Spring is the best time to apply nitrogen to citrus. Research has shown that the demand for nitrogen in citrus is highest from bloom through June and most of the supplemental nitrogen fertilizer should be applied during this time period.

Citrus growers commonly apply about one-tenth to one-fourth of the annual nitrogen requirement foliarly in pre-bloom and post-bloom sprays. Usually, the post-bloom foliar sprays are included with pesticide treatments for citrus thrips after petal fall. Additional nitrogen is applied through the irrigation system at intervals through the growing season beginning in March and usually ending sometime in July or early August.

Late summer and fall applications of nitrogen in the San Joaquin Valley and interior desert regions tend to retard winter dormancy and promote vegetative growth susceptible to freeze damage., Fall or winter applied nitrogen, especially on light sandy or sandy-loam soils is subject to loss through the soil profile as a result of winter rains and irrigation water run during frost protection.

Citrus responds readily to nitrogen nutrition. Current and past research shows that if orange fall leaf-tissue analysis is maintained between 2.4 and 2.6 percent nitrogen on a dry-weight basis for oranges, and between 2.2 and 2.4 percent for lemons, a good balance is struck between yield, size and fruit quality. The evidence linking nitrogen to puff, crease, smaller fruit size and staining does exist, but these negative effects are most significant at the higher levels of leaf-tissue analysis.

Nitrogen balance

Some growers have decreased nitrogen applications for several years in the hope of improving fruit size and quality and now may have leaf-tissue analysis below 2.0. Research has shown that nitrogen deficiencies this severe in oranges will result in considerable yield losses.

Nitrogen should not be applied in excessive quantities. Excessive nitrogen is not only associated with fruit size and quality problems, but also with problems of water contamination. How much nitrogen the citrus grove requires is a function of variety, rootstock, tree age, productivity, vigor, and the efficiency of how it is applied. For mature trees, at tree densities normally encountered in commercial groves, nitrogen requirement is most accurately calculated on a per-acre and not a per-tree basis. As a result of crowding and mutual shading, a closely spaced tree will use less nitrogen than one in a more open planting, but since there are more trees per acre the closely spaced trees will use a similar quantity per acre than the more open planting.

What would be the nitrogen requirement per acre of a grove of 25-year-old Washington navel oranges on Carrizo rootstock that yield 600 cartons per acre of fruit?

If we assume good irrigation efficiency and scheduling, growers who apply the bulk of their nitrogen through frequent but small injections of fertilizer through the irrigation system (six or more times through the season) with the rest applied foliarly, may maintain tree health, high fruit yield and quality of mature navels with as little as 80 pounds of actual nitrogen per acre. Those who apply nitrogen foliarly and, perhaps, split the nitrogen application among two or three injections will probably require 100 - 125 pounds of actual nitrogen per acre for navels.

Recent research by Drs. Lund and Arpaia with the University of California have shown that relying totally on foliar application of nitrogen will produce a tree with a thin leaf canopy.

In spring flush leaves, some empirical data suggests that each tenth of a percent of leaf nitrogen by dry-weight over and above 2.5 percent nitrogen is equivalent to the storage of approximately 10 - 15 pounds per acre of nitrogen in the trees. Trees in a high-yielding orchard in which leaves produced in the spring are analyzed and test 2.5 in September, will probably require about 120 pounds of nitrogen per acre the following season to produce a good yield and maintain 2.5 percent of nitrogen by dry weight in leaves the following fall when retested.

Trees in an orchard in which spring flush leaves test 3.0 percent in September, will probably require a total seasonal nitrogen application of only 60 pounds per acre the following season to produce a good yield of fruit and maintain leaf-nitrogen content of 2.4 - 2.6 percent when sampled that fall. Conversely, trees whose leaf tissue contains only 2.0 percent leaf nitrogen in the fall will probably require approximately 180 pounds of nitrogen per acre to bring leaf levels to 2.4 - 2.6 percent the following year.