California Crop Fertilization Guidelines

Recent research has resulted in the Pistachio Prediction Model, which uses nutrient analysis of spring leaf samples collected in May, 30-45 days after full bloom, to predict late summer nutrient concentrations of N and K. Sampling done early in the season provides information for in-season fertilizer adjustments.

The following leaf sampling protocol is recommended ^{[N6, N20]}:

• If variable, divide your orchards into uniform blocks and take separate samples for each block.
• Collect leaves from non-fruiting, exposed branches.
• Collect 10 leaves per tree at 6-7 feet height, from around the tree canopy.
• Sample at least 18 trees, at a distance of 25 yards from an orchard block.
• Collected leaves may be pooled in one bag for analysis.
• Leaf samples should be collected between 30-45 days after full bloom.
• The sample needs to be analyzed for N, P, K, Ca, Cu, and Mg.

The concentrations of all analyzed nutrients and the time of sampling (days after bloom) can be entered into the Pistachio Prediction Model, which can be found here.

The predicted July/August N and K concentrations can be interpreted as described for samples taken in July.

The following sampling procedure is recommended to determine the nutrient status of an orchard ^[N4]:

• If variable, divide your orchards into uniform blocks and take separate samples for each block.
• Take samples from late July through August.
• Randomly collect fully expanded sub-terminal leaflets from non-fruiting branches at about six feet from the ground.
• Sample 10-20 trees in each orchard block.
• Collect 4-10 leaves per tree.
• Do not sample leaves that have received in-season foliar nutrient applications for the elements of interest. This is especially important for micronutrients.
• Keep samples in labeled paper bags and submit them to the analytical lab within 24 hours of collection.

Leaf nutrient concentrations change during the season and may differ between leaves on the same tree depending on their location. The values in the table are based on leaves sampled from late July through August following the sampling procedure outlined above. When good trees are compared with poor ones, samples can be taken anytime. Critical and optimal nutrient concentrations in pistachio leaves sampled from late July through August ^{[N4, N5, N24]}.

When leaf nutrient concentrations at or below the critical values, yield is 95% or less than the maximum and deficiency symptoms may be present. When the nutrient concentration is in the suggested range, tree growth and yield should be optimal provided other growth parameters are not limiting ^[N4].

Siddiqui and Brown ^[N21] proposed the following stepwise approach:

• If variable, divide your orchards into uniform blocks. Nitrogen management should be planned for each orchard block separately.
• Establish a preseason N fertilization plan based on expected yield.
• Take into account N contributions from irrigation water and organic fertilizers, such as manure or composts). For example, 27.3 lbs N/acre are applied with one acre foot of water with a nitrate-N concentration of 10 ppm.
• Apply early-season N based on your pre-season plan.
• Take leaf samples in May and conduct in-season yield estimation.
• Adjust the rate of the remaining N applications based on your leaf analyses and yield estimates.

All decisions of fertilization are influenced by local environment and must be adjusted accordingly. N fertilization calculations for different scenarios can be found here.^{href="#N">[N21]}

In observational field trials established in four orchards with 9-15 year old 'Kerman' pistachio trees located in the southern San Joaquin Valley, Brown and Siddiqui ^{[N5, N6]} found that on average 28 lbs of N are removed from the orchard per 1000 lbs of marketable yield (CPC). This value includes all nutrients removed in hulls, shells and kernels, blank nuts and other non-marketable yield per 1000 lbs ^{[N5, N6]}.

Trials in almond orchards have shown that carefully managed fertigation of N can result in efficiencies of at least 70%. This is a feasible goal for pistachio given the high prevalence of micro irrigated and fertigated orchards ^[N5].

As an example, in an orchard with a marketable yield of 4000 lbs/acre, 112 lbs N/acre are removed from the orchard. Assuming 70% efficiency and no input from other sources such as irrigation water or organic fertilizers, about 160 lbs/acre of fertilizer N are needed (see Table). A model can be found here.

In addition, N is needed to support tree growth requirements. On average, N uptake exceeded removal with fruits and abscised leaves by 78 g N per tree and year ^[N17]. In an orchard with 120 trees per acre, this corresponds to about 21 lbs N/acre. However, most N accumulates in perennial tissues during 'off' years, while the N stored in perennial tissues decreases during 'on' years. The N stored during ‘off’ seasons is remobilized the following 'on' season mainly between early leaf out and early hull split ^{[N7, N17, N21, N22]}. Approximate N application rates based on yield and N use efficiency of the irrigation system (based on ^{[N5, N6]}.

