Nitrogen fertilizers need to be applied with care to young trees, as high N concentrations in soil solution may burn the roots. A good rule is to apply no more than one ounce of N per tree per year of growth with a single application ^{[N21, N13]}. One ounce of N per tree corresponds to 5-9 lbs N/acre, depending on tree density. For the first application to first leaf trees, even less than one ounce of N should be applied ^[N13]. The rate should also be reduced for stunted trees and in hot weather when the trees take up a lot of water. Under these conditions, lowering the rates per application and instead increasing the number of applications reduces the risk of leaf burn ^[N13].

A total annual application of up to 4 ounces/tree (20-35 lbs/acre) to first-leaf trees may increase growth, while higher rates have no benefits ^[N18].

Meyer ^[N23] suggested the following annual N rates for drip-irrigated trees on non-fertile soils:

Granular fertilizers should be placed at least 18 inches from the trunk, but within the wetting zone of the irrigation system ^[N21].

When N is fertigated, the fertilizer should be injected into the irrigation system in the middle third of the irrigation set ^[N25]. 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.

The risk of root burn is increased with liquid fertilizers, as their application may result in high concentrations in the root zone. Therefore, as a measure of caution, granular fertilizers should be used for first-leaf trees ^[N21]. Blends, such as 15-15-15 or 12-12-12 also supply K and P ^{[N13, N21]}. To second-leaf trees, liquid fertilizers, such as UN-32 or CAN 17 can be applied. To ensure sufficient P and K availability, a granular NPK fertilizer may be used for the first application of the year ^[N13]. In early spring, when the soil is still cooler, nitrate based fertilizers may be more efficient than urea or ammonia fertilizers ^[N13]. However, nitrate is very mobile and may move quickly below the still small root zone of the young trees. Therefore, careful irrigation management is important to prevent nitrate from leaching below the root zone.

Composition of common N fertilizers:

Fertilizer should not be applied to recently planted trees until spring after the trees leaf out ^[N21]. Further applications, if needed, can be made every 4-6 weeks ^[N13].

The effects of foliar applications to almonds have not been studies widely. The data available suggests that well fertilized trees do not benefit much from additional foliar applications. In a two-year study carried out in an orchard on an Arbuckle gravelly loam, Meyer and coworkers ^[N24] did not find consistent effects on kernel yield and leaf nutrient concentrations due to up to three spring foliar applications of different combinations of N, P and K, applying up to 35 lbs N/acre. Foliar fertilizers were applied in addition to 160-190 lbs/acre soil applied N. Almond meat yields ranges from 1500 to 2100 lbs/acre.

In a trial conducted by Duncan, fall applications of 25 lbs/acre of urea to healthy almond trees slightly increased yield, while higher application rates had no effect. None of the observed differences were statistically significant ^[N19]. However, fall urea application increased the resistance of peach trees to bacterial canker ^[N20].

Klein and Weinbaum ^[N22] found that a foliar application with a 0.5% urea solution did not cause noticeable leaf damage to mature Nonpareil almond trees, while a 1% solution caused slight marginal and tip necrosis. The same study showed that 85% of the urea was taken up within 24 hours. Meyer and coworkers ^[N24] also observed leaf tip necrosis in one out of three years with solutions containing 0.55% N, which corresponds to 1.2% urea.

Tip burn on ‘Butte’ almond leaves caused by foliar urea application. Leaves were samples after a post-harvest (October) application of 5.4% low-biuret urea solution containing 20 lbs N/100 gallons of water (photo provided by Franz Niederholzer).

Urea and fertilizers containing N in the form of urea are most often used. Using low-biuret urea reduces the risk of leaf damage.

Foliar N can be applied in spring or fall. Fall applications need to be done as long as the leaves are still active.

Meyer and coworkers reported applying foliar fertilizers pre-bloom, when the tip of the bloom was pink, 30 days later and 60 days after the pre-bloom application ^[N24]. The risk of leaf damage is greatest before harvest (Niederholzer, personal communication), but may accur at other times as well.

