California Crop Fertilization Guidelines

Leaf N should be monitored yearly, as N is very mobile in soil and may change quickly in soil systems ^[N25]. Leaf nutrient concentrations drop sharply from spring through spring and into summer, and stabilize around July. For this reason, University of California critical nutrient values have been established for July sampling, at which point nutrient concentrations are relatively stable. Samples taken earlier do not currently have any reliable interpretation ^[N26].

Leaf sampling should be done as follows ^{[N13,N25,N33]}:

• The orchard should be divided into uniform blocks with respect to soil type, tree age, variety, location and management.
• Unhealthy, stunted or injured trees are avoided and non-representative areas are sampled separately.
• Fully expanded leaves from non-bearing spurs 5-7 feet above the ground are chosen.
• One or two normal, healthy leaves are collected from each tree for a total of 50-75 leaves per sample.
• Visual evaluation of orchard health and vigor should be made and recorded. This will aid interpretation of test results.
• Samples are submitted to a lab as soon as possible. If a delay is anticipated, they should be refrigerated until they can be sent.

The critical values presented in the table are conservative; they represent the minimum concentrations necessary for optimal growth ^[N25]. Leaf concentrations greater than 2.8-3.0% indicate over-fertilization. Visual observations of tree vigor, crop load and leaf color may also be used to interpret test results.

If the test shows adequate leaf N, no N is needed until mid-April of the following year, at which point the rate may be decided based on the crop load. If the test N is low or marginal, a soil N application several weeks after petal fall may be needed, regardless of crop load ^[N24]. To correct an N deficiency mid-season, foliar urea sprays can be used (see Foliar N). An in-season soil application may also be used, but should be very light since stone fruit trees take up little N in fall and excess N may be leached with winter rains ^[N25]. Leaf critical nutrient values from nonbearing spur leaves of French prune sampled in July ^[N33].

The values in the table refer to a tree. In contrast, the results from leaf analyses refer to the average of all trees sampled in the orchard with some trees being below and others above the average. To minimize the risk of some trees in the orchard being deficient, the lower limit of the adequate range may be increased to 2.4-2.5% N may be more appropriate (Niederholzer, personal communication).

The critical values for N and P are also valid for Japanese plums ^[N1]. If prunes are produced for the fresh market the lower end of the optimal N range should be used, because of the importance of early maturity ^[N8].

Nitrogen is important to biomass growth, and young trees benefit from N fertilizer ^[N31]; however, immature trees require less N that mature trees as they are smaller and produce a smaller crop ^[N25]. In an orchard in Yolo County on a clay loam soil, Southwick and coworkers ^[N31] found that trees fertigated with an annual total of 0.25-0.5 lbs N/tree regularly during the season had good growth and high yields, while the incidence of bacterial canker was minimized. Based on these results and taking into account the increasing N requirements as trees grow, Niederholzer and coauthors ^[N25] recommended the application rates for young trees summarized in the table. These application rates are in line with earlier recommendations by La Rue and Gerdts ^[N16]. Optimal N application rates to young prune trees planted at a spacing of 14 x 17 feet (183 trees/acre) ^[N25].

In loam, clay loam or sandy clay loam soils, the soil may provide sufficient N to support the first year of growth ^[N15,N32]. In order to protect against fertilizer burn, rates should be reduced if applications are done in hot weather when trees are taking up more water ^[N9]. This is especially true where liquid fertilizers are used, as they can easily burn young trees ^[N11].

Young prune and plum trees have small root systems, and are inefficient at taking up applied N. Localized applications in a band or through the drip system are most effective ^[N16,N32]. Foliar N may also be used in addition to soil treatments (see Foliar N) ^[N34].

Experience with almonds suggests that liquid fertilizers more easily burn young roots and may be dangerous to use with first-leaf trees ^[N11]. A conservative approach is to use a granular fertilizer, placed at least 18 inches from the trunk but within the wetting zone of the irrigation system ^[N9]. Blends such as 12-12-12 and 15-15-15 also supply P and K. For fertigation, UN32 is commonly used ^[N25].

