Spring and early summer are the best times 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 1/10 to 1/4 of the annual nitrogen requirement foliarly in pre-bloom and post-bloom low-biuret urea sprays (when trees have tender new leaf flush on the trees, limit sprays to 10 pounds or less of low-biuret urea per 100 gallons of water). Post-bloom foliar sprays are commonly included with certain pesticide treatments after petal.

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 in fall-sampled citrus leaves between 2.4 and 2.6% nitrogen on a dry-weight basis for oranges, and between 2.2 and 2.4% 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 nitrogen levels greater than 2.6 % nitrogen. 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 can certainly be applied in excessive quantities. Excessive nitrogen is not only associated with fruit size and quality problems, but also with water contamination. How much nitrogen the citrus grove requires is a function of variety, irrigation system, 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.

Nitrogen requirements

What would be the nitrogen requirement per acre of a grove of 25-year-old drip–irrigated Washington navel orange trees on Carrizo rootstocks that yield 650 cartons/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 then split the remaining nitrogen application among two or three fertigations will probably require a total of 100 to 125 pounds of actual nitrogen per acre for mid-season and late navels and only 80 to 100 pounds for the relatively non-vigorous early navels. The use of significantly more nitrogen than this to maintain leaf nitrogen levels in the 2.4 to 2.6% range would suggest deep leaching of nitrogen through excessive irrigation may be occurring either through poor water scheduling or poor water distribution uniformity within the orchard.

Past 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. Application of nitrogen in furrow irrigated citrus is usually less efficient than with low-volume systems and annual nitrogen application rates may climb to 200 or more pounds per acre.

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 an efficiently irrigated high-yielding orchard in which leaves produced in the spring are analyzed and test 2.5 in September, will probably require about 120 lbs of nitrogen per acre the following season to produce a good yield and maintain 2.5% of nitrogen by dry weight in leaves the following fall when retested.

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

In some areas, well water can supply a significant amount of the nitrogen requirement, and growers should know the nitrogen content of their irrigation water. Nitrogen stored in the soil or present in organic amendments can substitute for chemical sources. However, some organic amendments can be sufficiently low in nitrogen (as those derived from yard waste) that their microbial degradation can actually induce a temporary nitrogen deficiency in a grove that would otherwise have sufficient nitrogen.

One of the biggest problems encountered by growers producing certified organic citrus is maintaining nitrogen nutrition. Fish oil, compost, and organic liquids that meet certification requirements, and that have high nitrogen to carbon ratios, are some of the products used by organic growers to meet tree nitrogen needs.

Critical levels for leaf-nitrogen for some varieties of citrus, like the grapefruits, pummelos, pummelo x grapefruit hybrids and the mandarins do not exist. The high-yielding grapefruits and their pummelo cousins and crosses, appear to have slightly higher nitrogen requirements than oranges, probably as a result of their vigorous vegetative growth habits and prolific yielding characteristics.

Trees of equivalent age will generally have a higher nitrogen requirement if grown on a vigorous rough lemon rootstock, for example, than if grown on trifoliate rootstock. Likewise anything that reduces tree growth and yield, such as water stress, late-stage rootstock/scion incompatibility, shallow soils, disease, or severe insect infestations, will reduce nitrogen requirements.

By keeping nitrogen as a nutrient and not a pollutant, the tree, the grower’s profit and the environment will all come out looking a lot better in 2011.