Fertilization Articles:

• Nitrogen fertility management for Arizona citrus 1-2
• Efficient nitrogen application is important. 1-3
• Phosphorus nutrition for Arizona citrus 1-4
• Groundwater contamination a concern in ag community 1-4*

Nitrogen Fertility Management For Arizona Citrus
Rick Gibson, Agricultural Extension Agent, Pinal County
Volume 1, Issue 2. December, 1993

The low, essentially non-existent levels of residual nitrogen in most Arizona soils means that nitrogen fertility management is an important responsibility for the citrus orchard manager.

In citrus, as in all commercial farming systems, adequate nitrogen nutrition is essential for optimum vegetative growth and for top fruit yield and quality. Heavy nitrogen requirements exist for citrus in the spring during flowering and fruit set and when 75 to 95% of all new leaves are produced. For maximum yields, it is absolutely necessary that sufficient nitrogen be in the leaves at the right time.

How do you tell if a citrus tree is getting enough nitrogen? Observations of annual shoot growth and the size and color of leaves are two critical management tools to answer this question. Leaf tissue analysis is another.

According to Tom Doerge, Arizona Cooperative Extension Soils Specialist, and co-authors Bob Roth and Bryant Gardner, August leaf samples are the best guide to determining the annual fertilizer needs of mature, over 5 years-old Arizona citrus. They say to sample 5 to 7 month-old, bloom cycle leaves from non-fruiting terminals on healthy trees. If the total nitrogen level is less than 2.2% then 3 to 4 lbs. of total nitrogen should be applied during the next year. For nitrogen levels in the leaf over 2.8%, no more than 1/2 lb. should be applied during the next year. For levels between 2.2 - 2.3, 2.4 - 2.6 and 2.7 - 2.8%, apply 2-3, 1-2, and 1/2-1 lbs. respectively.

These and recommendations for other Arizona crops are reported in Nitrogen Fertilizer Management in Arizona, available for $10.00 per copy from the University of Arizona College of Agriculture, Dept. of Agricultural Education, Agricultural Communications and Computer Support, 715 N. Park Ave., Tucson AZ 85719. Postage and handling charges are included.

Nitrogen sufficient leaves will show good color and size. Shoots of current year growth on mature trees of all types of citrus except lemon will range between 4 and 12 inches annually. Any more or less indicates too much or too little available nitrogen. Lemons often show more vigorous growth. In general, typical ranges of application rates for established citrus orchards over 5 years old in Arizona are 1 to 2 lbs. of nitrogen per mature tree. In very sandy soils, 2 to 3 pounds may be necessary.

Timing of nitrogen applications is also important. Because of the importance of the spring season, 1/2 of the required nitrogen should be applied in late winter, usually in February, with the remainder in 3 to 6 applications through mid-to-late summer. More frequent, lighter applications are recommended on sandy soils.

Ammonium forms of nitrogen, such as anhydrous and aqua ammonia and ammonium sulfate will become available for plant uptake with the second irrigation killing the scale following application. Nitrate and urea forms are readily available after the first irrigation. Caution should be exercised when using the ammonia forms of nitrogen as they can easily burn tender roots, especially on sandy soils. Burned roots will appear black and mushy after exposure.

Nitrogen fertilizers can be injected into the irrigation water, or placed dry around the tree and watered in. Foliar applications require 10 pounds of low biuret (less than 2%) urea per 100 gallons of water with at least one month allowed between applications.

Since citrus trees are now receiving applications of nitrogen fertilizer, the importance of efficient N fertilization cannot be overemphasized. Efficient N fertilizing involves proper leaf sampling, proper application rates and an understanding of the environmental factors that affect soil and tree N status. Efficient N fertilization will save money, improve fruit quality, and avoid groundwater contamination. In the last issue of this newsletter, Rick Gibson wrote on N fertility management for Arizona citrus. As he stated, N fertilization should be based on the results of leaf nutrient analysis. The U of A recommends collecting leaf samples in August, when N status is at a base level. Nitrogen levels in citrus are variable, and will be low in late summer, following the spring and early summer N demands of growth flushes and developing fruit. 5 to 7 month old, bloom-cycle leaves from non-fruiting terminals should be gathered, and 3 to 5 healthy trees should be sampled per 10 acre block, with about 20 leaves collected per tree. Nitrogen application amounts should then be based on the following table:

Used with permission from: Doerge, T. Fertilizing citrus trees in Arizona. University of Arizona Cooperative Extension Bulletin #8670.

