Fertilization Articles:

• Fertilizer response of mature 'Valencia' oranges converted from border-flood to pressurized irrigation 3-3*
• Citrus micronutrients 4-2
• Citrus micronutrients II 5-2
• Nitrogen management for newly-planted navel oranges 6-3
• Use of a slow-release Triazone-based nitrogen fertilizer on lemon trees 7-3/4

Although we are still in the grip of midwinter, it is not too soon to start thinking about citrus fertilization for the coming season. Micronutrients are always a topic for discussion, so this article will review three of the common micronutrients that are often deficient, yet are needed for proper citrus tree growth and production.

Iron
Iron (Fe) deficiency is the most common micronutrient deficiency in Arizona. The foliar symptom of Fe deficiency is chlorosis (leaf yellowing) characterized by a fine network of green veins. This is similar to nitrogen (N) deficiency. When Fe deficiency becomes severe, leaves may become light yellow to nearly white in color, and even the green veins will disappear. Iron deficiency occurs on the younger leaves, while N deficiency occurs on the older foliage.

Iron deficiency may have several causes. These include, high soil pH, poor soil drainage, inappropriate scion-rootstock combination or disease. When soil pH is between 7.0 and 9.0 (as is typical in Arizona citrus soils), little inorganic Fe is immediately available to the plant. In this case, additional Fe is made available to the plant by natural chelates found in the soil. These chelates may be derived from root exudates, soil organic matter or microorganisms. Common chelates include humic acids, organic acids, phenolics and hydroxamate siderophores. Any factor that reduces the natural chelation in the soil will cause the trees to exhibit Fe chlorosis. When soils are waterlogged, citrus will also be Fe deficient. This is likely because of the cessation of root growth, and the simultaneous reduction of Fe chelation noted above. Certain rootstocks, especially trifoliates (such as ‘Carrizo' and ‘Swingle') are prone to Fe deficiency because of poor adaptation to high pH soils. Roots of these rootstocks are less likely to exude Fe chelates. Finally, citrus may exhibit Fe chlorosis due to diseases that adversely affect the roots. Damaged roots may not grow, exude chelates, or absorb Fe properly.

Iron deficiency, unlike other micronutrient problems, is not easily corrected by foliar application of Fe. Iron is used within the leaf during the production of the green pigment chlorophyll. Often, leaf analysis may indicate sufficient Fe levels, but leaves still may appear chlorotic. This is because of high soil pH that leads to an increased bicarbonate content of the leaves, which then binds internal Fe and makes it unavailable for chlorophyll production. Nonetheless, foliar application of Fe is the most popular application method, because of its low cost compared to soil applications. Fe chelates or Fe lignin sulfonates are usually applied because of their efficiency, water solubility and low cost. Deficiency symptoms are usually alleviated 6-8 weeks after application.

For Arizona soils, all of which have a pH greater than 7.0, FeDDHA iron chelate may also be added to the soil. This is a very expensive procedure. Finely ground chelated Fe, should be mixed with an inert material and carefully applied to prevent burning of the fruit and leaves by drifting material. Application of Fe salts, such as ferrous sulfate is not recommended because they are rapidly oxidized to the less soluble ferric ion.

Ranges of Fe levels in leaves are as follows and are intended as general guidelines:

Manganese
Manganese (Mn) deficiency symptoms are common in Arizona, appearing as a mottled interveinal chlorosis on the young leaves. Leaves remain normal size. Leaf veins remain green, but appear fuzzy and thick, rather than fine, as is common with Fe deficiency. Contrast between the green and yellow portions of the leaves is greatest near the tip of the leaf, while at the base, little difference may be noted. This leads to what may appear to be a "Christmas tree" effect on the leaf. Mn deficiency often appears in the winter when the soil is cool, or during periods of rapid leaf expansion, and is usually temporary.

Mn deficiency may be induced when soils are exceptionally high in calcium carbonate (CaCO3), or when soils are not well drained. Deficiencies may be corrected by the application of manganese sulfate to the soil. It is not recommended to apply Mn chelates directly to the soil, as the Mn will be converted to unavailable forms. Mn chelates are commonly used as a foliar spray, often in combination with other micronutrients, because of their efficiency and lower cost.
Ranges of Mn levels in leaves are as follows and are intended as general guidelines:

Zinc
Zinc (Zn) deficiency is common worldwide, including the Arizona desert. Leaves of Zn deficient trees are smaller and narrower than normal and may occur in rosettes. Symptoms are readily distinguishable as distinct interveinal chlorotic regions on the young leaves, but veins appear neither as fine as with Fe chlorosis nor as fuzzy as with Mn deficiency. Leaves may also bronze. Prolonged deficiency leads to leaf abscission and twig dieback. Fruit may also be small and misshapen. Zinc deficiency symptoms commonly occur on trees affected by macrophylla decline, sieve tube necrosis, trees on high pH soils, especially those on citrange rootstock. Zn deficiency may also occur in poorly drained soils.

