- Author: Etaferahu Takele
Costs of Orchard Establishment and Production Summary for Avocados in California:
Based on 2011 studies: www.coststudies.ucdavis.edu
Etaferahu Takele, UCCE Area Advisor, Ag. Econ/Farm Management, southern California and Mao Vue, UCCE Staff Research Associate
In fall 2012, we completed and published sample costs and returns studies for establishment and production practices for conventional and organic avocados in the major producing counties. We divided the production regions into two parts to show the differences in production methods and costs especially as related to water prices. The northern part of the growing region includesVentura,Santa BarbaraandSan Luis Obispocounties. The southern part of the growing region includesSan DiegoandRiversidecounties. We developed four studies, two conventional methods and two organic methods, one for each region.
Data for the sample cost studies were obtained from growers, farm advisors, pest control advisors and other agricultural institutions including banks, agricultural appraisers and equipment dealers. The data we obtained were entered into a budget generator program for calculation and development of sample establishment and production costs.
The studies provide the detail assumptions used in the development of establishment and production costs (ref. the above website) Some common assumptions to all of the cost studies include average labor wages with benefits of $14 per hour for manual and irrigation labor and $18 per hour for machine labor. We used a price of $1.07 per pound which is the five-year average obtained from the California Avocado Commission (CAC) to calculate crop value and we assumed an additional $0.20 per pound premium for organic avocados, and used $1.20 per pound for organic avocados crop value.
Table 1 shows the summary of costs of establishment and production and returns per acre by production method, region, and county. Establishment costs include the accumulated net costs (gross returns less costs) during the orchard development period from year 1 to 6. The production costs estimates are for year 7 + when the trees are considered nearly mature and fully yielding. Gross returns are price per pound times yield. Gross margin are returns above operating and cash overhead costs and returns to management are returns above total costs.
Establishment and production costs are higher for San Diego County than any other avocado producing counties for both the conventional and organic production. In conventional production, establishment costs inSan DiegoCountyare higher by 8% above the northern growing region ofVentura,Santa BarbaraandSan Luis Obispocounties and 15% aboveRiversideCounty. Production costs inSan DiegoCountyare higher by 15% aboveVenturaandSanta Barbaracounties and 21% aboveSan Luis Obispoand 18% aboveRiversideCounty. The main difference is accounted for by water cost, which in our study was assumed $1,200 per ac-ft. forSan DiegoCounty, $650 per ac-ft. for Riverside County, $325 per ac-ft. for Ventura and Santa Barbara, and $200 per ac-ft. for San Luis Obispo. According to the local farm advisor, the high water cost in the southern growing region, especially inSan DiegoCounty, is due to various drought years and the loss of shares of water from theColorado River. Other influencing factors that resulted in higher establishment and production costs for San Diego County include lower yields due to wider space planting (145 trees per acre vs. 180 per acre in the northern region) and higher labor costs for material applications on steeper hillsides.
Organic avocado production costs are generally higher than conventional production regardless of the production region. Orchard establishment costs of organic avocados run 11-12% higher in the northern producing region and 12%-14% in the southern region than the conventional production practices. Similarly, production costs run 11%-12% higher in the northern producing region and 10%-12% higher in the southern producing region than conventional production practices. The main differences between conventional and organic productions include material costs, hours spent on labor, and yields. Organic material particularly in fertilization and pest management tends to cost more and require more labor hours for application than conventional materials. For example, organic fertilizers are usually in dry or granular form and have lower amount of nitrogen per pound (feathermeal 12% for organic vs. UN-32% nitrogen for conventional) therefore requires higher application rates and will take longer to apply by hand. Organic pest control such as spinosad ($34 per ounce) for thrips control cost more than conventional insecticide (abamectin $1 per ounce). In addition, organic growers spend more hours hand weeding in the northern region. Gypsum is hand applied for phytopthora root rot treatment in organic production and costs more to apply than potassium phosphite for conventional production. Overall, costs for organic production results in $3,453 more ($2,471 more for fertilization, $348 more for pesticide, $338 more for root rot treatment and $296 more in weeding) than the conventional method in the northern region and costs $3,804 more ($2,846 more for fertilization, $346 more for pesticide and $610 more for root rot treatment) than the conventional method in the southern region.