In the period from dormancy (January) through early leaf-out, pistachio trees depend almost entirely on N that is remobilized from perennial organs, and essentially no N is taken up from the soil ^[N21]. Nitrogen applications before March are therefore subject to leaching past the root zone ^[N2]. In 'on' years, approximately 30% of N is taken up during spring flush (mid-March to late May) and 70% during nut fill. N uptake was found to be negligible between harvest and leaf senescence ^[N17].

At least 80% of nutrients should be applied during the active tree growth period commencing in early spring (after leaf-out begins) and continuing through early hull split. Based on field trials, Siddiqui and Brown ^[N21] recommend applying 20% of the annual demand after leaf-out, 30% during fruit growth, 30% during kernel fill and 20% during fruit maturity or early post-harvest as long as leaves are still healthy.

A minimum of two fertilization events and, ideally, four or more should be initiated between leaf-out and hull split. Frequent fertigation with smaller amounts of N ensures adequate soil N concentrations for plant uptake while reducing the periods of high N concentration that may be subject to leaching loss in subsequent irrigation or rainfall events ^[N21].

Nitrogen demand and uptake during spring flush are larger in 'off' trees compared to 'on' trees. This suggests that 'off' trees need more N early in the season ^{[N16, N17]}. Half the season’s N may be applied to 'off' trees prior to shell hardening and the remainder in July and August. ^[N2]. Late applications should be avoided. Nitrogen applications in September to 'off' trees can ecourage excessive vegetative growth and delay dormancy, which in turn increases the risk of frost damage ^[N3].

Little research has been done to investigate the effects of fertilizer placement on pistachio trees. However, experience with other fruit and nut crops also applies to pistachios.

In microsprinkler irrigated orchards, fertigation is the most effective way to apply N fertilizer. 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 and does not remain in the irrigation system ^[N14].

Surface-applied dry or liquid urea and ammonium fertilizer must be incorporated as soon as possible to prevent ammonia volatilization losses ^{[N8, N14]}. Fertilizers are best 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 root density compared to the tree row ^[N15].

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. These N forms 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, especially in sandy soils. Urea is relatively mobile in the soil, but is generally quickly converted to ammonium. Ammonium is less mobile, but is generally converted quickly to nitrate by soil microorganisms 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 ^[N23]. Ammonium and urea may be lost through ammonia volatilization, especially when applied to the surface of dry and alkaline soils without being incorporated.

A common N fertilizer for fertigation is UN 32 ^[N2], Other relatively soluble N fertilizers are urea, ammonium sulfate, and potassium nitrate ^[N19]. Anhydrous and aqua ammonia can also be used for fertigation. However, they cause an increase in water pH, which may result in a precipitate if the water contains calcium or magnesium. The high water pH also increases the risk of ammonia volatilization losses When, lime may precipitate when calcium nitrate is used with water with a high bicarbonate concentration ^[N19].

Soil samples for nutrient analysis should be taken from the main root zone. Pistachio trees have an extensive root system with a taproot which allows them to extract water and nutrients from deep soil layers ^[P9]. However, most of the water and nutrients are taken up from the top 2 feet of the profile; especially in orchards with small but frequent irrigation water applications ^[P13]. When the soil samples are also used to assess soil salinity, sampling to a depth of 4 feet may be required ^[P16]. 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 ^[P10]. Using a test that is inappropriate for the soil analyzed may result in inaccurate results.

Interpretation of P and K soil test results in orchards ^[P10].

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 nuts each year (See Soil Applied P).

The following sampling procedure is recommended to determine the nutrient status of an orchard ^[P2]:

• If variable, divide your orchards into uniform blocks and take separate samples for each block.
• Take samples from late July through August.
• Randomly collect fully expanded sub-terminal leaflets from non-fruiting branches at about six feet from the ground.
• Sample 10-20 trees in each orchard block.
• Collect 4-10 leaves per tree.
• Do not sample leaves that have received in-season foliar nutrient applications for the elements of interest. This is especially important for micronutrients.
• Keep samples in labeled paper bags and submit them to the analytical lab within 24 hours of collection.