Zinc deficiency may be corrected using soil Zn applications, but is generally remedied by at least annual foliar application of ZnO or ZnSO4 or Zn chelates or lignin sulfonates following bloom.
Ranges of Zn levels in leaves are as follows and are intended as general guidelines:

Citrus Micronutrients, Part II
Glenn C. Wright, Citrus Specialist, Yuma Agricultural Center
Volume 5, Issue 2 April 1998

In the Arizona Citrus Newsletter, Volume 4, Issue 2, I reviewed the iron, manganese and zinc; three of the common micronutrients that are often deficient, yet are needed for proper citrus tree growth and production. In this issue, I will review Cu and B, two additionally important micronutrients.

Copper
Copper (Cu) is most commonly found in the plant as a component of proteins. Unlike most other micronutrients, deficiencies of copper (Cu) are not associated with leaf chlorosis. Instead, during the first stages, leaves are abnormally large and green, but fruit development is normal. When symptoms are more severe a distinctive dieback occurs; twigs become necrotic and will often curl and multiple buds will form where the live wood becomes necrotic. Growth will become bushy, as the multiple buds flush. Gum pockets will be found under the young green bark of live wood, and gum will extrude from the twigs and leaves. Fruits will also exude gum, gum will be found in the peel, often the peel will split and fruit will drop. Deficiency symptoms are usually not noticed until they are severe.

Cu deficiency is occasionally seen in Arizona citrus, especially on the Yuma Mesa, where it occurs chiefly in previously unfertilized areas where the soil type is calcareous sand. Citrus is also more sensitive to Cu than are some other crops. Many fungicidal sprays contain sufficient Cu to provide for the tree needs, but young trees are more likely to be deficient when they have not yet entered a regular fungicidal program. Cu should be applied to both young and mature citrus trees as a part of a regular foliar fertilizer spray regime.

Cu toxicity symptoms are not common in Arizona, but include sparse foliage, very small leaves and stunted feeder roots.

Ranges of Cu levels in leaves are as follows and are intended as general guidelines:

Boron
Boron (B) is a micronutrient that taken up into plant roots as H3BO3 (boric acid), and long-distance transport of B is restricted to the xylem. The complete functions of B within the plant are not well understood, but it is clear that B is necessary to maintain the structural integrity of the cell wall, as well as allowing for the synthesis of hemicellulose and pectin. Boron also limits the conversion of carbon into harmful phenolic compounds, and may stimulate the utilization and transport of sugars throughout the plant. Cell division and cell elongation are limited by B deficiency.

B regulation in mature trees may be a problem because of the narrow range between deficient and toxic levels. B is toxicity symptoms have been found in Arizona and California desert citrus, and are include dull green leaves with brown necrotic pustules. Leaf veins may be corky as well as the fruit peel. However, B deficiency is equally serious, and may be a problem in Arizona citrus, despite the fact that irrigation water often provides seemingly adequate B (0.2% in the water) to the trees. Coarse textured, well drained sandy soils that are low in organic matter, as found in southwest Arizona, are often low in B. Little of the B that is in this type of soil is bound to exchange sites; instead the bulk may remain soluble and is easily leached below the root zone. Leaching studies have shown that up to 85% of the B in the soil may be leached with only 5 inches of water. Typical flood irrigation practices may bring about this leaching problem. Additionally, B may be unavailable to plants during times of drought because of restricted release from the few soil exchange sites that do exist, as well as impaired transport to the roots via mass flow. The supply of B needed for reproductive growth in many crops is more than that needed for vegetative growth and the same may be true in citrus. Boron appears to accumulate in citrus peel to a much greater extent than in the leaves, and ranges in lemon from 1600 to 3500 ppm. Concentrations of B may also be higher in the flower parts as well. So, it is entirely possible that Arizona citrus that appear to have adequate B for vegetative growth may exhibit deficiency symptoms during flowering, fruit set, and while the fruit is maturing. In citrus, B deficiency leads to low sugar content, granulation and excessive fruit abortion; symptoms that are seen regularly in fruit grown here in Arizona. Ranges of B levels in leaves are as follows and are intended as general guidelines:

As always, any products, services, or organizations that are mentioned, shown, or indirectly implied in this publication do not imply endorsement by the University of Arizona.