There is also a yield difference between conventional and organic productions. Yield is generally higher for conventional than organic production methods regardless of the production region. The yield level provided by growers includes 15% more for conventional production methods than the organic production methods for the northern producing region and 14% more in southern producing regions.
Gross returns estimates for organic production in the northern producing region are higher than gross returns in San Diego and Riverside counties for both conventional and organic production. This is due to higher yield attained from the narrow space planting in the northern producing counties (22’x11’=180 trees per acre in the northern producing regions vs. 20’x15’= 145 trees per acre in the southern producing region).
Gross margin (returns above all cash costs) and returns to management (returns above total costs except management) are all positive for both the northern and the southern part of the growing region. However, gross margins are much lower than the northern growing region. Net margins are positive for the northern part of the growing region but are negative for the southern part.
We provided the breakeven prices analyses in Table 2. Given our costs of production estimates and yield assumption of 12,400 and 11,200 pounds per acre for conventional avocados, respectively for Ventura and Santa Barbara counties and San Luis Obispo County the breakeven prices to cover cash costs are $0.47/lb. in Ventura and Santa Barbara counties and $0.46/lb. in San Luis Obispo County. Whereas the breakeven prices to cover all costs are $0.88/lb. for Ventura and Santa Barbara counties and $0.91/lb. for San Luis Obispo County. This means given the $1.07 price per pound of avocados, there will be $0.60/lb. gross margin and $0.19/lb. net margin for Ventura and Santa Barbara counties; and there will be $0.61/lb. gross margin and $$0.16/lb. net margin for San Luis Obispo County.
For organic production, given the cost of production estimates and yield assumption of 10,500 and 9,500 pounds per acre, respectively for Ventura and Santa Barbara counties and San Luis Obispo County, the breakeven price to cover cash costs is $0.65/lb. for all counties (Ventura, Santa Barbara, and San Luis Obispo). The breakeven prices to cover all costs in organic production are $1.16/lb. for Ventura and Santa Barbara counties and $1.22/lb. for San Luis Obispo County. This means that given the $1.27 price per pound of organic avocados, there will be $0.62/lb. gross margin for all counties and $0.11/lb. net margin for Ventura and Santa Barbara counties and $0.05/lb. net margin for San Luis Obispo County.
In the southern producing region, given our costs of production estimates and yield assumption of 9,000 pounds per acre for conventional avocados for both San Diego and Riverside counties, the breakeven prices to cover cash costs are $1.01/lb. for San Diego and $0.79/lb. for Riverside. Whereas, the breakeven prices to cover all costs are $1.44/lb. for San Diego County and $1.18/lb. for Riverside County. This means that given the $1.07 price per pound of avocados, there will be $0.06 per pound gross margin for San Diego County and $0.28/lb. gross margin for Riverside County. However, the breakeven price falls short by -$0.17/lb. and -$0.11/lb. net margin, respective for San Diego and Riverside counties.
Given our costs of production estimates and yield assumption of 7,700 pounds per acre for organic avocados, for San Diego and Riverside counties, the breakeven prices to cover cash costs are $1.31/lb. for San Diego and $1.05/lb. for Riverside. The breakeven prices to cover all costs are $1.87/lb. for San Diego County and $1.57/lb. for Riverside County. This means that given the $1.27 price per pound of organic avocados, there will be a shortfall of -$0.04/lb. gross margin for San Diego County but a positive gross margin of $0.22/lb. for Riverside County. Net margins fall short of the breakeven prices by -$0.60/lb. and -$0.30/lb., respectively for San Diego and Riverside counties.
Overall, narrow space planting with higher yields and lower water costs are positive impacts on returns to management in the northern producing region with substantial profitability levels that keeps the industry moving forward. On the other hand, the wider spacing and lower yield and higher water costs have negatively affected the overall returns to management in the southern producing region. Currently, there is a trial in the southern producing region on narrow spacing with an emphasis on pruning methods. In the future, we hope to develop a thorough costs study on narrow spacing in the southern producing region after trials are completed. For detail information on our assumptions, costs tables, and profitability analyses copies of the avocado costs studies for both regions on conventional and organic avocados can be retrieve from the above referenced website.
