When young avocado trees have been in the ground for about four weeks, a fertilizing program should be started. If young trees are treated liberally with fertilizer, mulch, and water, it is quite possible for the tree to put on two year’s growth in one year.
Nitrogen fertilizers can easily pass through an emitter or a sprinkler. Fertilizers that are often referred to as “complete” (N-P-K), such as 15-15-15, 16-20-0, and 10-10-10, come in prilled or pelleted form and will clog the irrigation system. Liquid forms, which include P and K and are mixed in bulk by a fertilizer firm, are made from materials that are soluble and will pass through an irrigation system readily. These fertilizers are inserted into the irrigation water by a mechanism at the water head that injects them, either by a pump or by water pressure differential, into the system.
As a rule, drip irrigation systems include a fertilizer injector (also called an applicator), and if so, the nitrogen requirements can be divided into 40 applications per year to correspond to the typical number of irrigation weeks in the year. The same amount is applied once in each of those 40 weeks. This is a fertilizing plan trees will like. Ranches with systems that lack injectors should be encouraged to insert them in their systems because of labor savings and tree response.
Fertilize cautiously. If you correctly plant and carefully water young trees and the trees fail to exhibit good growth, you may need to apply nitrogen. You can supply this by following the fertilizer schedule for drip irrigation in table 12.
In a sprinkler system, regrettably, there is not apt to be a fertilizer injector to introduce the fertilizer into the system. This necessitates feeding by hand -- a costly, long-term labor proposition.
Hand feeding is the broadcasting of dry (prilled) fertilizing materials under the tree near the drip line and within the sprinkler irrigation pattern. Most of the tree’s feeder roots are out and away from the tree trunk, so fertilizer placed near the trunk will not benefit the tree. As the trees are irrigated, the fertilizer on the ground is dissolved by the water and is
made available to the tree.
February is a good month to fertilize avocado trees if you are on a twice-a-year program. University recommendations for hand feeding avocado trees irrigated by a sprinkler system are given in Table 13.
As the trees become larger and older, growers can reduce the frequency of feeding and increase the dosage of individual applications. You can satisfy the trees’ demand for (nitrogen) fertilizer through twice-a-year applications beginning in about the sixth year.
For adult Fuerte avocados, field research data suggests that about 150 pounds of actual nitrogen per acre per year will give the best production results. Too much nitrogen can be as detrimental to Fuerte production as too little is. Though research on the nitrogen requirements of Hass and other Guatemalan-type avocados has not been extensive, it is generally believed that twice the Fuerte recommendations may be about right for the Hass variety.
1st year - 1 tablespoon of nitrogen fertilizer each 3rd irrigation
2nd year - 1/8 lb. nitrogen each year
3rd year - 1/4 lb. “ “ “
5-7 years - 1/2 lb. “ “ “
8-9 years - 3/4 lb. “ “ “
10-14 years - 1 lb. “ “ “
15+ years - 1½ lbs. “ “ “
It should be noted that there is a body of thought called “organic growing,” or “natural growing,” in which the use of synthesized sources of nitrogen are held in distain. The use of bulky organic material and/or concentrated organics is preferred. Examples of each are rabbit manure and blood meal, respectively.
On thin, Cieneba-type soils, the use of organic materials especially improves the soil environment by promoting better tilth, hastening humus build up, and enhancing water infiltration to the plant. The cost of labor and materials is high compared to the use of inorganic fertilizers, but young trees really enjoy such an environment.
When considering the use of organics for fertilizing avocados, supplemental nitrogen sources (such as heavy applications of blood or fish meal) may be necessary, because all of the other bulky organic materials may not provide the necessary requirements for nitrogen.
Although classed as a “minor element,” the metallic element zinc is essential for plant growth, and any discussion of fertilizing avocados must include it. After soil applications, zinc remains immobile in the soil and will stay right there until the shallow rooted avocado comes to get it. Without small but essential amounts of zinc, the tree may decline and even die.
Zinc deficiency is often called “mottle leaf” and occurs in many orchards in southern California. The earliest symptoms are a mottling of the leaves on a few of the terminal branches. The areas between the veins are a light green to pale yellow. As the deficiency progresses, the yellow areas get larger and the new leaves produced are smaller. In the advanced stages, a marginal burn develops on these stunted leaves, twig dieback occurs, the distance between the leaves on the branches is shortened, and the terminal shoot takes on a “feather duster” appearance. Tree yield is reduced and some fruits may be more round-shaped than is normal.