Nitrogen Management for Newly Planted Navel Oranges
Tom Thompson, Tom Weinert, and Michael Maurer
Volume 6, Issue 3 April-May, 1999

Newly planted citrus groves are managed to reach production age in the shortest possible time. Management practices usually include applications of nitrogen fertilizers. Currently, the University of Arizona recommends up to ½ pound of N per tree for citrus during their first year after planting, depending on preplant soil N levels. However, recent studies from Florida suggest that optimum N fertilizer applications for young citrus may be lower than this.
We are conducting two experiments with microsprinkler irrigated ‘Newhall' navels on ‘Carrizo' rootstock, that were planted in January 1997 at the Citrus Agricultural Center. In the first experiment, we have treatments which include all possible combinations of three N rates (0.1, 0.2, and 0.3 lb N/tree/yr), and three N fertigation frequencies (weekly, monthly, and three times per year). The second experiment includes all possible combinations of two N rates (0.1, and 0.2 lb N/tree/yr), and the same fertigation frequencies as in the first experiment. In the second experiment, all N applied is isotopically labeled. Therefore, we can precisely trace the movement of this N through the soil and plants. All fertilizer in both experiments is applied as urea ammonium nitrate. Trees are watered when tensiometers buried one foot deep at the drip line reach 30 cbar.
Trunk diameter, leaf N concentration, and uptake of isotopically labeled N were used to evaluate the effects of the treatments. Trunk diameters more than doubled between January and August 1997. There were no differences in trunk diameter or leaf N after the first year of both experiments. In fact, even trees that received no fertilizer N had growth rates and leaf N concentrations that were equal to trees that received the highest N rate. The average leaf N in August 1997, seven months after planting, was 2.8%, even in the unfertilized trees. By using the isotopically labeled N fertilizer, we were able to determine that an average of only 4% of the fertilizer N was taken up by the trees in all the fertilized treatments.
These studies showed that newly planted navels may not need N during their first year after planting. These trees contained 2.4% N in their leaves at planting, which is above the 2.2% critical level. This appears to be an adequate amount of N to sustain the trees during their first year, even without applications of N fertilizer. We suggest that growers collect preplant soil and leaf samples before planting citrus. If soil NO3-N is above 5 ppm and leaf N concentration is above 2.4%, N applications during the first year are not needed.
These studies are continuing on two-year-old trees. Results to date suggest that the trees did not respond to N fertilizer during their second year. We will report complete results in a later newsletter.

Abstract
Trisert CB replaced conventional foliar applied low-biuret urea and liquid urea ammonium nitrate in a typical N fertilization regime, a urea triazone based N source. There was no yield decrease, change in fruit size or grade with the use of the Trisert CB. There were no differences in leaf P, K, Ca, Mg, Cu, Fe, Mn or Zn concentration. Occasionally, leaf N concentration of trees supplied with foliar applied Trisert CB was higher than that of the control treatment.

Introduction
Citrus producers in Arizona and California often apply between 1 and 3 lbs. N per mature tree per year, as a nitrogen solution in the water run or in a low volume irrigation system. Embleton and Jones (1974) found that pound for pound, foliar applied N was just as effective as soil applied N for producing citrus. Nonetheless, 100% foliar N application programs are not common because of their perceived cost. However, Embleton et al., (1986) also found that foliar nitrogen in combination with soil-applied nitrogen can be beneficial for maintaining production and quality.

Low biuret urea (LBU) sources have been typically applied as the foliar component of a typical N application program. Application of these N sources will limit leaching into the groundwater. Embleton et al. (1980) estimated that only 10% of the foliar spray reached the ground, and 75% of that was volatilized as ammonia. Winter foliar application of LBU sources has also been shown to significantly increase yield as compared to soil applied LBU (Ali and Lovatt; 1994).

Although many studies have been conducted in the San Joaquin valley on the benefits of a combination foliar and soil applied N program on oranges, citrus fertilizer programs in the Arizona and California desert face additional challenges that have not yet been answered through research. Desert citrus acreage is increasingly planted to lemons, which have a higher N requirement than do oranges. Desert citrus soils are often sandier than those in the San Joaquin, meaning that the potential for N leaching is higher. High air and soil temperatures lead to volatilization of soil applied granular N. Conventional foliar N sources cannot be applied when temperatures are high, since water evaporation occurs and leads to the formation of a crystalline residue. Some conventional N sources can also lead to leaf burn.

Urea-triazone N sources overcome the difficulties of using conventional foliar N fertilizer. Clapp and Parham (1991) report that urea-triazone N has slow release N properties, provides leaf burn protection, will not crystallize on the leaf, and offers more uniform coverage and superior absorption when compared to conventional N sources. Additionally, Lovatt at the University of California at Riverside reports that urea-triazone foliar applications at 12.5 lbs. N per acre were just as effective as 25 lbs. N per acre conventional LBU applications for navel orange yields.