This table should be used as a general guide, since application rates on sandy soil should be greater, and since lemons and ‘Minneola' tangelos would require somewhat more N, and grapefruit less. However, assuming that a grove's N status is adequate (leaf N levels between 2.4 and 2.6), the following conclusions can be drawn:
Normal N fertilization rates for oranges should range between 100 lb. actual N per acre (30 x 30 spacing) and 220 lb. N per acre (20 x 20 spacing).
Assuming 25% more N applied for lemon and tangelo, the normal N fertilization rates should range from 150 lb. to 330 lb. per acre.
If these or greater levels of N are being applied, and the trees exhibit N deficiency during the year, then fertilizer source, application timing, or other environmental factor may be the cause.

There are a number of reasons that N might be lost from the soil, leading to inefficient N application. Some of them are as follows:

Leaching. Since nitrate (NO3-) is completely mobile within the soil, it can move largely with the soil water. This occurs especially in soils with low cation exchange capacity. Yuma's sandy soils and our flood irrigation practices may predispose us to NO3- leaching. It is possible that a significant portion of the N applied may move with the irrigation water, past the root zone to lower soil levels. Improving the soil will help, since higher levels of soil organic matter will slow leaching. Leaching will also be less on sites using low volume pressurized irrigation systems. Fertilizer sources containing higher concentrations of NO3-, such as Calcium Ammonium Nitrate (76% NO3-) are more likely to leach than are forms such as UAN 32 (33.5% NO3-).

Denitrification. This process is probably not as important as leaching, but occurs when nitrate is converted to nitrogen gas (N2) and nitrous oxide (N2O), and is lost to the atmosphere. Denitrification is favored when the soil is waterlogged, as would be the case following flood irrigation. Clay soils would be much more susceptible to denitrification than would sandy soils. The maximum rate of denitrification also occurs when soil temperatures range from 25ºC to 60ºC, as is common in an Arizona summer. Therefore, it is important to consider soil type, irrigation practices and soil temperature so as to limit N losses due to denitrification.

Volatilization. This process occurs when ammonium reacts with (CaCO3) in the soil to form (NH4)2CO3, and finally ammonia gas (NH3). Volatilization loss can be greater when the soil contains a high level of CaCO3, has a high pH, or when temperature is high. Urea N sources are also subject to volatilization, since urea is hydrolyzed to ammonium following application.

Weeds. While we have no data for Arizona, a Japanese study suggests that almost 40% of the N applied to the floor of a citrus orchard was absorbed by the weeds, rather than the trees.

 

Phosphorus (P) is the second most important mineral nutrient that citrus growers should apply to their trees on a regular schedule. Only nitrogen application is more critical. Phosphorus deficiency is sometimes a problem in Arizona, but P toxicity is seldom a problem, and will not be addressed here. Soils in the western U.S. generally contain high to moderate levels of total P, yet some sandy soils contain lesser amounts of P. Virgin clay soils in Arizona usually contain between 0.10 and 0.19% P2O5 in the top 1 foot, while sandy soils are more likely to contain between 0 and 0.04%. Although soil P may be high, less P is available to the plant. Only P found in the soil solution, usually in the form of the orthophosphates H2PO4- and HPO42-, is available for plant acquisition. Concentrations of orthophosphate ions in the soil solution are of the greatest importance for plant growth. Phosphorus may be removed from the soil solution through adsorption onto clay minerals or onto organic matter. Adsorption of P can be enhanced when there are high soil concentrations of Ca2+, when soil pH is high or when soil temperature is high. Phosphorus becomes less available to plants in soils with pH levels less than 6.0. In Arizona, this situation may occur in areas that are heavily fertilized with ammonium sulfate.

Adsorbed phosphorus disassociates from the clay or organic matter particles and replenishes the soil solution continually. Transport of P from soil to the roots is occasionally by mass flow, yet mass flow accounts for only 1 to 20% of the total plant P requirement. Most P moves to the roots by diffusion. Diffusion is enhanced when there is adequate soil water. Research on other crops indicates that leaf P concentrations are lower in plants subject to periodic moisture stress. Diffusion is also enhanced when the diffusion pathway is not tortuous and when the soil temperature is low. Research also suggests that there is a 33% reduction in water-soluble soil P for each 15C increase in soil temperature above 5C.

Soil management system may also play an important part in determining tree P status. Studies in England show that P concentrations in apple fruit from orchards under clean-cultivation were reduced by about 10%, compared with P levels in fruit from orchards grown in sod with herbicide strips in the row. The author of the study attributed the decrease to decreased root growth. It remains to be seen if the same phenomenon occurs in citrus.

Rootstock also affects P status. In one study on citrus from the 1940's, researchers showed that lemons on rough lemon rootstock were more likely to be P deficient than lemons on sour orange rootstock. Other researchers report, however, that both sour orange and rough lemon are associated with less leaf P, compared with grapefruit or trifoliate orange as rootstocks.

Uptake of P occurs primarily at the root tip and areas immediately behind the root tip. Phosphorus as phosphate (PO3-), is found throughout plants as an important constituent of the nucleic acids found in DNA and RNA, as a component of the phospholipids found in membranes. It is also a major participant in energy transfer and is involved in the regulation of carbohydrate metabolism.

