In the minds of many gardeners, the word "nitrogen" is synonymous with "healthy plants." This is true; however, too much nitrogen can do more harm than good, especially if the goal is a bumper crop of ripe, red tomatoes. Tomatoes growing in soils with high nitrogen levels will certainly look leafy and healthy, but unless other nutrients are available in the proper ratios, the main show---plump, juicy fruits---will suffer as a result.
Most fertilizers sold for home garden use prominently display three numbers: nitrogen (N), phosphorus (P) and potassium (K). The nutrients are always listed in that order, and each of these numbers represent the respective nutrient's weight as a percentage of the total weight of the package. Nitrogen is essential for vigor and vegetative health; phosphorus encourages flowering and fruit set, and potassium is essential for strong root growth.
Home gardeners should always have a soil analysis performed to determine their exact soil composition before applying any type of fertilizer. Many types of soils have adequate levels of phosphorus and potassium, but over-fertilizing tomato plants with too much nitrogen can stymie fruit production for a number of reasons.
Choosing and Using the Right Fertilizer
University agricultural extension offices and tomato horticulturalists recommend using a fertilizer with lower ratios of nitrogen as compared with phosphorus and potassium. An all-purpose low-nitrogen fertilizer such as 5-10-10 will work well, but specialized tomato fertilizers with low nitrogen, high phosphorus and moderate potassium levels are optimal.
Apply 1 pound of 8-32-16 or 6-24-24 fertilizer for every 100 square feet and work well into the top 6 inches of soil. To encourage continued flowering and fruiting, side-dress plants after they begin to set fruit. Apply 3 pounds per 100 feet of row of a fertilizer such as calcium nitrate into the top inch of the soil around the tomato's base.
An obvious sign that a tomato is receiving too much nitrogen in its diet is the presence of copious quantities of huge limbs full of bushy green leaves and little in the way of flower production. While that wall of green may perform well to conceal your garden from the world, it probably means you're not getting many tomatoes. Plants with excessive vegetation lose more water through respiration, which creates problems with calcium supply in the plant, particularly to any developing fruits. This usually shows up as blossom-end rot.
This affliction causes the bottoms of developing fruits, or blossom ends, to soften and turn black, usually also rotting the inside of the tomato fruit. Calcium deficiency due to excessive plant respiration causes the cells at the ends of the fruit to collapse, which are then attacked by pathogens that rot the fruit. Side-dressing a tomato plant with a low-nitrogen fertilizer after it sets fruit can help alleviate incidences of blossom-end rot.
Blossom-end rot also manifests during periods of drought, so water tomatoes regularly during dry spells.
While cold nighttime temperatures are the main culprit behind blossom drop, high nitrogen levels in the soil can also cause tomato plants to drop flowers after they form, completely inhibiting fruit set. Excess nitrogen levels should be suspected if a tomato plant drops blossoms even when nighttime temperatures are adequate, which is generally between 55 and 75 degrees.