Therefore, our objectives in this study are to determine if a combination of foliar applied urea-triazone nitrogen (Trisert CB, Tessenderlo Kerley Inc., Phoenix, AZ) and water-run urea ammonium nitrate can maintain or enhance lemon leaf nitrogen status, fruit yield, packout and quality as compared to the current production practices of applying all or part of the nitrogen as UAN solution via the irrigation water and the remainder as conventional LBU solution.

Materials and Methods
This project was initiated at a citrus grove owned by Scheu properties, near Calipatria, California. Trees included in the project are mature ‘Limoneira 8A Lisbon' (Citrus limon Burm.) on Citrus volkameriana rootstock. For 1998, there were 124 trees per acre. The grower removed 50% of the trees during winter 1999, leaving 62 trees per acre. Trees were subject to normal grower practices, including regular microsprinkler irrigation, foliar application of both macro- and micronutrients, and applications of insecticides (chiefly for thrips) as needed.

For this experiment, total annual tree N requirements were met through the application of one of four treatments as follows:

• Control – Low biuret urea (46-0-0) applied as a foliar spray in 3 (1999) or 4 (1998) split applications annually (mid January, early March, early April and early May). For 1998, a total of 1.15 lb. N per tree or 143 lb. N per acre was applied. In 1999, a total of 46 lb. N per acre was applied.

Also, liquid urea ammonium nitrate 32-0-0 was applied through the irrigation system in 24 split applications. For 1998, a total of 1.47 lb. N per tree or 182 lb. N per acre was applied. For 1999, a total of 124 lb. N per acre was applied. Also, N-furic acid (15-0-0-49S) applied semi-weekly through the irrigation system in both 1998 and 1999, at a rate of 0.5 lb. N per tree.

Total N applied in this treatment for 1998 was 387 lb. N per acre, and for 1999 was 201 lb. N per acre.

• Liquid urea ammonium nitrate and N-furic acid applied as in the control treatment.

Also, for 1998, 43 gallons/acre Trisert CB (26-0-0-0.5B) was applied as a foliar spray in 3 split applications annually (mid January, early March, and early April) for a total of 0.94 lb. N per tree or 116 lb. N per acre. For 1999, 18 gallons/acre Trisert CB were applied as a foliar spray, providing 49 lb. N per acre.

Total N applied in this treatment in 1998 was 360 lb. N per acre, and in 1999 was 204 lb. N per acre.

• Low biuret urea and N-furic acid applied as in the control treatment.

Also, for 1998, 51 gallons/acre Trisert CB applied through the irrigation system in 24 split applications for a total of 1.11 lb. N per tree, or 138 lbs. N per acre. For 1999, 47 gallons per acre Trisert CB was applied through the system, for a total of 127 lb. N per acre.

Total N applied in this treatment in 1998 was 343 lb. N per acre, and in 1999 was 204 lb. N per acre.

• For 1998, 51 gallons/acre Trisert CB applied through the irrigation system in 24 split applications for a total of 1.11 lb. N per tree, or 138 lbs. N per acre. For 1999, 47 gallons per acre Trisert CB was applied through the system, for a total of 127 lb. N per acre.

Also for 1998, 43 gallons/acre Trisert CB (26-0-0-0.5B) was applied as a foliar spray in 3 split applications annually (mid January, early March, and early April) for a total of 0.94 lb. N per tree or 116 lb. N per acre. For 1999, 18 gallons/acre Trisert CB were applied as a foliar spray, providing 49 lb. N per acre.

Also, N-furic acid applied as in the control treatment.

Total N applied in this treatment in 1998 was 316 lb. N per acre, and in 1999 was 207 lbs. N per acre.

All foliar fertilizers were applied using a high-pressure air-blast sprayer in 100 gallons of water per acre in 1998 and 125 gallons of water per acre in 1999.

For this study, the experimental design was randomized complete block, with four blocks. Experimental units are double rows. Data are reported as cartons per tree, since the orchard had rows of varying length, and since 50% of the trees were removed in winter 1999.

Fruit were harvested on 13 October 1998 and 18 October 1999. For the 1998 harvest, trees were stripped, and fruit from the four treatments was kept in separate bins for transport to the packinghouse. There are 960 lbs. of fruit in a bin. At the packinghouse, fruit from each treatment, within each block were processed separately. Computer systems at the packinghouse generated a report showing the fruit yield and packout (fruit size and grade) for each of the four treatments.