Newly planted trees normally do not need to be fertilized until after growth has begun in spring or early summer ^[N15]. Frequent, small applications are preferred to large applications ^[N9,N31].

About 10-15 lbs of N are removed per dry ton of prunes ^{[N5,N24,N31,N37]}. For more information see Nitrate Uptake and Partitioning. In addition, based on values from other tree species it is estimated that about 30-40 lbs N/acre/year are required for vegetative growth in a mature orchard ^[N23]. Because not all N applied is taken up by the tree, total N input from all sources needs to be higher than this. The values in the table assume N application through drip or microsprinkers and an N use efficiency of 70%. Annual N application rates to prunes fertigated via low volume irrigation. N input includes fertilizer N as well as N in the irrigation water.

The values in the table are in line with Johnson and Uriu ^[N12], who found that 100-150 lbs N/acre are usually adequate for plums, peaches and nectarines.

Similar rates are recommended for mature plums and prunes grown for fresh market. Early season varieties should be fertilized less than those harvested later in the season ^[N8,N12]. Leaf analyses provide a means to evaluate the N fertilization program.

When well water is used for irrigation, it may contribute a significant amount of N ^[N28]. Well water samples should be taken after the pump has been run for several hours, to make sure that the water sampled is representative of what the orchard will receive ^[N23]. Nitrate in well water can be calculated as the ppm of nitrate-N x 2.72 = lbs N applied in one acre foot of water.

For example, a test value of 7 ppm nitrate-N in an orchard receiving 2 acre-feet of water per year would mean that 38 lbs N/acre are added annually with the irrigation water. If the concentration of nitrate rather than nitrate-N is given, multiply by 0.614 rather than 2.72. The proportion of irrigation water N available to the trees depends on how carefully irrigation water is managed ^[N5]. To estimate N credits, only the nitrate-N in the water taken up by the trees, which corresponds to the evapotranspiration (ET) rate, should be counted ^[N23].

Application through low-volume irrigation systems is more efficient than broadcasting or banding ^[N14].

When applied through fertigation, the fertilizer should be injected into the irrigation system in the middle third of the irrigation set. For example, in an 18-hour irrigation set, fertilizer is injected from hour 6 through hour 12. This prevents urea and nitrate from moving below the root zone but still ensures that the N is distributed well in the wetting zone and not overly concentrated, causing root burn ^[N23].

In orchards where N is not fertigated, the fertilizers should be applied in the herbicide strip along the tree rows and not broadcast over the entire area. Fertilizer applied between the rows is less efficiently used due to competition with weeds or cover crops and lower prune root density compared to the tree row ^[N22]. Surface applied N should be incorporated or irrigated into the root zone shortly after application ^[N21]. This is especially important when fertilizers containing ammonium or urea are used in hot, windy weather and on soils with an alkaline pH because these factors increase ammonia volatilization.

A number of mineral N fertilizers are available to growers. Mineral fertilizers contain N in the form of urea, ammonium, nitrate or a mix of them. Research with almonds suggests that when losses are minimized and equal amounts of N are added, the effects on yield of these fertilizers are similar, and this is likely true for plums and prunes as well ^{[N3,N4,N23,N25]}. The N forms, however, behave differently in the environment. Nitrate is very mobile in the soil and can easily be leached into deeper soil layers with irrigation water or rain. Ammonium is less mobile, but is generally converted quickly to nitrate by soil microorganisms, especially in warm and moist soils, unless they are water saturated. This process, nitrification, can lower soil pH. The acidifying effect is especially strong when ammonium fertilizers are applied by drip systems as they are concentrated in a small soil volume ^[N37]. Ammonium may volatilize as ammonia, especially when applied to the surface of dry and alkaline soils without incorporation. Urea is relatively mobile in the soil, but is generally quickly converted to ammonium.

More detailed information about the most common fertilizers can be found on the website of the IPNI.

