Written by Bernardita Sallato, WSU Extension, IAREC Prosser, Updated August 2023
Carbon (C), oxygen (O) and hydrogen (H), taken up in the form of carbon dioxide (CO2) and water (H2O), are the most important nutrients for tree fruit growth and development, representing more than 95 percent of the tree dry matter (Marschner, 2002). The rest of the nutrients represent the other 5 percent, and their uptake occurs mainly through the roots, between bloom and the rapid vegetative growth phase. In most perennial tree fruit, initial spring growth and early fruit development rely on reserves accumulated the previous season (Weinbaum et al., 1984). Thus, fall nutrient management strategies have had positive effects on building those reserves in apples (Nielsen et al 1996) and cherries (Lang, 2005). However the effectiveness of fall sprays will depend on the overall tree health, deficiency level and demand. Here are some considerations to decide whether fall sprays are needed.
In tree fruit, macronutrients such as nitrogen (N), potassium (K), phosphorous (P), calcium (Ca), magnesium (Mg) and sulfur (S) are required in much larger amounts than micronutrients (Table 1). In most intensive cropping systems, the demand of macronutrients can surpass soil availability (or soil supply), requiring nutrient correction or maintenance. For example, K demand in a ‘Gala’ orchard can be 400 times the demand for boron (B) (calculated from Cheng and Raba, 2009). Under adequate growing conditions; adequate root health and growth, pH between 5.5 to 7.5, well drained, moderate soil temperatures, etc. the most effective way to supply nutrients is via soil, following the physiologically natural pathway of nutrients from roots to fruit.
Table 1. Ranges of nutrient concentration (% d.w) in tree fruit. (Adapted from Silva and Rodriguez, 1995 and Faust, 1989)
Percent (% d.w)
40 – 50
42 – 44
6 – 7
1 – 3
0.1 – 0.3
1.5 – 3.0
1.5 – 2.5
0.35 – 0.6
0.008 – 0.02
Micronutrient uptake can be affected by root growth limiting factors including anoxia, nematodes, diseases, and alkaline soils (pH above 7.5) where zinc (Zn), cupper (Cu), manganese (Mn) iron (Fe) and boron (B) are precipitated and unavailable. Low soil temperatures (which also affect root growth) or when nutrients are needed early in the season before root uptake take place. However, micronutrients, required in small quantities (parts per million), can effectively be managed with foliar sprays, especially when there are some underground limitations (Fernandez et al., 2013). Despite the effectiveness of foliar sprays of micronutrient, keeping adequate levels in the soil will benefit root growth and tree health.
Fall nutrient sprays can be utilized for different purposes; to ensure adequate reserves for the following season (Johnson et al., 2001, Dong et al., 2002), to manage vigor and return bloom, or for disease control (Beresford et al. 2015, Burchill, 1968, Carreño et al., 1982). However, before making a decision, it is important to define your needs and management goals. In the following, I provide a few examples and things to consider when deciding which fall foliar nutrient sprays to apply.
Nitrogen is required in large quantities for tree fruit production systems and is a highly mobile nutrient in the plant and in soils (Table 2). In plants, nitrogen is a component of many compounds such as chlorophyll, amino acids, proteins, and nucleic acids, and is also part of many metabolic processes.
Table 2. Macronutrient demand per ton (US ton) of fruit.
lbs N / ton
1.2 – 2.6
Cheng and Raba 2009; Palmer and Dryden 2006; Silva and Rodriguez 1995.
6.1 – 13.6*
Silva and Rodríguez 1995; Fallahi et al. 1993.
3.3 – 12*
Silva and Rodríguez 1995; Weinbaum et al. 1984.
4.5 – 12*
Silva and Rodríguez 1995.
1.3 – 2.7
Silva and Rodríguez 1995.
*Includes vegetative growth.
The wide range in N demand reflects the wide variability that is dependent on cultivar, rootstock, tree density, vigor, tree shoot to fruit balance, among others. For example, ‘Gala’ on ‘Malling 26’ rootstock demands 2.6 lbs. of N per ton of fruit (calculated from Cheng and Raba, 2009) while preliminary data on ‘Cosmic crisp’ ® apple has shown a demand of 1.2 – 1.4 lbs. of N per ton of fruit over M9 and G41 respectively in a three-year study developed by Sallato (unpublished). Thus, the importance of determining demand needs to be addressed for specific cultivars, regions, and even, individual orchard blocks.
During the fall, nitrogen is mobilized and translocated to the roots where it remains stored until following season (Nielsen and Nielsen, 2003, Cheng et al., 2002, Dong et al., 2001, Dong et al., 2005, Ouzounis and Lang, 2011). In perennial trees, N and carbohydrate reserves are the main source of energy and nutrients for initial growth and early fruit development. This is particularly important in species where pollination and fertilization occur before leaves are fully expanded, such as cherries, apricots, peaches, nectarines, apples, and pears (Nielsen et al 1996, Lang, 2005). In apples, Nielsen et al (2001), showed that remobilized N contributed to 50% of the nitrogen in shoots the following season, 90% of N in spur leaves, and 60% of fruit N.
Since remobilized N is critical for establishing N reserves for developing flowers, leaves, and fruit, fall nitrogen applications can benefit to build up reserves for the following season crop (Sanchez et al., 1990, Johnson et al., 2001, Ouzounis and Lang, 2011, Sallato and Whiting, 2021). When needed, fall N sprays should be done after harvest, between October and November but before natural leaf fall when leaves starts to turn yellow, to ensure absorption and remobilization to the roots, trunk and buds (Sanchez et al., 1990, Johnson et al. 2001, Ouzounis and Lang, 2011). Before spraying, a decision is required to assess whether the block needs additional N reserves, because under adequate nutrient condition, foliar sprays are ineffective (Wojcik and Morgas, 2013, Faust, 1989, Guak et al., 2004, Sallato and Whiting, 2021) and can sometimes contribute to toxicity or excessive vigor.