In a four-year study carried out at multiple sites, Brown and coworkers found that the amount of N removed at harvest with hulls, shells, leaves, debris, and kernels ranges from 50 to 75 lbs/1000 lbs of kernels, averaging 68 lbs/1000 lbs of kernels ^[N28]. In addition, almond trees need nutrients for the growth of perennial parts, such as roots, trunk and branches. The N in the perennial parts of 9 to 13 year old trees increased annually by 25-30 lbs/acre, with 12 year old trees containing about 450 lbs of N ^[N3].

Based on these and earlier trials, the team led by Patrick Brown developed a computer based N management program ^{[N5, N9]}. The annual N application rates listed in the table below do not include adjustments for cover crops, organic fertilizers, N in irrigation water and high leaf N concentrations. The almond fertilization model can be accessed here for a detailed calculation of N application rates and the time of application.

Annual N application rates to almonds fertigated via low volume irrigation ^[N9]:

The N application rate needs to be adjusted for N contained in the irrigation water. In a study conducted in 2000 in selected orchards in the Sacramento Valley, the N input with the irrigation water averaged 9.3 lbs N/acre-foot with values ranging from 0 to 28 lbs N/acre-foot of irrigation water ^[N10]. Therefore, 27.9 lbs N/acre are applied with a seasonal application of three acre-feet of water containing 9.3 lbs N/acre-foot. Contact your local farm advisor for more information.

In orchards with flood irrigation or solid set sprinklers, 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 almond root density compared to the tree row ^[N26]. Surface applied N should be incorporated or irrigated into the root zone shortly after application ^[N11]. 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.

In orchards with microsprinkler systems, the fertilizer should be injected into the irrigation system in the middle third of the irrigation set. For example, in a 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 ^[N25].

A number of mineral N fertilizers are available to growers (see Table). Mineral fertilizers contain N in the form of urea, ammonium, nitrate, or a mix of them. When losses are minimized and equal amounts of N are added, the effects on yield of these fertilizers are generally quite similar ^{[N6, N7, N26]}. The N forms however behave differently in the environment. Nitrate is very mobile in the soil and can easily be leached below the root zone 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 ^[N34]. 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.

Common N fertilizers:

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

In a study carried out with FREP support, Brown and coworkers found that 13% of the total N is taken up during spring flush (January to mid-March), 42% between spring flush and nut fill (mid-May), and 45% during the summer ^[N5]. About 80% of the N in the fruits is taken up before mid-June ^[N7]. Two thirds of the N needed during spring flush is transferred from the trunk and roots, reducing the need for spring-applied fertilizer N.

Based on their research, Brown and coworkers developed an almond fertilization model which is available here ^{[N5, N9]}. The model recommends applying 30% of the total N in March/April, 40% in May/June, and 30% in June/July ^[N9].

The effects of foliar applications to almonds have not been studies widely. The data available suggests that well fertilized trees do not benefit much from additional foliar applications. In a two-year study carried out in an orchard on an Arbuckle gravelly loam, Meyer and coworkers ^[N24] did not find consistent effects on kernel yield and leaf nutrient concentrations due to up to three spring foliar applications of different combinations of N, P and K, applying up to 35 lbs N/acre. Foliar fertilizers were applied in addition to 160-190 lbs/acre soil applied N. Almond meat yields ranges from 1500 to 2100 lbs/acre.

In a trial conducted by Duncan, fall applications of 25 lbs/acre of urea to healthy almond trees slightly increased yield, while higher application rates had no effect. None of the observed differences were statistically significant ^[N19]. However, fall urea application increased the resistance of peach trees to bacterial canker ^[N20].

Klein and Weinbaum ^[N22] found that a foliar application with a 0.5% urea solution did not cause noticeable leaf damage to mature Nonpareil almond trees, while a 1% solution caused slight marginal and tip necrosis. The same study showed that 85% of the urea was taken up within 24 hours. Meyer and coworkers ^[N24] also observed leaf tip necrosis in one out of three years with solutions containing 0.55% N, which corresponds to 1.2% urea.

Tip burn on ‘Butte’ almond leaves caused by foliar urea application. Leaves were samples after a post-harvest (October) application of 5.4% low-biuret urea solution containing 20 lbs N/100 gallons of water (photo provided by Franz Niederholzer).