For soil application, growers should use zinc sulfate (ZnSO4) 36% granular, which comes packed in 50-pound bags. This is applied in an 18” wide band around the drip line of the tree and is watered into the soil. If any of the zinc fails to be in the watering pattern of the sprinklers, the rains will carry it into the root zone. The recommended application will sustain the tree for 3 to 5 years. Soil application is most effective during the months of June and July but may be applied anytime. The correction of zinc deficiency can usually be affected within one year when a soil application is used. Table 15 shows the recommended amounts of zinc sulfate containing 23-28% metallic zinc. Reduce amounts by 1/3 if zinc sulfate containing 31% or greater metallic zinc is used.
Age trees - years Pounds per tree
Foliar application requires the use of zinc sulfate dust, which is dissolved in water and sprayed on the foliage. Zinc deficiency can be corrected faster with foliar application provided the tree has sufficient foliage. Leaf uptake of zinc is most satisfactory when spring flush leaves have nicely expanded, which usually means spraying should occur in late May or June. With a continuous annual maintenance spray program, foliar application may prove the cheapest method of zinc application in the long run, even though in heavy crop years it may be desirable to apply zinc twice a year. Growers should be cautioned that zinc sulfate dust hardens into rock if an unused bag is left on the shelf, so it should be used immediately after opening.
The avocado industry has recently moved into using helicopters and fixed wing aircraft for foliar application of zinc because it tends to be cheaper and easier, and eliminates the grower’s need to own a spray rig.
Chelated forms of zinc may be applied through the drip watering system, but this is a relatively new method. The industry is still awaiting data on the benefits of zinc when applied to trees through an injector. Chelated forms of zinc may correct a deficiency, but appear to have no advantage over zinc sulfate and are more costly.
We have not recognized other nutrient deficiencies in California avocados, but as we move into more of the Cieneba-type soils or thinner soils, the need for more of every nutrient has become apparent. Be aware that needless applications of phosphorous can induce zinc deficiency or cause it to become more severe.
In the last 15 years in north San Diego County, growers have planted up the slopes and into the Cieneba-type soils that are excessively well drained and very shallow. These soils are quite satisfactory from an avocado point of view, but may require more of everything that the tree wants, i.e. moisture, nutrients, and mulch to improve the tilth of the soil. Under these conditions, growers have found more potassium deficiency than in the soils planted to avocados in the past. Therefore, when a leaf analysis is ordered, phosphorous and potassium should be part of the analysis report in addition to nitrogen and zinc.
Leaf analysis has progressively become a more useful tool in crop management, as it provides growers with a recommendation for the following year’s fertilization program. Although nitrogen is usually the deficient element in our soils, growers are urged to order annual analyses that include nitrogen, phosphorous, potassium, and zinc readings. A written report, which indicates the nutrient deficiencies, is returned to the grower. Leaf analysis is usually begun near the end of the tree’s fourth year in the ground.
RANGES OF ELEMENTS IN AVOCADO LEAVES
Ranges for mature trees*
Elements Unit Less than Adequate More than
Nitrogen (N) % 1.6 1.6 -2.0 2.0†
Phosphorus (P) % 0.05 0.08-0.25 0.3
Potassium (K) % 0.35 0.75-2.0 3.0
Calcium (Ca) % 0.5 1.0 -3.0 4.0
Magnesium (Mg) % 0.15 0.25-0.80 1.0
Sulfur (S) % 0.05 0.20-0.60 1.0
Boron (B) ppm‡ 10-20 50-100 100-250
Iron (Fe) ppm 20-40 50-100 ?
Manganese (Mn) ppm 10-15 30-500 1,000
Zinc (Zn) ppm 10-20 30-150 300
Copper (Cu) ppm 2-3 5-15 25
Molybdenum (Mo) ppm 0.01 0.05-1.0 ?
Chloride (Cl) % ? ? 0.25-0.50
Sodium (Na) % - - 0.25-0.50
Lithium (Li) ppm - - 50-75
*Based on analysis of the most recently expanded and matured, healthy, terminal leaves from nonflushing and nonfruiting terminals sampled during mid-August to mid-October. (These are normally leaves from the spring growth cycle.) Values expressed on a dry-matter basis.
