Nitrogen and Plant Nutrition

Published: March 1, 2012

While plants need all of the essential mineral elements in order to grow, nitrogen is considered by many growers to be the most important. This is due in part to the large role nitrogen plays in the growth and reproduction of a plant, and the (relatively) short-lived nature of nitrogen in the soil. Two of the more important roles of nitrogen in the plant include the fact it is a major component of protein, and it is part of the pigment chlorophyll. The latter is responsible for intercepting the energy from light in the process of photosynthesis. Therefore insufficient amounts of nitrogen results in plants with a pale green color due to the lack of chlorophyll and reduced yields due to the lack of photosynthesis.

As an element, pure nitrogen is a gas and cannot be taken up by plants. Instead, plants take up compounds that contain nitrogen. As a fertilizer, nitrogen is available in two forms: nitrate (NO3)- and ammonium (NH4)+. Given their preference, most vegetables (as well as other plants) would prefer to be supplied their nitrogen in the nitrate form. Nitrate nitrogen is readily available to the plant and typically has low salt index compared to fertilizers containing the ammonium form of nitrogen (e.g. ammonium nitrate or urea). Selecting fertilizers with low in salt indexes is especially important in high tunnel production because of the tendency for salts to build in the latter. Unfortunately, a majority (by percentage) of the nitrogen found in many commercial fertilizers is in the ammonium form, due to its lower cost.

Fortunately, ammonium forms of nitrogen are converted to the nitrate form by nitrifying bacteria in the soil. However, this conversion is temperature-related and, in cool soil conditions typical of high tunnels or the spring of the year, the ammonium form of nitrogen can build up. For many vegetables the result is poor nitrogen nutrition; for sensitive species such as tomato ammonium toxicity can result. While fertilizers high in the ammonium form of nitrogen are acceptable for summer crops such as sweet corn, they are not acceptable for many vegetables, including tomato. Therefore it is to a grower’s advantage to supply the majority of the nitrogen requirement of a crop such as tomato in the preferred nitrate form.

Along with a preplant fertilizer containing nitrogen, many commercial tomato growers choose to fertilize with calcium nitrate and alternate weekly with a fertilizer high in potassium (4-18-38). If phosphorus is adequate, then alternating calcium nitrate with potassium nitrate is another possibility. Calcium reduces the occurrence of blossomend rot and there is research that suggests potassium has a positive effect on fruit quality.

If a "general purpose"� fertilizer is the primary source of nitrogen, be sure to check its label to determine the amount of nitrogen it contains in the ammonium form. For example, 20-20-20 is was one of the first water-soluble fertilizers available. However, it contains 72 percent of its nitrogen in the ammonium form which is not acceptable for tomato production. Most growers "fertigate"�, or apply nitrogen via a drip system as they irrigate. Most tomato roots will be concentrated in a 24- to 30-inch-wide section of the bed or row. Multiplying the root zone width by the length of each row and the total number of rows of tomatoes equals the effective bed width. Additional nitrogen often is at a rate of 8 to 10 pounds per acre per week, or 2.9 to 3.7 ounces per 1,000 square feet per week. Plant tissue testing is the best way to determine if a crop is being supplied with adequate amounts of nitrogen and should be conducted on a regular basis.