Urea and fertilizers containing N in the form of urea are most often used. Using low-biuret urea reduces the risk of leaf damage.

Foliar N can be applied in spring or fall. Fall applications need to be done as long as the leaves are still active.

Meyer and coworkers reported applying foliar fertilizers pre-bloom, when the tip of the bloom was pink, 30 days later and 60 days after the pre-bloom application ^[N24]. The risk of leaf damage is greatest before harvest (Niederholzer, personal communication), but may accur at other times as well.

On the long term, the amount of P removed at harvest needs to be replaced with fertilizer when the soil or tissue P values suggest adequate P availability. Between 8 and 9 lbs P/acre (18-20 lbs P₂O₅/acre) are removed from the orchard with kernels, shells and hulls with each 1000 lbs of harvested kernels ^[P5]. Similar values have been reported from studies carried out in Spain ^{[P15, P16]}. For low soil or leaf P values, the application rate should be increased, while for high values it can be reduced (see Soil Test). Contact your local farm advisor for more information.

Broadcast applications of P have been found to be less effective than strip or ring applications at the drip-line of the tree ^[P2].

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 ^[P2].

Phosphorus can be fertigated; however care must be taken to prevent the formation of calcium phosphates which can plug the emitters ^[P6].

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.

For intermediate soil test K values, the amount of K removed at harvest should be replaced with fertilizer. With each 1000 lbs of harvested kernels, 70 to 80 lbs K/acre (85 to 95 lbs K₂O/acre) are removed from the orchard with kernels, shells and hulls ^{[K7, K25, K26]}. However, in a trial carried out in Arbuckle in an orchard with adequate soil test K levels but often low leaf K concentrations, Edstrom and coworkers ^[K15]found that increasing the application rate from 100 to 200 lbs K/acre had no effect on yield, which averaged 2600 lbs/acre. Both treatments however tended to increase the yield compared to the unfertilized control ^[K15].

For low soil test K values, the application rates should be increased, while for high values it can be reduced (see Soil Test). Contact your local farm advisor for more information.

Potassium can be applied as granular, dry fertilizer or dissolved in solution through the irrigation system.

Granular K fertilizer can be surface applied in a band approximately 1.2 m from the tree row, usually in the herbicide strip. The band needs to be in the area wetted by the irrigation system ^[K15]. However, surface applied K may be held in the top few inches of clay-rich soils, making it less available to the roots ^[K9]. Granular K fertilizer can also be drilled 6 to 8 inches deep in one or two bands on opposite sides of the tree row ^[K3].

In drip-irrigated orchards, applying K directly through the drip system is the most convenient approach. In-season applications of potassium chloride should be approached with caution due to chloride toxicity ^{[K13, K15]}. However, fertigation with potassium chloride is a successful practice in many orchards. When applied with the irrigation water, potassium thiosulfate (KTS) should be applied frequently at low rates, as high rates may damage the trees ^[K11].

In a trial carried out on a gravelly loam in Arbuckle, Edstrom and coworkers ^[K15] found no difference between a surface band application of granular fertilzer in fall and an in-season injection of 200 lbs K/acre in the form of potassium sulfate. In K fixing soils, band applied K should be placed in the same location each year to lower the need to overwhelm the soils ability to fix K ^[K20]. Fertigation with drip systems has the same effect. A discussion about K fixation in the San Joaquin Valley can be found here.

A number of K fertilizers are available to growers (see Table). Potassium from mineral fertilizers is readily available and the fertilizers differ little in their effect on yield. Brown and coworkers have not found any difference in yield and leaf K concentrations between trees fertilized with potassium thiosulfate, potassium sulfate, or potassium chloride ^[K6]. This is in line with Edstrom and coworkers ^[K15] who compared in-season injection of potassium sulfate, potassium thiosulfate and mono potassium phosphate. Mono potassium sulfate tended to increase yield compared to the other fertilizers. However, the difference was only significant in one out of four years ^[K15].