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- Author: Craig Kallsen
A sure way to generate controversy among citrus growers is to initiate a discussion on navel orange tree pruning. Some growers maintain that yield and fruit size is best maintained by minimal pruning, while others believe that the number of large fruit is increased when trees are severely pruned. A ‘standard’ manual pruning for navel oranges does not exist, but the closest thing to it is a procedure that involves pruning from the tree; 1.) shaded, dead branches 2.) branches which cross from one side of the tree to the other and 3.) green, triangular, juvenile shoots from the tree. This type of pruning commonly goes under the name of ‘deadbrushing’. Deadbrushing is a relatively light form of pruning, and a trained crew usually spends less than 15 minutes per tree performing it. In addition to any manual pruning, most navel orange orchards in California are mechanically ‘hedged’ and ‘topped’ to provide continued access to trees and their fruit by equipment and people involved in orchard cultural and harvest activities. Although growers have been growing navel oranges in California for over one hundred years, surprisingly few experiments have been conducted to determine the effect of pruning on navel orange yield and quality.
To assist in providing some guidance related to pruning and its possible effects on fruit yield and quality, an experiment was established in 2000 in northern Kern County in an orange orchard that was typically harvested in late December or in January. In 2000, 2001, 2002 and 2003, yield, fruit quality parameters and manual pruning costs were compared among mature “Frost Nucellar” navel trees (90 trees/acre) having one of three topping-height treatments (14 ft, 16 ft, and untopped trees). In addition to a topping treatment, the experimental trees were given one of three levels of manual pruning 1.) removal of several large scaffold branches in March of 2000 followed by deadbrushing in 2001, 2002 and no manual pruning in 2003; 2. dead brushing only in 2000, 2001, 2002 and no manual pruning in 2003; or 3. no topping or deadbrushing). Data were collected from experimental trees surrounded by similarly topped and manually pruned border trees. Fruit weight, numbers, size, grade and color were determined the day after harvest at theUniversityofCalifornia ResearchandExtensionCenterexperimental packline near Lindcove,California. The year, in this report, refers to the year that the crop bloomed and not to the year of harvest.
For the 2003 crop year, even after 4 years, trees that were severely pruned in the spring of 2000 produced less total yield and less fruit in the most valuable-size range (i.e. 88 to 48 fruit/carton) than trees that were deadbrushed or left unpruned. In 2003, differences in yield among manual pruning treatments were greater than in 2002 probably because of the higher yield potential that appeared to exist across the industry in 2003. The canopy of the severely pruned trees in 2003 had not yet retained the size of the deadbrushed or unpruned trees after four years, which limited their potential fruit production. In contrast, in 2001 only one year after the manual treatments were imposed and a year with high spring temperatures and very poor fruit set, no differences in yield were found among manual pruning treatments.
When the data of average individual tree performance are summed over the four years that this experiment was conducted, the treatment that included removal of some major scaffold branches in March of 2000 with deadbrushing in 2001 and 2002, was inferior in terms of yield, fruit number, and number of valuable-sized fruit in the range of 88 to 48 per carton than to trees that were only deadbrushed or those that had no manual pruning. Most of the detrimental effects of severe pruning on yield (and on fruit quality) occurred at the December harvest following the severe pruning in March 2000. Over the four years of the experiment, the trees that were not manually pruned produced equal or better cumulative yields of fruit, equal or more valuable sized fruit, and fruit with equal grade compared to deadbrushed or severely pruned trees. The percentage of the fruit on the tree larger than size 88 was greater in the severe pruning treatment, but because total fruit number per tree was less and more of this fruit was overly large (i.e. greater than size 48) the number of the most valuable-sized fruit/tree (sized 88 to 48) was less. Obviously, the trees that were not manually pruned had no associated manual pruning costs when compared to the other two pruning treatments. Manual pruning costs, from 2000 through 2003, not including stacking and shredding of pruned brush, were $8.50/tree for the deadbrushing treatment and $13.00/tree for the severe manual pruning treatment.