Very little soil N is taken up during dormancy and before bud swelling. While N is needed for flowering it is recycled from the perennial parts of the tree rather than taken up from the soil for immediate use ^[N35]. The majority of N uptake occurs during the period of rapid shoot growth, with about 60-70% of the total N requirement being taken up by June ^[N6]. The N taken up during the late summer supports flowering the following spring ^[N35]. Uptake slows during fall through leaf drop, and is essentially zero through winter ^[N22]. For this reason N fertilizers should not be applied after leaf drop, in winter or in spring before bud break as they have a high risk of being leached out before the crop can use them.

Based on the seasonal uptake pattern, the first N application in spring is best done around mid-April, before rapid uptake is beginning. Exceptions are marginal or N-deficient orchards where some N should be applied a few weeks after petal fall ^[N24].

Multiple small applications are more efficient than large applications; Niederholzer ^[N23] recommends not more than 20-30% of the annual fertilizer budget should be added per application. A general rule in a healthy orchard with adequate tree vigor is that one-third of the annual N requirement may be applied in spring (March or April), with the remaining two thirds applied in mid-to late summer (June, July and August). Where tree vigor is low, a relatively greater proportion should be applied in spring, and the remainder applied in summer ^[N22]. Applications after harvest should be small and applied well before leaf fall, so that the N can be taken up and stored in the tree's woody parts ^[N21,N25].

Rates must be kept low to avoid leaf burn and the multiple applications needed may become expensive ^[N17,N34]. Because of this, foliar N can supplement but not replace soil applications. Spring urea sprays of 1.8% or higher have been shown to damage leaves of stone fruits, and sprays of potassium nitrate exceeding 1.2% have damaged prune leaves ^[N34]. Higher rates, up to 4 to 5% of low-biuret urea, have been safely applied post-harvest ^[N17].

Low-biuret urea is the most commonly applied source of foliar N. Potassium nitrate supplies some N; however, urea is a more efficient source ^[N34]. Slow-release N fertilizers, which have less burn potential than urea, are also often used on fruit trees ^[N2,N19].

Soil samples for nutrient analysis should be taken from the main root zone. Most roots of plum trees are in the top 2 feet of the profile ^[P15]. When the soil samples are also used to assess soil salinity, sampling to a depth of 4 feet may be required ^[P8]. For more information on sampling procedure see Soil Sampling in Orchards.

Phosphorus availability strongly depends on soil pH. In acidic soils, P is immobilized by iron and aluminum minerals, while the predominant mechanism of P immobilization in alkaline soils is the formation of insoluble calcium phosphate compounds. For this reason, the recommended soil test depends on soil pH. For acidic to neutral soil (pH < 7.0), the Bray P1 test is generally used. For neutral and alkaline soils (pH > 6.5), the Olsen method is more appropriate ^[P7]. Using a test that is inappropriate for the soil analyzed may result in inaccurate results.

The University of California has not developed critical values for soil analysis in prune or plum orchards ^[P9]. The table below gives some general guidelines for soil test interpretation in orchards. Interpretation of P and K soil test results in orchards ^[P7].

When soil or leaf analyses suggest that P fertilization may be beneficial, the application rate can be based on the amount of P removed with harvested fruits each year (see P Fertilization of Mature Trees).

Leaf nutrient concentrations drop sharply from spring through spring and into summer, and stabilize around July. For this reason, University of California critical nutrient values have been established for July sampling, at which point nutrient concentrations are relatively stable. Samples taken earlier do not currently have any reliable interpretation ^[P10].

Leaf sampling should be done as follows ^[P6,P9,P14]:

• The orchard should be divided into uniform blocks with respect to soil type, tree age, variety, location and management.
• Unhealthy, stunted or injured trees are avoided and non-representative areas are sampled separately.
• Fully expanded leaves from non-bearing spurs 5-7 feet above the ground are chosen.
• One or two normal, healthy leaves are collected from each tree for a total of 50-75 leaves per sample.
• Visual evaluation of orchard health and vigor should be made and recorded. This will aid interpretation of test results.
• Samples are submitted to a lab as soon as possible. If a delay is anticipated, they should be refrigerated until they can be sent.

