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.