In mature citrus orchards, deficiency symptoms generally occur when leaf tissue P levels are less than 0.07%. Foliar symptoms include: reduced growth, premature defoliation, leaf bronzing, and reduced size of new leaves. Fruit yields are reduced because of lack of flower production and/or an abnormally large pre-harvest fruit drop. Deficiency symptoms on the fruit include: coarse, thick peel, pale color, spongy fruit and excessive juice acidity. The earliest visible effect of P deficiency is on fruit quality.

The U of AZ recommendations, as developed by Tom Doerge, for P are based on the age of the planting. For new plantings, a CO2 extractable test reading of less than 1 ppm P, or a HCO3 extractable test reading of less than 5 ppm indicates that P application may be necessary. Apply ½ lb. P2O5 per tree or 75 to 100 lbs. P2O5 per acre. For mature orchards, P applications should be based on the results of leaf analysis based on the table below. Apply 50-100 lbs. per acre to correct any deficiency.

Phosphorus sources normally applied in Arizona include liquid ammonium phosphate (10-34-0), phosphoric acid (0-52-0) and chicken manure (31.2-18.4-8.4). Any of these sources is suitable for citrus orchards. Other sources that could be used include superphosphoric acid (0-83-0), single superphosphate (0-20-0), and triple superphosphate (0-45-0).

When applying P fertilizers the following guidelines should be kept in mind:
1. The need for P application should always be based on soil or leaf analysis. Studies have shown that while tree foliage may appear healthy, it is fruit quality that will suffer first if P is deficient.
2. Consider your soil type and rootstock when making the decision to apply P. Sandy soils will usually need more P than clays, and trees on rough lemon or sour orange will need more P than those on trifoliate rootstocks, such as ‘Carrizo', ‘Troyer' or ‘Swingle'.
3. For young plantings, place P fertilizer in the tree row. Since P is comparatively immobile, P sources must be placed near or in the root zone. Water-run applications onto newly planted citrus groves are inefficient since roots occupy only a small fraction of the total soil volume.
4. Keep soil moist. Phosphorus diffusion to the root zone is possible only in the presence of water.
5. Maintain cultural practices that encourage healthy root growth. Only the actively growing portion of the root can take up P.

Groundwater is the source of domestic drinking water for approximately half of the population of the United States and nearly 90% of the rural residents. Protecting this vital resource has become one of the most pressing environmental problems in agriculture. The chemicals of major concern in groundwater contamination are pesticides and nitrates.

In some areas of California, three herbicides commonly used in citrus groves; simizine, bromicil, and diuron were found in groundwater supplies. These herbicides are also used, to a limited extent, on roadsides and on other crops produced in the citrus growing areas. But because of the geographic concentration of the citrus industry and the widespread use of these materials, citrus production has been implicated as the principal source of residues from these pesticides in the groundwater.

Contamination of groundwater by nitrate is a concern in the commercial production of many crops, including citrus, in both Arizona and California. A recent survey indicated that Arizona ranked third among the fifty states in the percentage of wells testing above the EPA standard of 10 PPM nitrate-N. Several strategies have been developed to minimize nitrate leaching from fertilizer use in citrus (and other crop) production. It is in the best interest of the public and growers alike to protect the groundwater quality by implementing these and other practices.

For example, in AZ the State Legislature has mandated the implementation of Best Management Practice's (BMP's) by growers to reduce the potential risk of nitrate-N contamination to groundwater supplies. These BMP's were developed and adapted as a result of this mandate and are documented in the AZ Revised Statutes under the Arizona Environmental Quality Act of 1987.

Guidelines for BMP's in Arizona are established for all the major field crops, citrus, turf grass, melons, grapes, vegetables and nuts. These guidelines are available to the public through the University of Arizona College of Agriculture or the local County Cooperative Extension office. The document is entitled "Nitrogen Fertilizer Management in Arizona" and is numbered 191025 in the Cooperative Extension publications list for 1994.

The University of California, Division of Agriculture and Natural Resources has just released a new document entitled "Protecting Groundwater Quality in Citrus Production". I was asked to review and announce the document in any appropriate newsletter. I have studied the book and believe it is an excellent publication that any citrus producer should find quite interesting as well as useful.

The document covers the magnitude of the problem, movement of chemicals to groundwater, weed management and nitrogen fertilization practices, various strategies for protecting groundwater quality, and integrating the solutions to the problems. I have contacted the information specialist at UC and requested flyers regarding this document. It is available for $5.00, and if you are interested in obtaining a copy write or call:

ANR Publications, University of California
6701 San Pablo Avenue
Oakland CA 94608-1239
(510) 642-2431
Ask for leaflet #21521