Fruit yield or quality was not different among topping heights in any of the four years of the experiment. Topping height did not affect yield, probably because of the wide spacing and tall trees in this orchard. The canopies of untopped trees had little fruit within 4 feet of the ground as a result of shading of the lower canopy by neighboring trees. Removing the top 4 feet from an 18-foot tall tree moved the fruit-bearing volume downward in response to greater light penetration into the lower canopy but did not decrease the volume of the tree that received sufficient light to produce fruit. This effect was in contrast to severe manual pruning, which reduced the volume of the unshaded canopy overall, limiting the volume available for fruit production. A highly significant positive-linear correlation was found in the data across the four years and treatments between the total numbers of fruit produced per acre versus the total number of fruit sized 88 to 48 per carton produced per acre. This functional relationship existed whether reductions in fruit numbers produced per acre were the result of severe pruning in March or from weather-related phenomena such as occurred in 2001, suggesting that anything that reduced fruit numbers below approximately 130,000 fruit per acre resulted in a decrease in the number of fruit sized 88 to 48 per carton in this orchard.
Of course, there are other reasons to manually prune orange trees, other than to improve fruit size. If certain insects, likeCaliforniared scale or cottony cushion scale have been a problem, pesticide spray coverage may be improved by making the canopy less dense through pruning and fruit quality may be improved by making this investment. In general, what this pruning research has reinforced is the concept that growers should know why they are pruning orange trees and that manual pruning is unlikely to increase the number of fruit in the most valuable size ranges.
- Author: Ben Faber
"We don't need to irrigate, it's winter." This is a commonly held idea, and many years it is true. Adequately timed rains will often meet the needs of avocado trees during the winter period, and in times like last year, even satisfy much of the spring requirement. And the calls are coming in – “What’s wrong with my trees, they have all these brown leaves?”. This from San Diego to San Luis Obispo.
In a low rainfall year, irrigation can be as necessary as at other times of the year. This is because a subtropical evergreen like avocado continues to use water regardless of rainfall patterns. At the time of writing this article in March, we have had a scant 4 inches in Ventura and this is on top of a low rainfall year in 2011-12. Rain is necessary to leach the salts that have accumulated from the last irrigation season.
The driving forces for plant water use are light intensity, wind and relative humidity, as well as temperature. Remember how cold, dry winds can dry your skin or freeze-dry backpack food. Even during the winter, the trees are quite capable of losing large amounts of water with clear skies and cold winds.
Dry Santa Ana conditions are also more common in winter than in the past. This winter, a time of drought, I went out to see an orchard to evaluate it for pruning. On arrival, my first concern was for the water stress in the trees. The grower, however, was unconcerned. The trees had been dutifully irrigated the previous Friday. But over the weekend, a Santa Ana had blown for three days and completely dried the soil in the top 10 inches. Digging around the roots convinced the grower of water stress. Do not take irrigation for granted.
Contributing to the problem is the determination of what amount of rainfall is effective. Effective rainfall is defined as the amount of water that is retained in the root zone after rain. Avocados, especially on shallow soils, do not have much of a root zone. Most soils can be expected to hold about 2 inches of available water in the top 2 feet, less the more sandy, more the more heavy.
If rainfall exceeds the holding capacity within the root zone, it is lost to the plant. Just imagine if all the year's expected rain fell during one storm. It would not be long before irrigation would be required with no more rain coming. The extra water may, however, perform the all-necessary function of leaching accumulated salts from the root zone. When the rain gauge says that 2 inches fell, it is quite possible that all that rain will not be available to the tree. This also goes for the quarter inch storms we get that do not even make it through the leaf litter. It is not effective rainfall, even though it may wash the persea mite off the leaves.
One of the best ways to assess the effectiveness of rainfall within the root zone is with tensiometers. These trusty instruments are most commonly used to schedule irrigations. A good rainfall should return the 8- and 18-inch depth gauges to close to 0 cbars. This will tell you whether the rain thoroughly wetted the root zone. It will not tell you how much may have passed through the root zone, however.
If you are using soil sampling to assess the depth of rain infiltration, simply squeezing a handful of soil can help. Regardless of soil texture, a wetted soil will form a ball or cast when thoroughly wetted. Water moves as a front through the soil. After a rain, take soil samples with depth to find where the potential to form a ball abruptly ends. This will tell you the depth of effective rain.
How well a soil holds together can also be an indication of when to irrigate. Even a sandy loam texture will retain a ball that does not hold together well when there is still adequate moisture for the tree. The possibility of forming a ball decreases with water content. When visible cracking of a soil ball is obvious, it is time to irrigate.