Leaf critical nutrient values from nonbearing spur leaves of French prune sampled in July ^[P14].

The values in the table refer to a tree. In contrast, the results from leaf analyses refer to the average of all trees sampled in the orchard with some trees being below and others above the average. To minimize the risk of some trees in the orchard being deficient, the lower limit of the adequate range may be increased to 0.15% P (Niederholzer, personal communication).

Leaves of Japanese plum have similar critical values for N and P ^[P1].

In the long term, the amount of P removed at harvest needs to be replaced with fertilizer to maintain adequate P availability. Approximately 5 lbs P₂O₅ (2.2 lbs P) are removed from the orchard per dry ton of prunes ^[P2,P12]. For stone fruits grown on P deficient soils, a large application of 350-450 lbs P₂O₅ per acre may be necessary; this should be effective for several years ^[P1,P13].

Strip or ring applications at the drip line are more effective than broadcast applications ^[P1].

As P is strongly immobilized by soil minerals, it cannot be leached into the root zone with irrigation or rainwater and should be incorporated into the soil for better availability. Phosphorus fertilizers can be drilled 6 to 8 inches deep in one or two bands on opposite sides of the tree row ^[P1].

Phosphorus can be fertigated; however care must be taken to prevent the formation of calcium phosphates which can plug the emitters ^[P3].

Facts sheets about the most common fertilizers can be found on the IPNI homepage.

Phosphorus is immobile in the soil and is barely leached. The time of application is therefore not crucial as it is for N. It may be applied with K in November after leaf drop begins.

Soil samples for nutrient analysis should be taken from the main root zone. Most roots of plum trees are in the top 2 feet of the profile ^[K31]. When the soil samples are also used to assess soil salinity, sampling to a depth of 4 feet may be required ^[K12]. For more information on sampling procedure see Soil Sampling in Orchards.

The University of California has not developed critical values for soil analysis in prune or plum orchards ^[K21]. The table below gives some general guidelines for soil test interpretation in orchards. Interpretation of P and K soil test results in orchards ^[K11].

Leaf nutrient concentrations drop sharply through spring and into summer, and stabilize around July. For this reason, University of California critical nutrient values have been established for July sampling, at which point nutrient concentrations are relatively stable. Samples taken earlier do not currently have any reliable interpretation ^[K23].

Leaf sampling should be done as follows ^{[K16,K21,K30]}:

• The orchard should be divided into uniform blocks with respect to soil type, tree age, variety, location and management.
• Unhealthy, stunted or injured trees are avoided and non-representative areas are sampled separately.
• Fully expanded leaves from non-bearing spurs 5-7 feet above the ground are chosen.
• One or two normal, healthy leaves are collected from each tree for a total of 50-75 leaves per sample.
• Visual evaluation of orchard health and vigor should be made and recorded. This will aid interpretation of test results.
• Samples are submitted to a lab as soon as possible. If a delay is anticipated, they should be refrigerated until they can be sent.

Leaf levels are used to determine fall application rates. If levels are low or marginal and a good crop was harvested, K fertilizer should be applied ^[K6]. If leaf levels are greater than 2% there is unlikely to be any benefit from additional K fertilization in that season. However, the K removed by the harvested crop should still be replaced to maintain soil K availability ^[K24] (see Soil Applied K). Leaf critical nutrient values from nonbearing spur leaves of French prune sampled in July ^[K30].

The values in the table refer to a tree. In contrast, the results from leaf analyses refer to the average of all trees sampled in the orchard with some trees being below and others above the average. To minimize the risk of some trees in the orchard being deficient, the lower limit of the adequate range may be increased to 1.5% K (Niederholzer, personal communication).

For Japanese plum, leaf K values of 1.1% are considered adequate ^[K1].

July leaf analyses are not very useful for in-season adjustment; and no reliable relationship between earlier leaf samples and July values has been found for prune. However, broadly, if leaf K is near deficiency (less than 1.3%) in May, trees will likely be deficient by July. If leaf K concentrations exceed 2.3% in May, trees are not likely to be deficient in July ^[K23].