Leaf nutrient concentrations change during the season and may differ between leaves on the same tree depending on their location. The values in the table are based on leaves sampled from late July through August following the sampling procedure outlined above. When good trees are compared with poor ones, samples can be taken anytime. Critical and optimal nutrient concentrations in pistachio leaves sampled from late July through August ^{[P2, P4, P21]}.

When leaf nutrient concentrations at or below the critical values, yield is 95% or less than the maximum and deficiency symptoms may be present. When the nutrient concentration is in the suggested range, tree growth and yield should be optimal provided other growth parameters are not limiting ^[P2].

On the long term, the amount of P removed at harvest needs to be replaced with fertilizer to maintain adequate P availability. Approximately 3 lbs P (7 lbs P₂O₅) are removed from the orchard per 1000 lbs of marketable yield (CPC). This value includes all nutrients removed in hulls, shells and kernels, blank nuts and other non-marketable yield per 1000 lbs of marketable yield ^{[P4, P5]}.

For low soil or leaf P values, higher application rates may be required, while for high values application rates can be reduced (see Soil P Test). Contact your local farm advisor for more information.

Dormant 21-year old trees contained roughly 270 g P in their permanent structures. Assuming a tree density of 120 trees/acre and a linear increase in P content over time, about 8.3 lbs P₂O₅/acre (3.6 lbs P/acre) are needed for new permanent structures every year. However, the P content has been found to be higher in trees after an 'off' than in trees after an 'on' year. The P accumulated in 'off' years is depleted in support of the large fruit demand during 'on' years ^[P6].

In 'on' years, only 5% of P was taken up during spring flush (mid-March to late May) while 95% was taken up during nut fill. In 'off' years, P uptake during spring flush accounted for 36% of total uptake. P uptake between harvest and leaf senescence was negligible ^[P14]. Thus, P must be available during the nut fill period ^[P15].

As P is immobile in the soil and is barely leached, the time of application is not as crucial as it is for N. Phosphorus is often applied in November after leaf drop begins. In acidic or alkaline soils, however, P can be strongly fixed by soil minerals. Under these conditions, applying P closer to the time of demand may be more effective. 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 ^[P3].

As P is immobilized by soil minerals, it is not 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 ^[P3].

Phosphorus can be fertigated; however care must be taken to prevent the formation of calcium phosphates which can plug the emitters ^[P7].

A number of granular and liquid P fertilizers are available. Fact sheets of the most common fertilizers can be found on the web site of the International Plant Nutrition Institute.

Soil samples for nutrient analysis should be taken from the main root zone. Pistachio trees have an extensive root system with a taproot which allows them to extract water and nutrients from deep soil layers ^[K8]. However, most of the water and nutrients are taken up from the top 2 feet of the profile; especially in orchards with small but frequent irrigation water applications ^[K12]. When the soil samples are also used to assess soil salinity, sampling to a depth of 4 feet may be required ^[K17]. For more information on sampling procedure see Soil Sampling in Orchards.

The ammonium acetate extraction method is the most common method to determine soil K availability (see Table). Interpretation of P and K soil test results in orchards ^[K9].

Pistachio trees growing on soils with extractable K concentrations of less than 150 ppm in the root zone are most likely to respond to K fertilization. Trees on soils with extractable K levels between 150 and 250 ppm are less likely to respond to K fertilization. Combining soil and plant tissue testing helps monitoring trends in K nutrition and guiding management decision ^[K9].

Recent research has resulted in the Pistachio Prediction Model, which uses nutrient analysis of spring leaf samples collected in May to predict late summer nutrient concentrations of N and K. Sampling done early in the season provides information for in-season fertilizer adjustments.

The following leaf sampling protocol is recommended ^{[K7, K16]}:

• If variable, divide your orchards into uniform blocks and take separate samples for each block.
• Collect leaves from non-fruiting, exposed branches.
• Collect 10 leaves per tree at 6-7 feet height, from around the tree canopy.
• Sample at least 18 trees, at a distance of 25 yards from an orchard block.
• Collected leaves may be pooled in one bag for analysis.
• Leaf samples should be collected between 30-45 days after full bloom.
• The sample needs to be analyzed for N, P, K, Ca, Cu, and Mg.

