Fertilization Articles:

Improving Freeze Hardiness
Michael Maurer, Extension Agent, Maricopa County
Volume 2, Issue 1. September, 1994

The role that citrus tree nutrient levels play in freeze hardiness has been examined for years in Florida. Post-freeze evaluations suggest that levels of Zn, Cu, Mn, Mg, P, K, and N are correlated with freeze tolerance of citrus trees. Conversely, others have reported that nutrient levels have a relatively small effect on citrus tree hardiness when obvious nutrient deficiencies are absent. Some studies suggest that late fall applications of N fertilizer should be avoided. This avoids stimulating growth and thus delays the onset of non-apparent growth which increases freeze susceptibility. Therefore, the opinion of many citrus growers is that fertilization should be discontinued in the late fall to improve tree hardiness.

Tree size and foliage density also appear to be important parts of citrus tree freeze hardiness as healthy trees had reduced radiation losses and were less damaged than smaller, less vigorous trees which may have had nutrient deficiencies. Increased tree vigor and canopy size may function to prevent radiation losses as temperatures within the tree canopy can vary up to 3.6 F. Citrus tree freeze hardiness appears to be related to N provided other nutrient levels are not deficient. Cheong and Moon found that of the various nutrients tested, only leaf N content showed a positive correlation with leaf freezing tolerance, but excessive N fertilization did not increase freezing tolerance. Likewise, less freeze damage was observed for mature "Marsh" grapefruit and "Valencia" orange trees with leaf N levels of 2.86% compared to 2.64%. Similarly, Koo observed an increase in freeze damage when leaf N content was below 2.5% compared to 2.6-2.8%. However, in a recent study using individual leaves, leaf freeze hardiness was similar for leaves with leaf N contents of 2.0-2.2%, 2.6-2.8%, and 3.6-3.8%. The leaves acclimated at similar rates as air temperatures below 50 F increased and deacclimated at about the same rate.

In a field study where minimum temperatures were at 20 F for 3 hours, young "Redblush" grapefruit trees received varying degrees of damage. Trees which had leaf N levels of 2.0% and had small canopies received the greatest freeze damage. However, two groups of trees which both had similar leaf N levels of 2.3% received different levels of freeze damage; as the most vigorous trees were less freeze tolerant compared to the less hardy trees, but both were more vigorous than the trees at 2.0% N. There is an indication that in the field that canopy size and density, as well as the leaf N level affect freezing tolerance.

The results from these experiments suggest that maintaining trees with optimum leaf N levels produces a healthy tree with a dense canopy which may have a greater freezing tolerance. Trees which are unhealthy due to disease or nutrient deficiencies and overly vigorous trees may decrease the freezing tolerance.


Now is the Time to Think About Flowering.
Glenn C. Wright, Citrus Specialist
Volume 2, Issue 2. December, 1994

Although spring flowering is still some weeks away, metabolic events leading to the appearance of flowers are occurring now.

Flower bud initiation occurs after vegetative growth ceases during midwinter. It is believed that several weeks of temperatures below 77 F are required for the initiation of flower buds. Trees under water stress will also initiate flower buds. Floral initiation begins as the vegetative meristem (growing point) undergoes morphological changes (differentiation) to become a floral meristem. Sepals form first, followed by carpels. Flowers are borne in clusters, known as cymes. Not all buds form flowers; some remain vegetative. If the terminal bud remains vegetative, the lateral buds will form thorns, but if the terminal bud is floral, the lateral buds will form flowers. These flower buds will become visible by February.

Flowering occurs after floral initiation and differentiation, and is dependent on temperature. Research in California suggests that the minimum threshold temperature for inducing flowering is 49 F. Generally, the terminal bud in the cluster opens first, followed by the bud that is farthest away from the terminal bud. The flower immediately below the terminal bud is the last to open. This final flower is likely to have the greatest percentage fruit set.

Five types of growth can occur during flowering. Some shoots contain only flowers borne on last year's vegetative growth, some shoots will have mostly flowers and only a few leaves, some having a few flowers and a few large leaves, some will have a few flowers and several leaves, and some shoots will have only leaves. Generally, leafy blooms have a greater percentage fruit set and carry a greater percentage of their fruit to maturity. Nonetheless, trees usually produce more leafless blooms. Winter seasons that have a long period of lower temperatures cause trees to produce more leafless blooms, while winter seasons characterized by higher temperatures result in the production of more leafy blooms. Low temperatures (less than 68 F) during bloom will produce a prolonged blooming period, while higher temps tend to shorten that period.

Factors which control flowering are carbohydrates, hormones, temperature, water relations, and nutrition. The effect of carbohydrates is obvious when trees are girdled due to animals or disease, and roots that would normally receive carbohydrates are cut off and those supplies are shunted to the fruit, resulting in increased fruit set. Gibberellic acid (GA), a naturally occurring plant hormone, is also known to inhibit flowering.

