Fertilization of Pistachio Trees

Pistachio trees consume nutrients from the soil, especially when they produce high yields. Therefore, do not hesitate to apply the appropriate fertilizers in quantities suitable for the production that you aspire to reach and advertise. Moreover, poor fertilization results in a lack of production, small size of nuts, and alternate fruit bearing. Trees may stop bearing permanently in poor soils and when nutrients are depleted from the soil after long periods of non-fertilization.

Basics

Try to avoid these factors that affect nutrients uptake:

• Drought or flooded soils
• Weed competition
• Poor root growth
• Soil nature: sandy, calcareous, alkaline
• Temperature
• Root disease

Nutrients are used most efficiently when you:

Apply the Right Rate:

• Match tree demand with fertilizer supply.

At the Right Time:

• Apply nutrients when root uptake is most active.

In the Right Place:

• Ensure delivery of nutrients to the active roots and not past the root zone.

Using the Right Source:

• Choose fertilizers sources that maximize uptake and minimize loss. In short: do not buy cheap products.

Organic Fertilizer

Fermented organic fertilizer is essential, especially for poor, clay soil. It is made from animal fermented manures and litters. Do not add unfermented manures.

Organic fertilizer is applied as long as the percentage of organic matter in the soil is less than 2%. The recommended amount to be applied is about 30 tons/ha or the equivalent of 20 kg per tree at least once every two years.

The amount of chemical fertilizer decreases as the amount of organic fertilizer used increases.

The field is tilled in spring under traditional orchard conditions. It is better for the fertilizer applied in winter to be mixed with the soil. Tillage and organic fertilizer help retain moisture in the soil. In organic and no-till farming, it is possible to till and rotate legume crops after they flower, or mow the grass and leave it under the trees as a thick insulating layer. Keeping remnants of straw, grass, and organic matter under trees helps preserve soil moisture and take greater advantage of the rain, provided that you don’t let harmful weeds grow to compete with the trees and that you keep them under control.

Modern and Young Orchard (up to 10 years old)

Apply 20g of nitrogen fertilizer per tree in March of each year (nitrogen deficiency makes the leaves small and pale green). Apply other nutrients based on soil analysis conducted over a period of 3-4 years.

Mineral Fertilizers

We must fertilize well during an “off” year in order to prepare for the bearing season.

The amount of fertilizer depends on the soil analysis and expected yield.

The initial approximate quantity of mineral fertilizers that a pistachio orchard needs each year:

Nitrogen

Deficiency symptoms

• Leaf yellowing and paling
• Smaller leaves
• Shrinkage of nuts and kernels

Unfortunately, correcting nitrogen deficiency becomes very difficult after these symptoms have appeared. Therefore, make sure to plan applying nitrogen before the symptoms appear. You can correct the deficiency when you notice the first symptoms (small leaves in early spring).

Soil requirement of N for two “on” and “off” years (kg/ha)

Nitrogen fertilizer should be applied in March each year under the tree (under the branches). Ammonium sulfate can be used as fertilizer for alkaline soils.

Nitrogen is associated with robust vegetative growth and nut fill and size. It doesn’t affect the number of flowers or fruit setting rate.

The first amount of N is applied in early spring (late March until the end of May) with the beginning of the flush period, while the second amount in June and July in order to meet the nut growth requirements and kernel fill.

Nitrogen fertilizer requirements for young pistachio trees depend on soil fertility. Application rates are best adjusted according to the results of soil or foliar analyses.

Recommended N Application Rates for Young Pistachio Trees

Fertilizer to young trees is best applied in mid-spring and early summer. Late applications after August can encourage excessive vegetative growth and delay dormancy, which in turn increases the risk of frost damage.

Little is known about the effects of different fertilizer types on young pistachio trees. For pistachios, it is recommended to use granular fertilizers for first-leaf trees because the risk of root burn increases with liquid fertilizers, as their application may result in high concentrations in the root zone. For second-leaf trees, liquid fertilizers may be applied. It is recommended to place granular fertilizers at least 60 cm away from the trunk, but within the wetting zone of the irrigation system.

N Fertilization of Pistachio Trees

Observational field trials have shown that an average of 28 kg of N is removed from the orchard per 1,000 kg of marketable yield. This value includes all nutrients removed in hulls, shells, kernels, broken nuts, blank nuts, and other non-marketable yield per 1,000 kg.

Trials have shown that carefully managed nitrogen fertilization can result in efficiencies of at least 70%. This is a feasible goal for pistachio given the high prevalence of irrigated and fertigated orchards.

As an example, in an orchard with a yield of 7200 kg/ha, 125 kg N/ha are removed from the orchard. Assuming 70% efficiency and no input from other sources such as irrigation water or organic fertilizers, about 180 kg/ha of N fertilizer are needed.

Additionally, N is needed to support tree growth requirements. On average, N uptake exceeds removal with fruits and abscised leaves by 78 g N per tree and year. In an orchard with 300 trees per hectare, this corresponds to about 23 kg N/ha. 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.

Approximate N application rates are calculated based on the expected yield, N use efficiency in the soil, and the irrigation system. Therefore, observe your orchard after fruit setting and nuts appearance. Estimate the expected yield and calculate the amount of N to apply to your trees based on the bearing amount and the amount of N you applied during the dormancy phase.