The concentrations of all analyzed nutrients and the time of sampling (days after bloom) can be entered into the Pistachio Prediction Model, which can be found here.

The predicted July/August N and K concentrations can be interpreted as described for samples taken in July.

The following sampling procedure is recommended to determine the nutrient status of an orchard ^[K5]:

• If variable, divide your orchards into uniform blocks and take separate samples for each block.
• Take samples from late July through August.
• Randomly collect fully expanded sub-terminal leaflets from non-fruiting branches at about six feet from the ground.
• Sample 10-20 trees in each orchard block.
• Collect 4-10 leaves per tree.
• Do not sample leaves that have received in-season foliar nutrient applications for the elements of interest. This is especially important for micronutrients.
• Keep samples in labeled paper bags and submit them to the analytical lab within 24 hours of collection.

Leaf nutrient concentrations change during the season and may differ between leaves on the same tree depending on their location. The values in the table are based on leaves sampled from late July through August following the sampling procedure outlined above. When good trees are compared with poor ones, samples can be taken anytime. Critical and optimal nutrient concentrations in pistachio leaves sampled from late July through August ^{[K5, K6, K20]}.

When leaf nutrient concentrations at or below the critical values, yield is 95% or less than the maximum and deficiency symptoms may be present. When the nutrient concentration is in the suggested range, tree growth and yield should be optimal provided other growth parameters are not limiting ^[K5].

In field trials established in four orchards with 9-15 year old 'Kerman' pistachio trees located in the southern San Joaquin Valley, Brown and Siddiqui ^{[K6, K7]} found that on average 24 lbs of K (29 lbs K₂O) are removed from the orchard per 1000 lbs of marketable yield (CPC). This value includes all nutrients removed in hulls, shells and kernels, blank nuts and other non-marketable yield per 1000 lbs marketable yield ^{[K6, K7]}.

In addition, K is needed to support tree growth requirements. On average, K uptake was found to exceed removal with fruits and abscised leaves by 82 g K per tree and year ^[K13]. In an orchard with 120 trees per acre, this corresponds to about 26 lbs K₂O. Estimated K requirements as affected by yield ^{[K6, K7, K13]}.

Higher K application rates may be required for trees grown on soils with a low K availability or on K fixing soils (see K fixation in the San Joaquin Valley). In a study in Madera County with 15-year old 'Kerman' pistachio trees grown on a soil with low K availability, K application rates of 240 lbs K₂O/acre (200 lbs K/acre) resulted in the highest yields (3200 lbs/acre). Higher application rates, however tended to decrease nut yield, possibly due to a decreased uptake of Mg and Ca caused by high soil K availability ^{href="#K">[K21]}. A negative relationship between pistachio yield and high leaf K concentrations in combination with a low plant Mg status was also observed in recent trials (Siddiqui, personal communication).

In contrast, young alluvial soils such as those on the west side of the San Joaquin Valley are very high in exchangeable K, and a response to K fertilization is less likely, unless confounded by salinity or extremely light texture ^[K4].

Potassium demand and uptake are high during nut fill (late May to early September). In 'on' and 'off' years, more than 90% of K is taken up during nut fill. Therefore, a sufficient supply of K to the tree during this period is critical for satisfactory nut filling ^{[K13, K14, K21]}.

Independent of crop load, seasonal K uptake and removal are balanced, indicating that little K is stored in permanent structures to be used in the subsequent year ^[K14].

Beede ^[K2] suggested K applying 40% of the seasonal demand in May, 40% in June, and 20% in July through the drip system.

In a study carried out in three orchards with low K availability over two seasons, nut yield tended to be higher when K was applied via microsprinklers as compared to banding on the soil surface ^{[K18, K19]}.

Band applications are more effective than broadcast applications, especially on soils with a high K fixation capacity. Applying the K in a band saturates the exchange complex of the clay and provides more K in soil solution for uptake ^[K3].

Potassium sulfate (K₂SO₄), potassium chloride (KCl) and potassium nitrate (KNO₃) did not differ in their effects on leaf K concentration, nut yield, and quality of 'Kerman' pistachios grown in Madera County. The use of KCl as a K source for 3 years did not increase leaf Cl concentration ^[K21]. However, orchard health, soil permeability, salinity, stratification and deficit irrigation need to be considered before applying large amounts of KCl ^[K3].