Nutrition and Fertilization

Julian W. Sauls, Ph. D.
Professor and Extension Horticulturist
Texas Cooperative Extension

January, 2008

There are 16 nutrient elements essential to plant growth and development, a deficiency of any one of which will limit growth and production. Thirteen of the essential elements are normally provided by the soil. Most Rio Grande Valley soils are quite fertile and contain ample levels of most elements to sustain optimum citrus growth and development.

Nitrogen is the only major nutrient which routinely must be applied to Valley citrus orchards to supplement existing soil nutrient levels. Longterm fertility studies in Texas citrus have not shown a meaningful response to supplemental applications of either phosphorous or potassium fertilizers.

Iron deficiency sometimes occurs in the early spring in Valley citrus orchards. The deficiency is usually transient, disappearing as soil temperatures increase in the spring. Swingle citrumelo and Carrizo citrange rootstocks in heavier soils will exhibit persistent iron chlorosis severe enough to cause die-back. For that reason, both rootstocks are limited to very sandy soils.

Minor zinc deficiency patterns are often noted in Valley orchards, but the symptoms are usually limited to just a few trees and to just a few leaves within a tree. Research into foliar applications of zinc has corrected the deficiency symptoms, but has not resulted in any apparent increase in growth or production. Growers who note zinc deficiency symptoms normally include zinc in one or more of their normal sprays as a precautionary measure.

Deficiencies of other micronutrients have rarely been reported in Texas citrus. A documented deficiency should be corrected, either by soil application or or by appropriate foliar sprays. Routine foliar sprays containing various micronutrients are not necessary for fruit set or general orchard condition. Moreover, repeated application of some micronutrients can result in potentially toxic levels in citrus.

Nitrogen Fertilization

There are a number of dry and liquid formulations of nitrogen available to growers, each containing a different percentage of actual nitrogen. The grower's choice commonly is based on the cost per unit of nitrogen and the method and ease of application. No yield differences have been correlated with the different sources of nitrogen. Some urea fertilizers may contain biuret impurities that can cause toxicity symptoms in citrus. The most commonly used dry nitrogen fertilizer is ammonium sulfate (21-0-0), although other materials are sometimes available.

The rate of nitrogen application is based upon tree age, tree size or yield. The table below lists nitrogen rates for different tree ages. Tree size becomes the determining factor following severe rehabilitative pruning and nitrogen application rates should be reduced to levels corresponding to the age category which best matches existing tree size. Growers who use yield as the key to fertilization rate should apply 5 pounds of nitrogen per ton of fruit produced the previous season, unless the previous season's yields were reduced due to natural causes.

Nitrogen rates for bearing citrus

Neither the time of application nor the number of applications of nitrogen appear to matter. However, consistency in both timing and number of applications from year to year is important. A single application should be made pre-bloom. In split applications, two-thirds of the total nitrogen is applied pre-bloom and the balance in either May or August. A three-way split application of one-third of the total nitrogen each during pre-bloom, May and August is also acceptable, particularly on sandy soils. Dry fertilizers should be applied uniformly to the orchard soil surface. All fertilizer applications should be followed within a few days by irrigation or rainfall to move the fertilizer into the soil moisture system for use by the trees.

Some growers have long been accustomed to applying higher rates of nitrogen, perhaps with some justification in terms of higher yields. However, the shift to Rio Red grapefruit and its proclivity to produce sheepnosed fruit has resulted in the use of somewhat lower rates of nitrogen than those recommended above.
Because the severity of sheepnosing results in part from excessive vigor, it seems logical to reduce the nitrogen rate slightly so as to avoid excessive vigor. In addition, split applications are highly recommended for Rio Red grapefruit in order to more closely adjust total nitrogen application to crop load to try to alleviate sheepnosing. Essentially, if fruit set is about normal, the second application would be applied in May. If, however, fruit set is especially high, apply a slightly higher rate than initially intended. On the other hand, if fruit set is somewhat light, the second application should probably be omitted.

Fertilization through a low-volume irrigation system is an effective technique for Texas citrus. The annual total rate can be split into equal applications at weekly, biweekly or monthly intervals from February to November. Because of increased nitrogen use efficiency of fertigation, total nitrogen per year should not exceed 125 pounds per acre. Injection into the irrigation stream should be near the end of the irrigation cycle to avoid leaching the nitrogen through the root zone. Most growers prefer to inject downstream from the filters.

Soil Amendments

Soil amendments have generally not proven beneficial to Texas citrus production. In situations where soils have become compacted, with attendant drainage and salinity problems, deep chiseling to break up the compacted layers is the recommended recourse. While deep chiseling will break off a lot of shallow roots in the row middles, the improvement in internal soil drainage and salinity abatement will quickly overcome the damage.

Soil and Leaf Analysis

Soil and leaf analyses are not essential for a citrus fertilization program in Texas, as the general recommendations for nitrogen fertilization and close observation of visual deficiency symptoms are adequate for the average orchard. However, analyses of soil and leaf samples, properly taken over several seasons, can lead to a more precise and economical nutritional program and optimum citrus production. Long-term trends in nutrient levels and soil salinity allow the grower to alter production practices before a nutritional or salinity problem can become limiting to citrus production.

Composite soil samples should be taken annually for each soil type and each variety in each orchard. The composite sample is comprised of 10 to 20 individual soil cores taken across the orchard. The individual cores should be thoroughly mixed in a plastic bucket, from which a 1-pint sample is taken for analysis. The sample should be air-dried overnight before delivery to the analytical laboratory.

The depth of sampling is one-half to one foot for routine analysis, but occasional subsurface samples to 3 or 4 feet in one-foot increments can be useful for salinity analysis. Sampling time is not so critical as is consistency in sampling time from year to year. It is common practice, however, to take soil samples in late July or August to coincide with leaf sampling time.

Soil analysis indicates the levels of various nutrients in the soil, but it does not indicate availability of those nutrients to plants. Because some nutrients may be unavailable in the soil, leaf analysis is useful to determine nutritional status based upon what the plant can extract from the soil.

Leaf analysis assumes that there is a given range of concentrations of each nutrient element which correlates well with optimum production-even under different soil types, climatic conditions and rootstock/scion combinations. The ranges which have been developed for Florida citrus are presented in the table below:

Satisfactory ranges in concentration of nutrient elements in Florida citrus leaf samples
(from Nutrition of Florida Citrus Trees, SP 169. University of Florida,
Institute of Food and Agricultural Sciences).

The standard citrus leaf sample consists of 100 leaves from 20 trees across the orchard. Leaves should be four to five months old, taken from nonfruiting twigs of the spring growth flush. Thus, leaf samples should be collected in late July or August. Each sample should represent only one soil type, one variety and one orchard. Leaves should be air-dried prior to delivery to the analytical laboratory. The results for any element should be ignored if that element has been foliarly applied to the trees after the spring growth flush emerged, as the results will be artificially high because of its presence in the waxy cuticle of the leaves.

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This page revised January 7, 2008