For the 1999 harvest, the trees were stripped, and fruit from each treatment were kept in separate bins. From each bin, a 70 lb. sub-sample was collected. Fruit from the subsample was processed at the experimental site, through an automatic, portable fruit sorter (Autoline, Inc., Reedley, CA). The fruit sorter provided fruit weight, size, color and grade information.

Leaf samples for mineral nutrient analysis were taken once a month from February through August. Thirty-two leaves from each experimental unit were collected, and were dried at 60C for 48 hr and were ground with a Wiley mill. Samples were digested using a method developed by Parkinson and Allen (1975). Nitrogen and phosphorus were determined colorimetrically at using a spectrophotometer. Potassium, Ca, Mg, Fe, Cu, Mn and Zn were determined using atomic absorption spectrophotometry.

Results and Discussion
Application of Trisert CB in combination with foliar LBU, in combination with water-run urea ammonium nitrate, or without additional N source did not suppress or improve overall yield, fruit size or fruit grade compared to the control treatment (Tables 1 and 2). While there were occasional differences among fruit sizes or fruit grades, no discernible trend was evident.

The treatments had a greater effect on leaf nitrogen level (Figure 1). Although there was no continuous trend, for five of the 14 dates leaf N concentration of the foliar Trisert CB and water-run 32-0-0 treatments was significantly higher than that of at least one of the other treatments. Likewise, for five of the 14 days, the leaf N concentration of trees receiving the control treatment was significantly lower that at least one of the other treatments, often within the deficient range.

Treatments with Trisert CB had little effect on leaf P, K, Ca, Mg, Cu, Fe, Mn and Zn concentrations (data not shown).

In conclusion, Trisert CB may prove to be an alternative to foliar LBU applications or water-run urea-ammonium nitrate applications, particularly if point-source nitrate contamination of the ground water is an issue. Our research shows that for at least 2 years, application of Trisert CB will lead to equivalent lemon yields and packouts compared with conventional N fertilization regimes.

Literature Cited
Ali, A.G. and C. J. Lovatt. 1994. Winter application of low biuret urea to the foliage of ‘Washington' navel orange increased yield. J. Amer. Soc. Hort. Sci. 119:1144-1150.

Clapp, J.G. and T.M. Parham, Jr. 1991. Properties and uses of liquid triazone-based nitrogen fertilizers. Fertilizer Research 28:229-232.

Embleton, T.W., C. O. Pallares, W.W. Jones, L.L. Summers and M. Matsumara. 1980. Nitrogen fertilizer management of vigorous lemons and nitrate pollution potential of groundwater. University of California Water Resource Center. Contribution 182.

Embleton, T.W. and W.W. Jones. 1974. Foliar applied nitrogen for citrus fertilization. J. Environ. Qual. 3:388-392.

Embl eton, T.W., M. Matsumara, L.H. Stolzey, D.A. Devitt, W.W. Jones, R. El-Motaium, and L.L. Summers. 1986. Citrus nitrogen fertilizer management, groundwater pollution, soil salinity and nitrogen balance. Applied Agric. Res. 1:57-64.

Parkinson, J.A. and S.E. Allen. 1975. A wet oxidation procedure for the determination of N and mineral nutrients in biological materials. Comm. Soil Sci. Plant Anal. 6:1-11.

Table 1. 1998 Yield and Packout of lemons treated with foliar and water-run nitrogen sources.

z - Mean separation by Duncan's Multiple Range Test, =0.05
y - Yield expressed as numbers of 37.5 lb. cartons of fruit per tree.
x - Fruit size expressed as the percentage of fruit in each size category as a portion of the total number of fresh-packed cartons. The size categories are indicative of the number of fruit per carton.
w - Fruit grade expressed as the percentage of fruit in each size category as a portion of the total number of cartons. Grade categories #1 and #2 are fresh-packed fruit, and #3 are fruit destined for juice. The three values may not add up to 100% because about 2% of the fruit was discarded.

Table 2. 1999 Yield and Packout of lemons treated with foliar and water-run nitrogen sources.

z - Mean separation by Duncan's Multiple Range Test, =0.05
y - Yield expressed as numbers of 37.5 lb. cartons of fruit per tree.
x - Fruit size expressed as the percentage of fruit in each size category as a portion of the total number of fresh-packed cartons. The size categories are indicative of the number of fruit per carton.
w - Fruit grade expressed as the percentage of fruit in each size category as a portion of the total number of cartons. Grade categories #1 and #2 are fresh-packed fruit, and #3 are fruit destined for juice. The three values may not add up to 100% because about 2% of the fruit was discarded.

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