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:

Once trees produce nuts, the optimal N application rate depends highly on the yield level (see Soil Applied N).

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 ^[N26]. 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].

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 ^[N29]. 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]. In young trees, the annual increase in N in perennial parts is higher ^[N9].

The annual N application rates listed in the table below do not include adjustments for cover crops, organic fertilizers, residual soil nitrate, N in irrigation water and high leaf N concentrations. For example, 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.

Nitrogen demand of trees depending on age and expected kernel yield. Expected kernel yield and N demand are in lbs/acre ^[N9].

As an example, the N demand from all sources of a 6-year-old tree with an expected kernel yield of 3,000 lbs/acre is 244 lbs/acre (40 + 3000 x 0.068). More detailed information on best nutrient management practices for almonds can be found here.

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 ^[N26].

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 ^[N27]. 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.

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, N27]}. 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 ^[N35]. 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.

During bloom, the N demand is supported by remobilization of N stored in perennial tissue. Uptake of N from the soil starts after leaf out. By 100% hull growth, about 30% of the seasonal N uptake from the soil has taken place, by the end of kernel fill, 55% of the total N uptake has occurred and by the end of kernel weight accumulation 85% of total N uptake has taken place. Depending on the N status of the tree, N uptake is completed between hull split and three weeks post shake ^[N9].

It is recommended to apply 20% of total annual demand in early-spring (end of bloom through full leaf expansion), 30% during fruit growth (from full leaf expansion through shell hardening), 30% at kernel fill (shell hardening through early hullsplit), and 20% from hullsplit through early postharvest ^[N29].

Frequent applications of small amounts of N can reduce the risk of nitrate losses.

Nitrogen and other nutrients move from the roots to the leaves and fruits with the water taken up. Transpiring leaves create a suction which results in an upward movement of water and nutrients from the roots to the shoots ^[N15]. While almond trees may take up some nitrate pre-bloom during bud-swell, the amount of N taken up between leaf fall and leaf-out is minimal due ot lack of transpiring leaves ^[N32]. Therefore, most of the N applied between late fall and spring will not be taken up by almond trees before leaf-out. This increases the risk of nitrate leaching below the root zone. The risk of leaching losses is especially high in sandy soil and areas with higher winter rainfall.

In order to minimize nitrate leaching losses, the first application should be done after leaf-out ^{[N14, N15]}.

The N demand of almonds is highest between bloom and mid-June ^{[N3, N5, N27]}. During this period it is very important to provide the trees with sufficient N. Most of the N is taken up by the end of June. For the subsequent kernel growth, most N is translocated from the hull and shell to the kernels ^[N31].

Nitrogen applied late in the season after kernels have developed will be directed into the hull and has no effect on yield. In contrast, it may increase the risk of hull rot ^[N12].

When yields exceeded expectations or leaf N levels were low, a post-harvest application may support the development of next year’s blossoms, immature fruits and earliest leaves ^[N32]. The application should be made as soon as possible after harvest to ensure that the N is taken up before leaf fall ^[N32]. However, if a significant leaf loss has occurred already due to pest pressure or drought, the last application should be reduced or skipped ^[N27]. The post-harvest N application should also be reduced or skipped when the July leaf N concentrations are higher than 2.5-2.8% ^{[N9, N16]}.

The effects of foliar applications to almonds have not been studied 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 ranged 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 while 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.