Winter irrigation is something we do not commonly perform, but in low rainfall years it is an activity we need to consider, especially for controlling the salts that accumulate from our previous irrigation season.
Salt damage due to lack of leaching
- Author: Ben Faber
There have been a lot of new avocado orchards planted during the last few years. These often have been in old ‘Valencia’ orchards or lemons that had poor production. In order to save money, growers have just cut the trees at ground level and replanted the avocados near the stumps. Avocados have recognition of being resistant to Armillaria, but in this environment of high disease pressure, they can fail.
Armillaria root rot is common, yet is an infrequently identified and poorly understood disease. It is capable of attacking most species of trees and other woody plants growing in California. It is sometimes called “shoestring root rot” and the causal fungus is often referred to as the “honey mushroom.” Because oak is one of the preferred hosts, it is also called “oak root fungus.”
If a tree undergoes a slow to rapid decline without any obvious reason, suspect Armillaria as the cause. Certain areas, such as drainage areas from chaparral or woodlands are likely areas for this disease. Old roots left underground provide a food base for continued fungal growth and survival.
General symptoms of Armillaria resemble those of other root disorders. These symptoms are disrupted growth, yellow foliage, branch dieback, and resin or gum exudates at the root collar. Trees may die rather abruptly without showing any decline symptoms. Avocados typically have a rather protracted death, but in citrus it can be rapid.
Only rarely can the disease be diagnosed without examining the larger buttress roots and root collar of the tree. After carefully removing the soil, examine for the presence of:
1) Rhizomorphs, or fungal ‘shoestrings’ attached to the wood under the bark. These may occur beneath the bark for some distance above the soil line in advanced cases, rarely they may radiate from the wood into the soil. Rhizomorphs may also grow out from the larger roots, resembling feeder roots in appearance. They are about the diameter of pencil lead and vary in color from black to reddish brown. The interior consists of white mycelial tissue.
2) Decayed areas of wood at the root collar or on the crown roots. Armillaria causes a white rot and the wood develops a stringy texture. Roots in advanced stages of decay may be soft, yellowish and wet.
3) Veined, white mycelial fans between the bark and wood where the cambium has been killed. Sometimes this fan (or fans) extends quite far above the soil line beneath the bark.
4) Soil remaining attached to the roots.
5) Characteristic mushrooms on the lower trunk or on the ground near the infected roots. These short-lived annual fruiting structures of the disease-causing fungus may develop during the fall or winter rainy season and may occur in small clusters or in large numbers. The stalk is typically yellow and 3 inches or more long. Usually a ring is connected to the stalk just below the cap. The cap is 2-5 inches across and often honey-yellow. It may be dotted with dark brown scales. The underside is covered with loosely spaced white or yellow gills radiating from the stem.
After the disease has been identified, the grower should study the situation to determine the role Armillaria root rot has played in causing the decline or death of the tree. Frequently the fungus is only involved in a secondary manner by invading and destroying roots after the tree has been exposed to stress of some form, such as severe drought, water logging, or soil fill over the roots. The fungus can also act as a saprophyte feeding on dead wood. It is frequently involved in the decay of old tree stumps and roots.
Many oaks are lightly infected with the disease for years with no resultant damage except for isolated pockets of buttress root rot which are walled off by the tree and have no ill effects. Other infected trees show no damage until subjected to stress. Accumulating evidence suggests the type of root exudate that is produced influences the susceptibility of the tree. Certain forms of stress cause a shift in exudates that promote rapid development of the fungus and may hasten tree invasion and decay.
Spores are produced by the mushroom fruiting structures (mushrooms) and disseminated by air currents and introduced into new area. Once the fungus enters the cambium and bark tissues, mycelial fans develop during the parasitic phase of the attack. Subsequently, mycelium invades and decays the woody tissue of the roots and sometimes also the base of the trunk. Under proper conditions the fruiting structures form at or near the base of the infected tree, completing the life cycle.
Direct control of the fungus in a diseased tree is not possible with present technology. However, in many instances the fungus is incapable of causing severe damage unless the tree is first subjected to substantial stress. Thus, keeping the tree healthy and avoiding severe stress is one important approach in preventing loss of trees to Armillaria.