Tree nitrogen and phosphorus status also affects flowering. Japanese researchers have shown that trees grown at lower temperatures produced more flowers and fewer shoots than trees grown at higher temperatures, but that this relationship did not occur in N or P deficient trees. Thus, severely N deficient trees produce few flowers, but trees that are slightly deficient produce extensive flowers. These trees often have poor fruit set and yields. Trees with adequate N have moderate numbers of flowers and are able to maintain their fruit. Trees with adequate N have moderate numbers of flowers and are able to maintain their fruit. Trees with excessive N lack flowers because vegetative growth is promoted at the expense of floral growth. Therefore maintaining optimal leaf N (2.5 - 2.8%) and P (0.1 - 0.3%) is necessary for moderate floral growth and adequate fruit set and yield.


Fertilizer Choices for the Yuma Area
Mark Wilcox, Yuma Cooperative Extension
Volume 2, Issue 2. December, 1994

There are two basic broad categories of dry commercial citrus fertilizers: traditional synthetic quick-release and slow-release fertilizers. The traditional dry fertilizers include such products as ammonium nitrate, ammonium phosphate, potassium phosphate, and calcium nitrate. The basic properties of these fertilizers include high solubility, a somewhat high salt index and low cost per unit of N. Basically these fertilizers are an inexpensive and readily available source of plant nutrition. Since these sources are highly soluble, multiple applications are necessary throughout the active growing season to insure adequate levels of N are available to the trees.

One promising alternative to conventional dry fertilizers are the slow-release fertilizers. Slow-release fertilizers are products which are either slowly soluble, slowly released, or held in a natural organic form which requires mineralization and nitrification in the soil. The slowly soluble forms include isobutylidene diurea, (IBDU) and methylene urea. In these two fertilizers, the N is released by hydrolysis or microbial activity in the soil so hydrolysis and/or microbial activity determine the activity rate of the material. Other sources of slowly soluble fertilizers are sulfer-coated urea and resin-coated products.

With the sulfur-coated products, the amount of sulfur and the size of the pores formed in the coating process determine the solubility of the urea within. As water penetrates the sulfur shell, the urea is dissolved and leaks through the shell slowly. Resin-coated fertilizers are a most recent development. With these products, the resin forms a barrier around the N and release of the fertilizer can be a result of time, hydrolysis or heat. Urea-form fertilizers are combinations of various methylene-urea polymers. The longer the polymer, the more time required for nutrient release. Natural organics like activated sewage sludge and processed manure are very slow in their release of nutrients and quite expensive per unit of N. However, these products do offer a very low leaching potential.

IBDU is a product that is not dependent upon microbial activity and has a low water solubility. For this reason, this product is best suited for and most effective with cool soil conditions. Methylene urea, natural organics and processed manure products are all dependent upon microbial activity and heat for release of the nutrient, so they are best suited to warm soil conditions. Most citrus grown in Yuma county is grown on sandy soils which are inherently low in natural fertility. These soils generally have a low cation exchange capacity and poor retention of applied plant nutrients against the leaching action of frequent irrigations. For these reasons, citrus must be fertilized frequently with soluble fertilizers to ensure good growth, fruit quality, and profitable yields. Growing concern over the introduction of farm chemicals (i.e. fertilizers) into the groundwater supply merits greater attention to efficient fertilizer management practices.

Recent studies in Florida have proven that these controlled release materials are effective tools in fertility management and can greatly reduce the potential for leaching in sandy soils. It has been documented that with the use of slow-release fertilizers, application frequency can be reduced by at lease one-half compared with that of the soluble fertilizers without compromising fruit quality, production or growth. Slow-release fertilizers offer tremendous potential in citrus nutrition under conditions of very sandy soils. N is released over a longer period of time than that of the soluble forms and therefore also reduces the leaching potential.


Good Grower Response to Nutrition Survey
Kathy Taylor
Volume 2, Issue 3. March, 1995

In a recent newsletter, I announced a nutrition survey that would be received by many of you. I am happy to say that we had a good response. Although only 16.3% of those receiving the mailing responded, 32,422 acres of citrus was represented in the survey! Your responses are summarized as follows:

Twenty-nine percent of the respondents owned or managed 1000 or more acres of citrus. They handle 84% of the acreage. Another 29% of the represented growers own 200 or fewer acres. Production acreage greater than 200 and less than 1000 was acknowledged by 18.5% of the respondents. The remaining 18.5% declined to give their production acreage.

Forty-nine percent of you own or manage citrus in Yuma county, 35% in Maricopa county, and 13.5% in Pinal county. Lemons are produced by 71% of the growers responding, with grapefruit, navel, Valencia, and mandarin production by 61%, 37%, 58%, and 71% of you, respectively. This survey did not determine the distribution of that acreage for each grower. We merely determined the major types of citrus in production.

