All plants draw nutrients from the soil through their roots and, unless nutrients are replenished, can deplete the soil of nutrients, resulting in poor growth and heightened susceptibility to disease. Tomatoes are heavy feeders, meaning that they extract large quantities of nutrients from the soil as part of the growth process, so replacing the nutrients they need is essential for a bumper crop.
Nitrogen is one of the most important nutrients for plant development, regulating the production and use of energy, growth and fruit production and the formation of chlorophyll. Although nitrogen is abundantly available in the air, it is not a form where it is usable by plants. Gardeners must, therefore, add nitrogen as part of a fertilizer or use planting practices that restore nitrogen naturally, such as composting or crop rotation. The University of Missouri Extension website recommends using a fertilizer with 6 to 8 percent nitrogen. Overfertilization with nitrogen produces big, bushy plants with few tomatoes.
Phosphorus encourages plant growth, aids in photosynthesis and protects plants from stress. Phosphorus occurs naturally in some rocks but, like nitrogen, requires replenishment when growing tomatoes. The University of Missouri Extension recommends using fertilizers high in phosphorus when growing tomatoes. Fertilizers should contain 24 to 32 percent phosphorus. Organic phosphorus sources include bone meal and rock phosphate.
After nitrogen, plants absorb more potassium than any other minerals. In tomatoes, adequate potassium helps to produce quality fruits, in addition to powering certain metabolic processes and protecting plants from disease. Fertilizers containing medium levels of potassium, 16 to 24 percent, help tomatoes to thrive. Greensand, kelp meal and wood ashes provide natural sources of potassium.
Tomatoes are prone to a disorder called blossom-end rot, where the part of the tomato opposite the stem becomes dark and sunken, appearing rotted. Blossom-end rot is caused by a calcium deficiency, often exacerbated by inadequate water. Because calcium moves into the plant through water, if the ground is dry, the tomato cannot absorb the calcium it needs. Although adequate watering prevents blossom-end rot, "Rodale's Ultimate Encyclopedia of Organic Gardening" suggests ensuring that enough calcium is available by throwing a handful of bone meal or crushed eggshells into the hole when transplanting tomato seedlings.
Magnesium is needed to produce chlorophyll and is essential for photosynthesis. Tomatoes deficient in magnesium will develop yellow leaves that eventually wither and die. Because potassium inhibits the uptake of magnesium, and tomato fertilizers tend to be high in potassium, magnesium deficiencies are common in tomatoes. Adding a pinch of Epsom salts to the hole when planting tomato seedlings increases the availability of magnesium for your plants.