Prunes remove an estimated 23 lbs K₂O per dry ton harvested per acre. Assuming that 50% of the fertilizer applied is plant available, about 100 lbs of potassium sulfate fertilizer (0-0-50) per acre would be needed to replace K removed by each ton of yield ^{[K2,K24,K28,K32]}. A common recommendation for K applied by fertigation to mature, heavily cropped prune trees is 150 lbs K₂O/acre as K₂SO₄ or KCl ^[K7]. Since more of the K applied through drip systems is plant available, the application rate can be reduced when fertigation is used compared to when dry fertilizer is banded in fall (240-250 lbs K₂O/acre) ^[K7].

Fruit is the driver of K uptake, so in years where crop load is light (for example, if hot spring weather reduced fruit set) in-season K can be reduced^[K7,K24,K32]. If prunes are being grown for the fresh market, the K requirement will be lower because of the lighter crop load ^[K5].

Soil properties can affect K availability. Higher rates may be required for heavy soils, where more K is held on soil surfaces, compared to lighter soils ^[K21]. In areas where soils are known to fix K, such as the east side of the San Joaquin Valley, analyzing the soil for its K fixation capacity before the orchard is planted helps adjust the K fertilization program. A discussion about K fixation in the San Joaquin Valley can be found here. For more information about soil K fertilization in your area,contact your local farm advisor specializing in prune or href="https://ucanr.edu/sites/ucanr/Find_People/Academic_Directory/?appointment=ceadvisor&commodity=204">plum production.

Broadcast or sprinkler applications do not deliver K directly to the root zone and are less effective than band applications or fertigation through the drip system ^[K32]. Fertigation is more efficient than banding on fine-textured soils. On a clay loam near Colusa in the Sacramento Valley, Uriu and coworkers ^[K29] found that mature prune trees receiving 10 lbs drip-applied K₂SO₄ per tree maintained higher leaf K than trees receiving 20 lbs trenched granular K₂SO₄ per tree. In addition, when K is repeatedly applied in the same place, K binding sites become saturated and availability is increased ^[K7,K32].

Since soil-applied K fertilizer may take several months to affect tree K status, dry fertilizers are best applied in fall ^[K7]. If a heavier crop is set or large prunes are desired, fall fertilization may be supplemented by in-season fertigation or foliar application ^[K7]. If KCl is used banding the dry fertilizer in fall is the safest method, so the chloride ion can be washed out of root zone over winter, if adequate rainfall occurs. Irrigation water may be needed to leach chloride if insufficient rain falls. See Fall Applied K for information on applying dry fertilizers.

Many types of K fertilizer may be used. Potassium sulfate (K₂SO₄) and potassium chloride (KCl) are the most common ^[K20].

Although KCl fertigation is possible (see Table above), this practice is risky, as chloride may burn the roots ^[K7]. KCl should not be used in orchards with fluctuating water tables or significant drainage obstructions ^[K20,K23].

Chlorine toxicity on plum and prune appears as a leaf tip burn and marginal scorch ^[K25]. Where KCl is used, it is a good idea to test July leaf samples for Cl^- to monitor status over time. A value of 0.3% Cl^- is considered excessive ^[K16].

Potassium thiosulfate (KTS, 0-0-25) is a liquid fertilizer sometimes used in fertigation. Rates higher than 12 gallons per acre per application may injure or kill trees. Potassium thiosulfate lowers the soil pH ^[K19].

When fertigated through the drip system, K is best applied throughout the growing season, starting in late April or early May and going through July ^[K3,K32]. Prune fruits accumulate K from bloom until harvest ^[K20]. In-season application methods allow growers to adjust rates if unforeseen events like spring frost or hot weather at blooming lead to a lighter crop than was expected.

A total of 100 lbs potassium nitrate (KNO₃) per acre in multiple sprays is recommended to avoid K deficiency in heavily cropped trees ^[K21,K27]. 20-30 lbs KNO₃ in 100 gallons of water per acre may be applied at 2 to 3 week intervals, starting in late April to early May, before deficiencies show, and going through July ^[K7,K16,K27].