Liquid potassium chloride, potassium sulfate and potassium thiosulfate are commonly applied with the irrigation water during the growing season.

Large quantities of granular potassium chloride should be applied in fall so that chloride, which may cause phytotoxicity, can be leached during the winter (see time of application) ^[K10]. Potassium carbonate buffers acidic soils, while potassium thiosulfate acidifies the soil ^[K13].

Commonly used K fertilizers

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

Granular potassium chloride and potassium sulfate are most commonly applied in November after leaf drop begins ^[K10]. When only low amounts of K are needed, K can be applied every 2 to 3 years. In sandy soils with a low cation exchange capacity, K may be leached during the winter. In these soils, K is better applied in multiple smaller applications made throughout the year, matching the N program ^[K13].

To avoid chloride toxicity, granular potassium chloride should be applied when no active leaves are left on the tree ^[K10]. Potassium chloride should not be used on weak trees, young trees, or in orchards with water tables, hardpans, stratified soils or any restriction which would prevent chloride from leaching out of the root zone ^[K10]. Approximately 10 inches of rainfall or winter irrigation is recommended to leach chloride out of the root zone ^[K10].

To micro-irrigated trees, K can be applied during the growing season with the irrigation water. 70% of K is accumulated in the almond fruit by mid June ^[K7]. Therefore, most of the K should be applied between bloom and mid June.

For intermediate soil test K values, the amount of K removed at harvest should be replaced with fertilizer. With each 1000 lbs of harvested kernels, 70 to 80 lbs K/acre (85 to 95 lbs K₂O/acre) are removed from the orchard with kernels, shells and hulls ^{[K7, K25, K26]}. However, in a trial carried out in Arbuckle in an orchard with adequate soil test K levels but often low leaf K concentrations, Edstrom and coworkers ^[K15]found that increasing the application rate from 100 to 200 lbs K/acre had no effect on yield, which averaged 2600 lbs/acre. Both treatments however tended to increase the yield compared to the unfertilized control ^[K15].

For low soil test K values, the application rates should be increased, while for high values it can be reduced (see Soil Test).

Potassium can be applied as granular, dry fertilizer or dissolved in solution through the irrigation system.

Granular K fertilizer can be surface applied in a band approximately 1.2 m from the tree row, usually in the herbicide strip. The band needs to be in the area wetted by the irrigation system ^[K15]. However, surface applied K may be held in the top few inches of clay-rich soils, making it less available to the roots ^[K9]. Granular K fertilizer can also be drilled 6 to 8 inches deep in one or two bands on opposite sides of the tree row ^[K3].

In drip-irrigated orchards, applying K directly through the drip system is the most convenient approach. In-season applications of potassium chloride should be approached with caution due to chloride toxicity ^{[K13, K15]}. However, fertigation with potassium chloride is a successful practice in many orchards. When applied with the irrigation water, potassium thiosulfate (KTS) should be applied frequently at low rates, as high rates may damage the trees ^[K11].

In a trial carried out on a gravelly loam in Arbuckle, Edstrom and coworkers ^[K15] found no difference between a surface band application of granular fertilzer in fall and an in-season injection of 200 lbs K/acre in the form of potassium sulfate. In K fixing soils, band applied K should be placed in the same location each year to lower the need to overwhelm the soils ability to fix K ^[K20]. Fertigation with drip systems has the same effect. A discussion about K fixation in the San Joaquin Valley can be found here.

A number of K fertilizers are available to growers (see Table). Potassium from mineral fertilizers is readily available and the fertilizers differ little in their effect on yield. Brown and coworkers have not found any difference in yield and leaf K concentrations between trees fertilized with potassium thiosulfate, potassium sulfate, or potassium chloride ^[K6]. This is in line with Edstrom and coworkers ^[K15] who compared in-season injection of potassium sulfate, potassium thiosulfate and mono potassium phosphate. Mono potassium sulfate tended to increase yield compared to the other fertilizers. However, the difference was only significant in one out of four years ^[K15].

Liquid potassium chloride, potassium sulfate and potassium thiosulfate are commonly applied with the irrigation water during the growing season.

