A collaboration between

Acknowledgments

Acknowledgments

Funding provided by:

Guidelines and Webpage Design:

Daniel Geisseler, Ph.D.; UCCE Specialist in Nutrient Management; Department of Land, Air and Water Resources, University of California, Davis

Reviewers:

Ben Faber, Ph.D.; Farm advisor; University of California Cooperative Extension Ventura County

William R. Horwath, Ph.D.; Professor of Soil Biogeochemistry and James G. Boswell Endowed Chair in Soil Science; Department of Land, Air and Water Resources, University of California, Davis

Support:

Patricia Lazicki, Department of Land, Air and Water Resources, University of California, Davis

Amadou Ba, Ph.D.; Branch Chief Feed, Fertilizer, and Livestock Drugs Regulatory Services, California Department of Food and Agriculture

Amrith Gunasekara, Ph.D.; Science Advisor to the Secretary; California Department of Food and Agriculture

Last Update: September, 2015

Additional Information:

Citrus Nitrogen Uptake and Partitioning

Citrus Production in California

California Fertilization Guidelines

Citrus

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Young
Trees

Dormancy

Spring Flush;
Bloom

Fruit
Development

Harvest

Applying some of the N with foliar sprays and the rest by fertigation has been found to be a very effective N fertilizer program, which ensures high yield and fruit quality, while reducing the risk of nitrate leaching ^{[N5, N7]}.

Rate based on Rate Trials

Rate based on N in Fruits

Mode of Application

Fertilizer Type

A number of mineral N fertilizers are available to growers. Mineral fertilizers contain N in the form of urea, ammonium, nitrate or a mix of them. These N forms behave differently in the environment. Nitrate is very mobile in the soil and can easily be leached below the root zone with irrigation water or rain. Urea is also relatively mobile and is generally quickly converted to ammonium. Ammonium is less mobile, but is generally converted quickly to nitrate by soil microorganisms in warm and moist soils, unless they are water saturated. This process, nitrification, can lower soil pH. The acidifying effect is especially strong when ammonium fertilizers are applied by drip systems as they are concentrated in a small soil volume ^[N44].

Citrus trees are sensitive to biuret, which is a byproduct of urea production. Biuret toxicity is characterized by chlorosis of the leaf tip (yellow tip). The leaves do not regreen ^{[N19,

N20]}. Urea containing up to 2.5% biuret has been found to be safe for soil applications ^[N20]. This concentration is higher than the biuret concentration found in urea fertilizers currently produced in North America, which have a biuret concentration of less than 1.0-1.3% ^[N30]. Biuret does not accumulate in the soil and is broken down in about 6 weeks ^[N20]. Therefore, for soil applications, biuret is generally not a concern.

Studies carried out in Florida on sandy soils showed that controlled release fertilizers may reduce the risk of nitrate leaching losses. However, when leaching is minimized with careful timing of fertilizer applications and irrigation, the benefits of controlled release fertilizers are likely small ^{[N4, N39]}. Furthermore, in one study controlled release fertilizer applications in early February resulted in reduced yields. The authors concluded that N release was too slow to meet the high N requirement during the initial 3–4 months after its application ^[N6].

Fertigation

Many soluble N fertilizers, such as UAN, urea and ammonium sulfate work well with fertigation. Anhydrous and aqua ammonia can also be used for fertigation. However, they increase water pH, which may result in precipitates if the water contains calcium or magnesium. The high water pH also increases the risk of ammonia volatilization losses ^[N42].

Time of Application

Foliar N

Foliar Nitrogen

Foliar fertilizers can correct nutrient deficiencies more rapidly than soil applied fertilizers and can be properly timed to key developmental stages. However, the amount of foliar fertilizers that can be supplied with one application is limited to prevent leaf damage. Therefore, foliar fertilizers are best used to supplement soil applied fertilizers ^[N27].

Applying N as foliar sprays may also reduce N leaching. Arpaia and Lund ^[N7] reported that foliar applications resulted in lower nitrate-N concentrations in the soil profile below the root zone than N applications through the irrigation system. The same study found that trees receiving N only with foliar sprays tended to have lower yields compared to trees receiving the same amount of N applied to the soil or as combined soil and foliar applications; however, the specific proportion of soil- to foliar-applied fertilizer did not have a consistent effect on yields ^[N7].