†Values above 2% N will not increase yield in most varieties; however, a reduction in yield of the Fuerte variety may occur above that level.
‡ppm, parts per million.
Many agriculturists are available to aid growers in the area of plant nutrition. Prudence suggests securing the services of a specialist well before August because specialists and the testing labs are likely to be very busy producing reports until the Christmas season. Because testing and writing reports is a time consuming process, growers should not be dismayed if the report does not reach them before 15 January. This time lapse should not present any problem because growers should not begin fertilizing before 15 February in any case. So, to be on the safe side, arrange for your testing in July so that the firm performing the service can properly book the time.
Leaves for testing are selected from 15 August through 15 October from the most recently expanded and mature, healthy, terminal leaves from non-flushing and non-fruiting branches. Leaves must be from the spring growth cycle and not from the summer growth flush. These leaves are sent to the laboratory where they are dried and analyzed for nutrients, the report giving the fertilizer recommendations for the next year is derived from this analysis for the grower. Of course, growers can train themselves to make the leaf selections, but it is usually best to employ a firm specializing in this field so that the material will give a more accurate reflection of the needs of the grove.
The major consideration for the typical avocado growing area will be nitrogen expressed as a percent (%) of dry matter. Table 17 shows the percentages for several varieties of avocados.
SUGGESTED NITROGEN LEVELS IN AVOCADO LEAVES
Variety Percent nitrogen in leaf*
Fuerte 1.6 to 2.0
Zutano 1.6 to 2.0
Hass about 2.0
Bacon about 2.0
MacArthur 2.0 (if attainable)
*Levels in 5- to 7-month-old spring-cycle leaves from mature trees.
Weeds are the bane of growers of all crops (Fig. 20). They rob moisture and nutrients from the cultivated plants, obscure gopher mounds, and provide a haven for snails -- all things that are detrimental to healthy crops. On the other hand, some crops benefit from the companion planting effect of weeds that seem to enhance the growth of a particular crop. They provide significant erosion control by breaking up heavy raindrops, while their roots permit the water to percolate to lower depths than would otherwise be the case. Over the years, weeds will provide mulch for the soil that will greatly improve the tilth. As the mulch breaks down, it will add humus to the soil and gradually build a layer of topsoil. Weeds also provide a habitat for beneficial insects. When the overhead tree canopy becomes so dense that sunlight cannot penetrate to the grove floor, weeds and grasses tend to die out.
In our southern California climate, excessive weed growth may pose a fire hazard during the dry summer and fall months, so weeds must be controlled. On some of the very steep slopes that are now being planted, this creates a serious problem. Several methods of weed control will be explored, but all are basically manual or chemical non-tillage methods that reduce damage to the tree’s shallow roots and control soil erosion.
Figure 20. Bermuda and other grasses and weeds are out of control in this Macadamia grove.
On slopes that are not so steep as to preclude the use of equipment, weeds may be mowed. Mowers come in many types: the sickle bar, the gas powered, hand pushed lawnmower, or the heavy duty machine, such as the “Bush Hog,” which can cut 6’ or more at a time and is tractor drawn. A relatively new type of mower, called a “Weed Eater” or “Green Machine,” is gas-powered, handheld, carried with a strap over the shoulder, and equipped with saw blades or mono-filament nylon cords. Some of the more expensive models can use both interchangeably. While using mechanically powered machines definitely saves time over the hand control methods, it is prudent to be very careful around young trees, irrigation risers, drip tubing, and anything else that can be quickly damaged when the mower gets too close to it.
Hand control methods may include cutting weeds with a scythe, a sickle, a weed whip, or a hoe. This writer is one of those growers who favor the use of a “push hoe,” especially around young trees, because there are fewer disturbances of the roots and mulch as contrasted with weed pulling or conventional hoeing. It is also easier, requiring one to go only 1/8” below the soil surface to do a good job. On steep hillsides it is tough to beat a very short man with a very long machete.
The primary method of herbicide application is a sprayer. Sprayers range in size from hand-held units and backpacks to truck-towed or self-propelled rigs capable of spraying a large area with each fill. Regardless of the size, most sprayers have some type of an agitator to keep the materials in suspension and provide even coverage. After use, the sprayer should be emptied of any residual spray material and thoroughly washed out before storage, even for so short a period as overnight. Otherwise, the spray will clog the lines, making it nearly impossible to clean them out sufficiently so that sprays may move through them again.