Drought and leaf defoliation are two major forms of stress that favor Armillaria. In dry years it is advisable, as in all years, to make sure irrigation scheduling is appropriate. Stresses such as defoliation from persea mite, soil compaction and physical injury can exacerbate the disease. Nutrient management may minimize Armillaria effects, although little research information exists on this subject.
The second most important means of minimizing Armillaria damage is to avoid or eliminate the fungus inoculum before planting. Trees planted in former orchards will quite possibly be exposed. Since these sites cannot be avoided, here is a suggestion that will be helpful: remove stumps and old roots from the old orchard to the greatest extent possible.
Below:
Armillaria mushrooms and hyphal plaques under the bark
- Author: Craig Kallsen
Many citrus trees in the southern end of the San Joaquin Valley are grown on moderately calcareous soils and frequently have high levels of boron in the leaf tissue. Citrus is sensitive to boron. Boron, when excessive, may cause defoliation and significant yield loss. At high, but nontoxic concentrations, leaf symptoms are similar to those caused by excessive salt, deficient potassium, heat stress, or biuret toxicity from urea foliar sprays. Therefore a leaf tissue analysis is important for delineating causes.
Excessive levels of boron produce a yellowing of the tip of leaves and yellow spotting of the leaf surface. Death of the leaf tissue may occur along the margins. Higher levels of boron may cause brownish, resinous gum spots on undersides of leaves but this symptom is not always present. Leaf symptoms are most severe on the “hot” south side of the tree. Boron accumulates in the leaves as they age so symptoms usually appear on older leaves first. Older leaves with high concentrations of boron are relatively short lived compared to trees that have boron at optimum concentrations. Often excessive boron and sodium appear together in leaf tissue analyses. Boron is associated with a decreased distance between leaf nodes. Trees with high leaf tissue boron concentrations appear to be less vigorous with shorter branches, probably as a result of the association of boron with decreased distance between leaf nodes.
Discussion of levels of boron which would be considered excessive in September-sampled spring-flush leaf tissue may be misleading because the particular leaves that are selected for the sample can greatly influence results. If only leaves with the most severe symptoms are sampled, such as leaves that are mostly yellow with dead margins, concentrations of boron can reach into the thousands of parts per million (ppm). A truer picture of the boron status of the grove can be gained by pulling leaves with ‘average’ symptoms. Using this sampling technique, the highest level of boron in orange leaves seen in this office over the past eight years has been 600 ppm from an isolated and particular calcareous part of an orchard located near the town of Edison in Kern County.
Standards from citrus in Florida for the concentration of boron in leaf tissue (4-6 month old leaves on nonfruiting terminals) correlate well with observations made in the San Joaquin Valley as follows:
Deficient <20
Low 21-35
Optimum 36 - 100
High 100 - 200
Excess > 250
Leaf boron concentrations greater than 250 ppm are excessive, but in older orange, lemon and grapefruit trees visible leaf symptoms are not usually manifested until leaf-tissue boron concentrations exceed 300 ppm. A range of 300 to 400 ppm show little outward sign of boron toxicity except for some slight tip yellowing and some reduction in vigor. Excessive defoliation does not usually begin in most citrus until concentrations of approximately 450 ppm are reached. Trees at 450 ppm and greater will, generally, exhibit a thin-canopied, unthrifty, somewhat stunted appearance. The yield of the tree does not appear to be affected nearly as rapidly as the appearance of the canopy. At least one large lemon grove in Kern County, that characteristically produces excellent yields of early-maturing, good quality fruit, has elevated leaf-boron levels. Moderate levels of leaf boron, in the 300 to 400 ppm range in this orchard appear to reduce tree growth, reducing the need to prune, while yield remains relatively unaffected.
Leaf boron concentrations greater than 300 ppm probably warrant further investigation as to the source of the boron. Orange leaf tissue samples taken from trees planted in the 1960’s or early 1970’s in Kern County routinely show levels of 300 to 400 ppm. Young trees appear to increase in boron concentration rapidly but at about 300 to 400 ppm the concentration tends to plateau. Why boron levels tend to plateau is not known. Chandler pummelos appear to be the most sensitive to excess boron, followed by lemons, grapefruits and oranges. Leaf boron concentrations of 400 ppm in Chandler pummelos appear to have caused severe stunting of the trees in several orchards in Kern County, while similar levels in Melogold (a pummelo x grapefruit hybrid) resulted in only some tip burn.
