers, fruit, and seeds, and stiffens the stems of plants. Much of the phosphorus in the soil is not available to the plant, and phosphorus that is applied sometimes becomes quickly bound and unavailable for plant use. Phosphorus is most available at a soil pH between 5 and 7. Because phosphorus does not move readily in the soil, it must be applied in the root zone, where it can be quickly taken up by the plant.
Phosphorus deficiency delays plant growth and maturity and causes the stems to become thin and short. A purple-pink color develops on the undersides of the leaves, then progresses through the entire leaf.