No more than 25 lbs KNO₃ per 100 gallons per acre should be applied in hot weather, to avoid leaf damage ^[K21]. If zinc fertilizer is applied in the same spray, no more than 10 lbs KNO₃ in 100 gallons water per acre should be used.

Potassium nitrate is most commonly used. Other foliar K fertilizer are available; however, if the entire K demand is being supplied foliarly, annual application rates should be equivalent to a total of 100 lbs KNO₃ per acre (45 lbs K₂O/acre) ^[K20].

Prunes remove an estimated 23 lbs K₂O per dry ton harvested per acre. Assuming that 50% of the fertilizer applied is plant available, about 100 lbs of potassium sulfate fertilizer (0-0-50) per acre would be needed to replace K removed by each ton of yield ^{[K2,K24,K28,K32]}. A common recommendation for mature, heavily cropped prune trees is 240-250 lbs K₂O/acre banded as K₂SO₄ or KCl. Since more of the K applied through drip systems is plant available, this application rate is higher than when in-season fertigation is used (150 lbs K₂O/acre) ^[K7].

July leaf K analyses and the previous year's crop are used to determine the rate of fall fertilization ^[K6]. Fruit is the driver of K uptake, so in years where crop load is light less in-season K is necessary, and as long as tissue K levels are acceptable fall K application may be reduced ^[K7,K24,K32]. In plums or prunes grown for the fresh market, the K requirement will be lower because of the lighter crop load ^[K5]. If trees are K deficient, higher application rates may be used.

Soil properties can affect K availability. Prunes are not commonly planted on sandy soils, but where they are, lower rates of fall applied K are suggested as the soil has less K holding capacity. Experience with almonds suggests that when soil CEC is less than 15 meq/ 100 g soil, K applications should be reduced ^[K10]. Conversely on heavier soils higher rates may be required, as more K is held on soil surfaces ^[K20]. In areas where soils are known to fix K, such as the east side of the San Joaquin Valley, analyzing the soil for its K fixation capacity before the orchard is planted helps adjust the K fertilization program. A discussion about K fixation in the San Joaquin Valley can be found here. For more information about soil K fertilization in your area, contact your local farm advisor specializing in plum or href="https://ucanr.edu/sites/ucanr/Find_People/Academic_Directory/?appointment=ceadvisor&commodity=204">prune production.

Broadcast or sprinkler applications do not deliver K directly to the root zone and are less effective than band applications or fertigation through the drip system ^[K32]. In-season fertigation is more efficient than fall banding on fine-textured soils. On a clay loam near Colusa in the Sacramento Valley, Uriu and coworkers ^[K28] found that mature prune trees receiving 10 lbs drip-applied K₂SO₄ per tree maintained higher leaf K than trees receiving 20 lbs trenched granular K₂SO₄ per tree. In addition, when K is repeatedly applied in the same place, K binding sites become saturated and availability is increased ^[K7,K31].

Dry fertilizers should be banded or shanked along tree rows, and washed into the soil by rainfall or irrigation water ^[K6]. On heavier soils, which bind more K, gypsum (CaSO₄) is sometimes added in the same band at rates of 1000-4000 lbs per acre ^[K24]. The calcium ion competes with K for binding sites on soil surfaces, helping it move deeper into the root zone ^[K4]. Bands should be made in the same place every year ^[K24].

Many types of K fertilizer may be used. Potassium sulfate (K₂SO₄) and potassium chloride (KCl) are the most common ^[K21]. Because prunes and plums are sensitive to chloride, KCl must be used with care. Usually it is applied in late fall or winter so that winter rains can flush the chloride ion (Cl^-) out of the root zone. If there has not been much rain by January, supplementary irrigation to wash it down is advised. KCl should not be used in orchards with fluctuating water tables or significant drainage obstructions ^[K21,K24].

