Does Nitrogen Ripen Fruit?

Nitrogen in the Soil

• Nitrogen is not naturally abundant in soils because this element is not a major constituent of the parent rock materials from which soil is formed. The fundamental source of nitrogen is the atmosphere, which supplies nitrogen to special types of bacteria that carry nitrogen down to the soil. This atmosphere-to-soil method provides plants with a balanced source of slowly available nitrogen. Because refined fertilizers are not part of this balanced system, supplemental nitrogen has the potential to knock a plant's growth out of balance, which can impair fruit production, delay ripening or reduce quality.

Nitrogen in Plant Nutrition

• Nitrogen is essential for the production of proteins that plants need for building healthy tissue. It is also a component of chlorophyll, the primary substance involved in photosynthesis and which makes a plant green. Consequently, nitrogen encourages lush, vigorous vegetative growth characterized by vibrantly green foliage. This is beneficial for production and ripening of fruit because leaves manufacture the sugars that are stored in fruits. The fruit depends on the foliage. But excess nitrogen, or nitrogen applied at the wrong time, can cause a plant to focus on vegetative growth to the detriment of fruit production.

Nitrogen for Garden Fruits

• In general, excess nitrogen is more problematic for garden fruits than for tree fruits. If tender crops such as tomato (Lycopersicon esculentum) or winter squash (Cucurbita spp.) delay fruit production because of excess nitrogen, much of the fruit may fail to ripen before frost. On the other hand, nitrogen can over-stimulate fruit formation, possibly resulting in fruits with inferior flavor, aroma, firmness and nutritional content. Also, extensive foliage production resulting from improper nitrogen fertilization contributes to a shady, moist environment conducive to disease organisms.

Nitrogen for Tree Fruits

• Though tree fruits are susceptible to problems related to excess nitrogen, it is also likely that tree fruit production will be seriously limited by inadequate nitrogen levels. As with garden fruits, excess nitrogen encourages extensive dark-green foliage and vigorous shoot growth, leading to humid, shady conditions that reduce yield, delay ripening, lower quality and promote disease. Nevertheless, proper nitrogen fertilization is critical for fruit trees because this nutrient is required in the largest quantities yet is rarely abundant in the soil.

Supplying Nitrogen

• Nitrogen needs vary widely from one garden to another because factors such as rainfall, soil temperatures and organic matter significantly influence levels of available nitrogen. As a general guideline, gardens should receive about 1 pound of elemental nitrogen per 1,000 square feet. This could be supplied by 17 pounds of soybean meal or 5 pounds of ammonium sulfate broadcast and mixed into the top few inches of soil. A rule of thumb for fruit trees is to add 0.1 pounds of nitrogen per year for each inch of trunk diameter measured twelve inches above the soil line. This means that a trunk 6 inches in diameter requires 0.6 pounds of nitrogen, which corresponds to 10 pounds of soybean meal or 3 pounds of ammonium sulfate mixed into the soil around the base of the tree.