Large quantities of granular potassium chloride should be applied in fall so that chloride, which may cause phytotoxicity, can be leached during the winter (see time of application) ^[K10]. Potassium carbonate buffers acidic soils, while potassium thiosulfate acidifies the soil ^[K13].

Commonly used K fertilizers

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

Granular potassium chloride and potassium sulfate are most commonly applied in November after leaf drop begins ^[K10]. When only low amounts of K are needed, K can be applied every 2 to 3 years. In sandy soils with a low cation exchange capacity, K may be leached during the winter. In these soils, K is better applied in multiple smaller applications made throughout the year, matching the N program ^[K13].

To avoid chloride toxicity, granular potassium chloride should be applied when no active leaves are left on the tree ^[K10]. Potassium chloride should not be used on weak trees, young trees, or in orchards with water tables, hardpans, stratified soils or any restriction which would prevent chloride from leaching out of the root zone ^[K10]. Approximately 10 inches of rainfall or winter irrigation is recommended to leach chloride out of the root zone ^[K10].

To micro-irrigated trees, K can be applied during the growing season with the irrigation water. 70% of K is accumulated in the almond fruit by mid June ^[K7]. Therefore, most of the K should be applied between bloom and mid June.

In an orchard with low soil B in Fresno County, post-harvest B applications enhanced pollen germination, tube growth and fruit set. The optimal application rate was 0.7-1.5 lbs B/acre applied with 350 gallons/acre of water (0.2-0.4 lbs/100 gallons)^{[B16, B18]}.

It appears that the B concentration in the water applied is also important ^[B6]. With 100 gallons of water applied per acre, a foliar application of 0.2-0.4 lbs B/acre has shown good results, while concentrations above 0.4-0.6 lbs B/acre may actually reduce fruit set and yield ^{[B6, B12, B15]}. Contact your local farm advisor for more information.

The best results have been obtained with 1-2 lb of Solubor^® (20% B) per 100 gallons of water or the same amount of actual B (0.2-0.4 lbs B) with other products ^[B6].

In contrast to other trees, such as pistachio and walnuts, B is mobile in almond trees. For this reason, foliar B applied post-harvest is absorbed by the leaves and translocated to flower buds ^[B2]. Nyomora and coworkers ^[B17] found that September (post-harvest) B applications were more effective in increasing tissue B concentration, fruit set, and yield in almond than applications in December (dormancy), February (budbreak) or August (early hull split). Pre-bloom applications at bud swell may be an alternative ^[B13], but seem to be less effective than post-harvest applications ^[B6]. Spraying flowers with boron at early or full bloom is not effective and can reduce nut set ^[B12].

The optimal time for a fall foliar B application is from well after hull split until the leaves become inactive ^[B6]. Foliar boron applied before hull split is translocated to the hulls and therefore removed from the orchard at harvest ^[B6]. Post harvest foliar applications need to be repeated annually ^{[B5, B12]}.

With a kernel yield of 2500 lbs/acre approximately 0.5 lb B/acre are removed from the orchard ^[B14]. Hull boron concentrations below 80 ppm indicate the need for a ground application ^[B9]. Boron deficiency can be corrected by broadcasting agricultural borax at a rate of 50 to 75 lbs/acre every 3 to 5 years ^[B3]. This application rate corresponds to 5.5 to 8.3 lbs B/acre. In sandy soils with a high leaching potential, lower but more frequent applications of 2-4 lbs B/acre are more efficient ^[B9].

Granular B fertilizers should be broadcast, not banded ^[B6].

Liquid or soluble B fertilizers can be applied through the irrigation system or sprayed on the ground. Applying B together with an herbicide is possible; however fertilizers containing B in the form of polyborate may increase solution pH ^[B19]. Therefore, when applied with glyphosate or other pH sensitive herbicides, the water needs to be buffered before the addition of the herbicide ^[B9].

A number of B fertilizers can be used ^[B4]:

Granular B fertilizer is most commonly applied to the soil in fall. Fall application of soil applied B can increase hull B levels the following year and flower B levels in the second year if appropriate rates are used ^[B15].

Liquid formulations can be applied in spring or fertigated.