In the period from dormancy (January) through early leaf-out, the tree depends almost entirely upon N that is remobilized from perennial organs, and essentially no N uptake occurs from the soil. Nitrogen applications before March are therefore subject to leaching past the root zone. 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.

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, it is recommended to apply 20% of the annual demand after leaf-out, 30% during fruit growth, 30% during nut fill, and 20% during fruit maturity or early post-harvest as long as leaves are still healthy.

Frequent fertilization with smaller amounts of N and at intervals 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, especially in spring.

Table showing the distribution of the pistachio tree requirements for N, along with the optimal addition dates and percentages

You can spray N or add it to the irrigation water postharvest only if the trees are healthy and the N application rate was very low during the season.

Remember that late application of N in September may increase vegetative growth, especially in “off” years, delay branching, and increase frost danger.

Phosphorus

Deficiency symptoms

• Poor root growth, which means poor growth of the tree as a whole
• Weak flowering and fruit setting, and cluster degradation
• Leaf yellowing and dropping, especially those near the fruit cluster
• Poor growth in general

Fertilizing Young Trees

Phosphorus deficiency has rarely been observed in deciduous orchards. A soil test conducted before the trees are planted will indicate whether P applications may be beneficial (see Soil P Test and Leaf P Analysis).

In compound fertilizers, the amount of fertilizer applied depends more on the need for N than for P.

Since P is an immobile component in the soil, it should be placed closest to the roots. Research conducted on almond, pistachio, and walnut trees have shown that applying P fertilizer in 20-cm deep trenches is more effective than broadcast application. The trenches can be 60 or more centimeters away from the trunk, depending on tree size. They also need to be within the wetting zone of the irrigation system.

Soil Application Rate

On the long term, the amount of P removed at harvest needs to be replaced with fertilizer to maintain adequate P availability. Approximately 3 kg of P (7 kg P2O5) are removed from the orchard per 1,000 kg of marketable yield. This value includes all nutrients removed in hulls, shells, kernels, blank nuts, and other non-marketable yield per 1,000 kg of marketable yield.

For low soil or leaf P values, higher application rates may be required, while high values application rates can be reduced (see soil P test).

Dormant 20-year-old trees contain roughly 270g of P in their permanent structures. Assuming a tree density of 300 trees/ha and a linear increase in P content over time, about 8.5 – 9 kg P2O5/ha (4 kg P/ha) are needed for new permanent structures every year. However, P content has been found to be higher in trees after an “off” year 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.

Time of Application

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. Thus, P must be available during the nut fill period.

As P is immobile in the soil and barely leached, the time of application is not as critical as it is for N. Phosphorus is often applied in November after leaf drop begins. In acidic or alkaline soils, P can be strongly fixed by soil minerals. Under these conditions, applying P closer to the time of demand may be more effective. Therefore, it is preferable to use soluble P fertilizer at the beginning of March every 2-3 years and to apply it in at least 30-40-cm trench at the drip line under the tree.

Potassium

Deficiency symptoms

• Symptoms appear after midsummer
• Smaller old leaves with scorched margins
• Sparse foliage with pronounced dieback
• Yields will decline (in quantity and quality) as potassium levels decline
• If you notice these symptoms, this means that a quantity of the crop has been lost, the yields have been damaged, and there is no going back.

Potassium is important for enhancing plant growth and nutrients uptake from the soil, and it is associated with the movement of nutrients in plant tissues.

In field trials, it was found that on average 24 kg of K (29 kg of K2O) are removed from the orchard per 1,000 kg of marketable yield. This value includes all nutrients removed in hulls, shells, kernels, blank nuts, and other non-marketable yield. Potassium is very important for the growth of strong and healthy leaves, as they consume 80 g of K per tree and year. Additionally, 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 of K per tree and year. In an orchard with 300 trees/ha, this corresponds to about 29 kg of K2O. Thus, the orchard needs, in total, 55 kg of K to produce 1,000 kg of good quality pistachio nuts and good trees. Remember that the roots only uptake a proportion of the surface application.

Estimated K requirements as affected by yield

Higher K application rates may be required for trees grown on soils with a low K availability or on K fixing soils. K application rates of 270 kg K2O/ha (224 kg K/ha) resulted in the highest yields (3500 kg/ha). A rate of 227 kg K/ha is recommended in “on” year and 113 kg in “off” year. Higher application rates, however, tend to decrease nut yield, possibly due to a decreased uptake of magnesium and calcium caused by high soil K availability. In contrast, young alluvial soils are very high in exchangeable K, and a response to K fertilization is less likely, unless confounded by salinity or extremely light texture. The need for K will be less for heavy, clay soil.

Time of Application

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.

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. It is suggested that K applying rates be 40% of the seasonal demand in May, 40% in June, and 20% in July through the drip system.

Mode of Application

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 sprinklers compared to banding on the soil surface.

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

Notes

• Remember that the figures listed in this article are approximate, and it is better to analyze the soil and fertilize according to the analysis results.
• The applied quantities change according to the amount of the expected yield per unit area and according to the size and strength of the trees.

By Wael AM.

by W. Al Metni