The majority of the respondents (82%) use flood irrigation, with 16%, 21% and 26% of you using furrow, drip or microsprinkler irrigation. Many respondents use their irrigation system in fertilizer application (76%), with 8% water running their fertilizer, 24% using fertigation, and the other 44% unspecified. The majority (68%) of the respondents also apply their fertilizers foliarly. Since 66% of you apply nitrogen as UN32, this was not surprising.

Seventy-three percent of you apply phosphorus, primarily as a 10-34-0 formulation. Only 63% of the respondents applied sulfur, primarily for soil acidification. A number of the respondents (45%) did not report the use of several micronutrients or potassium. Those that did, applied them foliarly and often in chelated forms.

Most fertilization decisions (74%) are made on the basis of observation and previous successful seasons. Half of the growers responding use soil and tissue analyses for determining when and how much to fertilize. Only 5% of you feel that current recommendation can be relied upon for your fertilizer decisions.

Almost half of the respondents admitted their dissatisfaction with their current fertilization programs. They expressed concern over the use of micronutrients, balance of materials applied, timing of application, cost of application, and ultimately the effect of fertilization on orchard productivity.

Clearly, there is a need for our careful consideration of the fertilization requirements of growers in Arizona. It is also apparent that we need to take a leadership role in meeting this need. I will soon be contracting a number of you to discuss your programs in greater detail. After a careful assessment of the current fertilization programs in Arizona, I will begin field trials. These trials will be based upon your experience and successes, as well as the recommendations of California and Arizona.

Thank you for your thoughtful responses to this survey. I know that it came in the middle of your harvest season. Your prompt attention was greatly appreciated!


Foliar Applied Nitrogen for Citrus
Michael Maurer, Extension Agent, Maricopa County
Volume 2, Issue 4 June, 1995

Foliar nitrogen (N) applications have been suggested as an alternative to conventional soil fertilization. One of the advantages to foliar applied N is the reduction in nitrate leaching. The use of foliar N application substantially reduced pollution of ground water, even when used to only partially replace soil-applied N. Recent research indicates that foliar N application timing is critical for optimum N uptake and may increase yields. Foliar applications of N may be advantageous not only environmentally, but also for production of citrus.

Research over the years has shown that citrus trees require 1 to 2 lb. N/tree/year regardless of application method to obtain maximum yields. The disadvantage to foliar applied N has been that it required 3 to 6 foliar applications of urea per year to apply similar rates of N as conventional methods. However, Sharples and Hilgeman were able to obtain yields on 'Valencia' oranges over 7 years with only 0.5 lb. N/tree/year split between two foliar applications of urea, one in February and a second in late April to early May, that were statistically equal to yields obtained with 1 to 2 lb. N/tree/year as trees receiving soil applied ammonium nitrate.

In Florida, research indicates that timing of foliar applications of urea has a significant impact on uptake of N into citrus leaves. The study compared the effect of leaf age (2 vs. 6 month-old leaves) on the uptake of foliar applied N. In addition, it was determined that a 11,200 ppm solution of urea could be safely applied to both the 2 and 6 month-old leaves. When N was applied to the leaves at 11,200 ppm the percentage uptake of N was 1.6 to 6 times greater for 2 month-old leaves as compared to 6 month-old leaves. However, 15% of the N was translocated from the young leaves as compared to 40% for the old leaves. The results from this research would indicate that foliar applications applied when leaves are in their early stages of development increases the uptake of N by the leaves. It may be important to consider leaf age when determining foliar N applications.

Many questions have arisen about the use of winter foliar applications of low-biuret urea. Recent articles report significant yield increases from foliar applications of urea in the winter. A study conducted in California on ‘Washington' navel oranges evaluated foliar winter applications low biuret urea. In this study all trees received 1 lb of urea each year. In addition, treated trees received one foliar application of low biuret urea at a rate of 0.35 lb. N/tree in either mid-November, mid-December, mid-January or mid-February. Only the mid-January and mid-February applications of low biuret urea significantly increased yield per tree above the control trees that only received soil applied urea. The mid-January foliar application of low biuret urea significantly increased the number of fruit per tree for all three years. Although fruit yield was increased, there was no decrease in the fruit size. The results of this study demonstrate that properly timed winter applications of low biuret urea could increase yield of citrus. Further research is currently needed to determine if similar results could be obtained in Arizona.

The use of foliar applied N may have some advantages to conventional fertilization methods, such as reducing the amount of N required per tree based on Sharples and Hilgeman and also reducing the risk of nitrate contamination of ground water. In addition, timing foliar applications to the stage of leaf growth may increase the effectiveness of foliar applied N. Likewise, specific timing such as winter foliar applications of low biuret urea may enhance yield.