Herbicides that control unwanted plant growth are available from chemical supply houses and are generally applied with mechanical sprayers (Fig. 21). These spray rigs are mounted on a truck or trailer and are capable of being used for herbicides and pesticides, and give the operator a selection of nozzle pressures and patterns.
When purchasing herbicides, be sure to buy only materials that have been registered for avocados. Some hazardous chemicals, like Paraquat®, will also require a use permit from the Agricultural Commissioner’s office in your area.
Dorothy and Larry Bean preparing to apply herbicides to the weeds in their grove. Larry is driving his pride and joy, his homemade tractor.
Three classes of herbicides are useful to the avocado grower: 1. Contact killers such as weed oil and Paraquat®, which kill only what they touch. 2. Systemic killers that penetrate the plant tissues, translocate to the roots, and cause the whole plant to die. An example is Roundup®. 3. Princep, a pre-emergent spray, will kill seeds in the soil that would germinate the next season. Pre-emergent herbicides must be watered in, so their use should be late in the season to take advantage of the fall rains.
Surfactants or spreaders are added to the herbicides to avoid the “dewdrop” effect, permitting the material to better adhere to the plant and give coverage. Some of the contact and pre-emergent herbicides are compatible and may be used together to good advantage. Pre-emergents should not be used around young trees under two years of age. If weeds can be mowed a week before spraying with weed oil or Paraquat®, they will dry enough that less spray material is needed to effect a better kill. Growers on steep hillsides tend to strip spray the tree rows, leaving some weed growth in the aisles for erosion control. A good spray application around piping that emerges from the soil will make it more visible and thus avoid breakage and damage from machinery.
Since Roundup® has been registered for avocados and the “Herbie” spray system has come on the scene, growers have another weed control option to benefit from. Roundup® is less effective when plants are dormant but provides a good kill when they are in active growth. Read the label for best results.
Pesticides are poisonous and must be used with caution. READ the label CAREFULLY BEFORE opening a container. Precautions and directions MUST be followed exactly. Special protective equipment as indicated must be used.
STORAGE: Keep all pesticides in original containers only. Store separately in a locked shed or area. Keep all pesticides out of the reach of children, unauthorized personnel, pets and livestock. DO NOT STORE with foods, feeds or fertilizers. Post warning signs on pesticide storage areas.
USE: The suggestions given in this publication are based upon best current information. Follow directions: measure accurately to avoid residues exceeding tolerances, use exact amounts as indicated on the label or lesser amounts given in this publication. Use a pesticide only on crops, plants or animals shown on the label.
CONTAINER DISPOSAL: Consult your County Agricultural Commissioner for correct procedures for rinsing and disposing of empty containers. Do not transport pesticides in vehicles with foods, feeds, clothing, or other materials, and never in a closed cab with the vehicle driver.
RESPONSIBILITY: The grower is legally responsible for proper use of pesticides including drift to other crops or properties, and for excessive residues. Pesticides should not be applied over streams, rivers, ponds, lakes, run-off irrigation or other aquatic areas except where specific use for that purpose is intended.
BENEFICIAL INSECTS: Many pesticides are highly toxic to honey bees and other beneficial insects. The farmer, the beekeeper and the pest control industry should cooperate closely to keep losses of beneficial species to a minimum.
PROCESSED CROPS: Some processors will not accept a crop treated with certain chemicals. If your crop is going to a processor, be sure to check with the processor before making a pesticide application.
POSTING TREATED FIELDS: When worker safety reentry intervals are established be sure to keep workers out and post the treated areas with signs when required indicating the safe reentry date.
PERMIT REQUIREMENTS: Many pesticides require a permit from the County Agricultural Commissioner before possession or use. Such compounds mentioned in this publication are marked with an asterisk (*).
PLANT INJURY: Certain chemicals may cause injury or give less than optimum pest control if used: at the wrong stage of plant development; in certain soil types; when temperatures are too high or too low; the wrong formulation is used; and excessive rates or incompatible materials are used.
PERSONAL SAFETY: Follow label directions exactly. Avoid splashing, spilling, leaks, spray drift or clothing contamination. Do NOT eat, smoke, drink, or chew while using pesticides. Provide for emergency medical care in advance.