Dry fertilizers are best applied in fall, since soil-applied K fertilizer may take several months to affect tree K status ^[K7]. If a heavier crop is set or large prunes are desired, fall fertilization may be supplemented by in-season fertigation or foliar application ^[K7].

Leaf nutrient concentrations drop sharply from spring through spring and into summer, and stabilize around July. For this reason, University of California critical nutrient values have been established for July sampling, at which point nutrient concentrations are relatively stable. Samples taken earlier do not currently have any reliable interpretation ^[Zn10].

Leaf sampling should be done as follows ^{[Zn2,Zn6,Zn8,Zn13]}:

• The orchard should be divided into uniform blocks with respect to soil type, tree age, variety, location and management.
• Unhealthy, stunted or injured trees are avoided and non-representative areas are sampled separately.
• Fully expanded leaves from non-bearing spurs 5-7 feet above the ground are chosen.
• Leaves which may have received or been contaminated with Zn from a foliar fertilizer or Zn- containing pesticide spray should not be used.
• One or two normal, healthy leaves are collected from each tree for a total of 50-75 leaves per sample.
• Visual evaluation of orchard health and vigor should be made and recorded. This will aid interpretation of test results. This is particularly important for Zn, for which leaf concentrations may be reduced in a vigorously growing tree.
• Samples are submitted to a lab as soon as possible. If a delay is anticipated, they should be refrigerated until they can be sent.

If levels are at or around deficiency and a good crop was harvested, Zn fertilizer should be applied, either in fall or in the following year ^[Zn3].

Leaf Zn levels may be lower in vigorously growing orchards, and leaf Zn may also be affected by past foliar fertilizer history. A good practice is to keep records from past years and track Zn levels over time. Past experience, and visual observation of orchard appearance and current growth should be used alongside leaf analysis to make fertilization decisions ^[Zn2]. Leaf critical nutrient values from nonbearing spur leaves of French prune sampled in July ^[Zn13].

In a study with Fortune Plums at the Kearney Agricultural Center, Johnson and coworkers ^[Zn4] found that Zn deficiency symptoms were only present in trees with leaf Zn levels below 10 ppm. These results suggest that the deficiency threshold for Zn may be lower than the published threshold ^[Zn4].

A variety of Zn fertilizers, including basic zinc sulfate, zinc nitrate, zinc oxide, zinc EDTA and various other zinc chelate products may be used ^[Zn8].

A general recommendation is for 3-5 lbs of zinc oxide or basic zinc sulfate per 100 gallons of water. This application may be combined with an early-season potassium nitrate application; however, a smaller rate of K fertilizer (10 lbs KNO₃ in 100 gallons water per acre) should be used ^[Zn8]. If spray adjuvants are used the rates may need to be decreased, as they increase uptake and may cause marginal leaf burn ^[Zn8].

In spring, Zn should be applied preferably before leaves reach their full size and no later than mid- May ^[Zn9]. Applications for both prune and plum should be avoided if there is rain forecast in the next 48 hours, as leaf rewetting may cause too much Zn uptake, leading to "shot-holing" in the leaves ^[Zn8].

Normally, only zinc sulfate is applied in fall and during dormancy.

Late Zn applications are done at the beginning of normal leaf drop, or during dormancy. Higher rates may be applied than in spring; from 10-15 lbs zinc sulfate per acre in 100 gallons water is recommended, with a maximum of 20-25 lbs per acre.

Aphid control materials may be sprayed with fall zinc sulfate application, if no oil is included in the mix. Oil should not be applied within 30 days of zinc sulfate ^[Zn3,Zn8].

Zn applied in fall before leaf drop may accelerate defoliation. If the orchard is being irrigated or rain has recently fallen, Zn sulfate sprays at fall-recommended rates will remove leaves over the course of a week or so (Niederholzer, personal communication). However, this should not affect the following year's buds and may be an advantage in areas where tree blowover or foliar disease overwintering is a concern ^[Zn3,Zn8]. In dry orchards, when humidity is low and irrigation has stopped, even high Zn sulfate applications don't cause defoliation.