Further reading:
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(6):1144-1150.

Lea-Cox, J.D. and J. P. Syvertsen. 1995. Nitrogen uptake by citrus leaves. J. Amer. Soc. Hort. Sci. 120(3):505-509.

Sharples, G.C. and R.H. Hilgeman. 1969. Influence of differential nitrogen fertilization on production, trunk growth, fruit size and quality and foliage composition of ‘Valencia' orange trees in central Arizona. Proc. 1st. Intl. Citrus Symp. 3:1569-1578.


Combating Citrus Tree Yellows.
Glenn Wright, University of Arizona Fruit Crop Physiologist
Volume 2, Issue 4 June, 1995

Yellowing of mature citrus trees can usually be attributed to nitrogen (N), zinc (Zn), manganese (Mn), or iron (Fe) deficiency.

N deficiency is probably the least likely to appear at this time, since most citrus producers are well aware of the tree's N requirments. The majority of the N used in the most recent growth flush was translocated from the older tissues, rather than derived from the most recent application. Thus, if leaves show N deficiency now, it would be because of lack of N fertilization early in the spring, rather than from a June N application. Since N is highly mobile within the plant, the deficiency symptoms appear on the older leaves. If deficiency is severe, N is recycled from the older leaves to the newer ones. Thus the life span of the older leaves is shortened from 2 to 3 years to 6 months. Older leaves will turn yellow, then will fall off, leaving only the pale green young leaves at the end of a terminal. If deficiency is moderate, recycling still occurs. Leaf life span is 1218 months and leaves will turn yellow just before abscising. When trees are adequately supplied, leaves remain green even after abscission, and leaves will remain on the tree from 2-3 years. The canopy will appear dense.

Generally, rates of 100-350 lbs. per acre actual N per year are recommended for mature citrus. Use lower levels on grapefruit, median rates on oranges and mandarins (except tangelos) and higher rates on lemons and tangelos. Also, higher rates are needed on sandy soils. Fertilizer applications should be based on leaf N analysis (Table 1) of 5 to 7 month old leaves on non-fruiting shoots, collected in July or August.

Table 1. Leaf nutrient content of citrus leaves collected in July or August
Nutrient Element
Low Range
Optimal Range
High Range
N 2.2% or less 2.2% - 2.5% 2.5% - 2.8 % 2.8% - 3.0% 3.0% or more
Mn 16 ppm or less 16 - 24 ppm 25 -200 ppm 200 - 500 ppm 500 ppm or more
Zn 16 ppm or less 16 - 24 ppm 25 - 100 ppm 100 - 200 ppm 200 ppm or more
Fe 35 ppm or less 35 - 50 ppm 50 - 120 ppm 120 - 200 ppm 200 ppm or more

Mn deficiency sypmtoms are common on young, expanding leaves appearing as yellow patches between veins. Mn is often unavailable in high pH, calcareous soils. In this case Mn should be applied as a foliar spray (chelate or lignate) in the spring and again in the fall.

Zn deficiency symptoms are common worldwide, especially in the desert, and are readily distinguishable as distinct interveinal chlorotic regions. Leaves are smaller than normal and may be found on shortened internodes. Prolonged Zn deficiency leads to yield reductions. Zn is an immobile element, so deficiency symptoms occur in the young leaves. Zinc deficiency is not readily corrected using soil applications, but is generally remedied by at least annual (semiannual is better) foliar application of Zn chelates or lignates. Zinc deficiency symptoms commonly occur on trees affected by macrophylla decline and on leaves of trees on citrange or citrumelo rootstock especially in high pH soils.

Iron deficiency is probably the most common cause of citrus tree yellowing. Distinct interveinal chlorosis of the young leaves is symptomatic of Fe deficiency. Severely Fe-deficient leaves may become light yellow to white.

Solubility of Fe in the soil solution reaches a minimum between pH 7.4 and 8.5, and in these cases Fe deficiency appears. Trees on soils that have a caliche (calcium carbonate) layer are especially susceptible to Fe deficiency symptoms, because the calcium carbonate reacts with water and CO2 to form bicarbonates that raise soil pH to within the appropriate range. Poorly drained soils will also lead to Fe deficiency symptoms because CO2 accumulates and because root growth is limited when oxygen is limited. Soils with greater amounts of organic matter may have greater iron availability, while any factor that limits root growth (such as nematodes) may lead to Fe deficiency. Iron deficiency is not easily corrected by foliar application of Fe. Iron chelates or lignates may be applied as a foliar application.

Deficiency symptoms are usually alleviated 6-8 weeks after application. However, sometimes deficiency symptoms will show, but leaf analysis indicates sufficient levels of Fe. This is because of bicarbonate-inducing high leaf pH that limits Fe mobility within the leaf.