The yield effect of foliar urea applications was found to be most pronounced in orchards with low leaf N concentrations, while it was small or absent when N leaf concentrations exceeded 2.6% ^[N41].

Rate and Mode of Application

Fertilizer Type

Time of Application

Winter prebloom foliar applications of low-biuret urea have been shown to increase yield ^{[N1, N3]}, while summer applications can increase fruit size ^[N25].

The optimal time for pre-bloom applications in southern California is from December 15 to February 15 ^[N27]. Winter foliar application of 0.35 lbs N/tree in the form of low-biuret urea significantly increased yield of mature navel orange trees in Riverside. The foliar application was in addition to 1.1 lbs N/tree of soil-applied urea ^[N3]. In a Florida study, prebloom foliar applications of urea (28-31 kg/ha) to mature Valencia trees increased yield and soluble solids in juice compared to the control ^[N1]. In a study carried out with ‘Nules’ Clementine mandarins in a Fresno orchard, a January prebloom application of low-biuret urea (23 lbs N/acre) tended to increase yield and significantly increased yield of large fruits. In contrast, three applications of 28 kg KNO₃/ha in February, late May and July had no effect on yield and fruit size ^{[N26, N45]}.

Summer foliar urea should be applied before the end of the cell division stage, characterized by maximum peel thickness, which is between early June and late July in California. Low-biuret urea is applied at the rate of 50 lbs in 200 gallons of water per acre (0.35 lbs/tree; 23 lbs N/acre) ^[N25]. In Valencia oranges, urea applications close to harvest can result in regreening ^[N27].

Foliar fertilizers are best applied early in the morning when the plant’s stomata are open ^{[N8, N27]}.

Phosphorus (P₂O₅)

Soil Test

Soil Analysis

Soil Sampling and Analysis

Soil samples taken before an orchard is planted provide valuable information about soil conditions, salinity and nutrient availability. Soil analyses for nutrient availability are less common in existing orchards, as decisions are generally based on leaf analyses ^[P10]. However, periodic check-ups help evaluating the fertilization program ^[P3].

While soil samples for salinity analysis may be taken to a depth of 4 feet or more ^[P21], nutrient availability is generally determined in the top two feet of the profile, which is the zone where most of the fibrous roots are located. However, studies in Florida involving a range of rootstocks showed that citrus roots may reach to a depth of 7 to 12 feet. Poor drainage and a compacted or poorly aerated soil layer severely limit vertical root development ^{[P16, P17]}.

For sampling purposes, the orchard should be divided into uniform blocks with respect to soil properties, tree appearance, age, variety, and rootstock. Interpretation of the results is most informative, when samples are taken from the same blocks used for leaf analysis.

Samples should always be taken around the same time of the year to better detect long-term trends. The same samples can be used for nutrient and salinity analyses. Sampling for P and in fall post-harvest gives ample time to detect and correct deficiencies and is also most informative for salinity evaluation. However, P and K samples can be taken at any time of year ^[P21]. For more information on sampling procedure see Soil Sampling in Orchards.

Phosphorus availability strongly depends on soil pH. In acidic soils, P is immobilized by iron and aluminum minerals, while the predominant mechanism of P immobilization in alkaline soils is the formation of insoluble calcium phosphate compounds. For this reason, the recommended soil test depends on soil pH. For acidic to neutral soil (pH < 7.0), the Bray P1 test is generally used. For neutral and alkaline soils (pH > 6.5), the Olsen method is more appropriate ^[P11]. Using a test that is inappropriate for the soil analyzed may lead to inaccurate interpretation.

Interpretation of Test Results

Leaf Analysis

Sampling Procedure

Interpretation of Results

P to Young Trees

Soil Applied P

Soil Applied Phosphorus

There are currently no University of California recommendations for P fertilization of citrus orchards. Soil and leaf analyses can be used to monitor the P status of an orchard over the years to ensure that P availability is adequate (See Soil P Test and Tissue P Analysis).

Application Rate