There are actions the grower can take to reduce the amount of boron in the tree. First the source of the boron should be determined if possible. If boron levels are increasing in the leaf tissue, analyze both surface water and well water. Avoid using water with greater than 0.5 ppm of boron for irrigation of citrus. Levels of boron that are beneficial to cotton or pistachio can cause severe problems with citrus. Surface water comes from diverse sources in Kern County. Surface delivered water may have started out as well water, or in some instances as diluted oil-field waste water which may contain relatively high concentrations of boron. Water districts will know if oil-field waste water is being diluted in irrigation water. Use of oil-field waste water can be seasonal and irrigation derived in part from oilfields may fluctuate in boron concentration. If boron is in the water even at slightly elevated levels, avoid spraying it directly on the trees when treating for insect pests or when applying foliar fertilizers. Fertilizers are foliarly applied because of the quick uptake of dissolved minerals through the leaves. If boron is in the spray solution, it will be absorbed quickly by the tree along with the potassium, zinc, manganese, nitrogen and other foliar nutrients. Organic matter, manure, composted materials, and mulches on the ground have been shown to reduce boron uptake by the plant from irrigation water with high concentrations of this element.
In the southern San Joaquin Valley, soils should be tested before citrus is planted. Areas of soil with high boron are found in the most unexpected places. Boron may have accumulated on some properties when high-boron well water was used before the advent of easier access to water from Sierra snow melt.
If leaf-tissue boron is high and the water or soil is not, check the foliar fertilizer blends being used. Often, boron is included in many micronutrient mixes because boron can be deficient in acid soils. Determine how much boron soil amendments may contain. Pit gypsum can have varying quantities of boron in it. A ton of this gypsum may contain as much as 20 pounds of boron.
Discovering the cause of high boron in citrus leaves may require an extra soil test in addition to the typical saturated pest extract. Soil tests for ‘available’ boron using a saturated pest extract can be deceiving. In many instances where the concentration of boron in a ‘typical’ leaf averaged greater than 300 ppm, plant-available boron in the soil and water frequently averaged less than 0.25 ppm. However, total soil boron in these same orchards was at very high levels. Total soil boron estimates both available and unavailable boron. To help determine where the boron in the trees originates, both readily available and total soil boron should be sampled. This disparity between plant-available and total boron suggests that boron moves between the relatively small plant-available pool in the soil and the much larger ‘unavailable’ pool tied up in these calcareous soils. Soil acidifying agents and acid-forming fertilizers probably increase the availability of boron to citrus trees by making boron that is relatively unavailable to the trees at high pH, more available at lower pH. At any given time, plant-available boron may be relatively low but its constant replacement from the unavailable pool keeps the boron concentration in trees relatively high. In orchards where total soil boron is elevated; soil pH should probably be kept as high as tree health permits. Where the total amount of soil boron is moderate and soils are relatively well-drained and topography is flat, acidifying and leaching is probably the preferred option for reducing boron levels. Acidifying the soil and not supplying sufficient water to leach the boron from the root zone can compound the problem by making more boron readily available to the tree.
If boron is not found in the upper soil profile, but is found or suspected to exist deeper, irrigations could be scheduled that are more frequent but of shorter duration so that most of the citrus roots remain in the upper, lower-boron portion of the soil profile.
Actively growing, vigorous trees may dilute the concentration of boron in the leaf tissue through the production of a thick canopy. Old leaves tend to accumulate boron and drop. Adequate nitrogen ensures that enough nitrogen is present for production of new leaves. Increasing the nitrogen fertilization rate can encourage vegetative production and enhance this effect, but too much nitrogen may be associated with adverse fruit quality characteristics like regreening of Valencias, later maturity of early navels or higher yields of smaller fruit. Keeping other nutrients in the leaf in balance is important if boron is present at excessive concentrations. Maintaining high concentrations of phosphorous and calcium in the leaves through an appropriate fertilization program should be beneficial as these nutrients have been shown to reduce absorption of boron.