Conditions that indicate the need to buildup reserves.
Nitrogen deficiency: nutrient deficiency can be diagnosed with leaf tissue analyses, visual symptoms and shoot growth or vigor assessment (Righetti et al. 1998) Figure 1. Leaf tissue concentration in recently mature leaves (mid-summer) below 1.7% in apples (Nielsen and Nielsen, 2003), 1.8% in pears and 2.0% in cherries are indicative of deficiency (Sallato et.al, 2019).
In apples, nitrogen deficiency can lead to biennial bearing. If you are in your “off year” and you expect higher cropping the following year, fall nitrogen can increase the ration between shoot and fruit, and vice versa, in “on years” avoid fall nitrogen sprays to prevent excessive growth the following season.
Highly productive orchards; when trees have been cropped heavily, the demand for N increases and trees can decline in vigor. Table 1 can be used as a guide to estimate N demand based on yield. If the demand surpasses all the inputs: soil O.M, N in the irrigation water, compost u others), fall nutrient sprays can provide up to 20 lbs to the acre.
Conditions that indicate no need for building reserves.
Adequate N levels; nitrogen levels in the leaf during the summer in combination with vigor assessment of the block are good indicators of nitrogen status in the trees. If tissue analyses are within adequate values, and trees have balanced and adequate vigor, additional N sprays would be ineffective (Wojcik and Morgas, 2013). Adequate levels for N in recently mature leaves, mid-summer should be in the rage between 1.7 – 2.5% (for more information visit fruit-tree-nutrition)
Excessive vigor; blocks with excessive vigor have sufficient N for the following spring and additional N could be counter-productive (Figure 1). In blocks with excessive vigor, leaf tissue analysis can be deceiving. In highly vigorous tissue, N concentration can be diluted, thus, assessment of vigor should consider tissue test plus visual assessment. Excessive vigor can affect return bloom in apples, fruit quality, and susceptibility to diseases such as fire blight (Van der Zwet and Keil, 1979 cited by Nielsen and Nielsen, 2003) and powdery mildew in cherries.
Boron (B)Boron is a micronutrient that is fundamentally important for meristematic growth (shoots and roots new growth), pollen germination and pollen tube growth, fruit set, xylem and phloem development, and consequently, fruit quality (Marschner, 2002, Brown and Hu, 1995). Thus, its demand is particularly important for fruit set early in the season (Wojcik and Wojcik, 2006, Cheng and Raba, 2009). Uptake of B by the roots is in the form of boric acid with mass flow (water movement), having low mobility in the xylem, although in Prunus sp., Malus sp. and Pyrus sp. has adequate mobility in the phloem (Brown and Hu, 1995).
Deficiencies have been reported widely in the PNW region due to extensive areas with high soil pH, cold soils, excessively drained soils, and dry soil conditions (Nielsen et al., 2004, Peryea et al., 2003). According to Callan et al., 1978 (cited by Faust, 1989), fall application of B is more effective than spring applications in increasing B levels on flowers and improving fruit set, however effectiveness depends on the orchard deficiency levels. Effectiveness has most consistently been reported in trees with confirmed deficiency prior to the application. Peryea et al. (2003) reported that boron maintenance sprays in apples and pears are more effective at pink flowering stage, and that postharvest sprays in the PNW have not been widely adopted in apples due logistics and reduced efficacy in late ripening apple cultivars. More recently, Karlidag et al (2017) demonstrated that the application of B (1000 ppm) plus 3% Urea during the fall were effective in augmenting N and B in buds, improving flower development and fruit set.
Most soils in WA tree fruit growing area are low in B and most growers manage the deficiency with foliar sprays. Thus, to monitor B deficiency in the trees, leaf tissue samples are better indicators than soils. Samples should be collected during mid-summer in leaves that have reached maturity, with adequate levels ranging between 20 to 80 mg/Kg (ppm). Although adequate or high levels of boron in leaf not always correlated with fruit boron concentrations, if boron is low in leaves deficiency in the fruit is most likely. Precautions should be taken with boron sprays because there is a small margin between deficiency and toxicity. Faust (1989) indicated that one spray per season should be sufficient to prevent deficiencies in apples.
Zinc is a micronutrient involved in many enzymatic processes. Deficiency symptoms are very distinctive with smaller leaves, yellowing, rosetting, and short internodes (Figure 2) (Silva and Rodriguez, 1995). Among different tree fruit crops, cherries and apples appear to be highly susceptible to Zn deficiency (Faust, 1989, Silva and Rodriguez, 1995). Levels in recently mature leaves below 25 mg/kg in are indicative of a deficiency. However a clear indicator of Zn deficiencies are visual symptoms of chlorosis, rosetting and blind wood (Figure 2).
In the PNW, deficiencies are most frequently in cold, wet soils, in soil with a high pH (above 7.5) where Zn becomes unavailable, sandy soils or poor root growth. Under these conditions, the deficiency relates more to a lack of uptake rather than a lack of supply, therefore, soil application of Zn is inefficient. Foliar sprays with Zn sulfate (ZnSO4) during the fall and thought the season have shown to be effective in managing Zn deficiencies in apples (Nielsen and Nielsen, 1994), peaches (Sanches et al., 2006) and sweet cherries (Sallato and Whiting, 2021) and tart cherries (Wojcik and Morgas, 2015). However, Zn has low re-translocation and high binding capacity in the plant (Zhang and Brown, 1999, Marschner, 2002), and as noted by Wojcik and Morgas (2015) in sour cherry and Sallato and Whiting (2021) in sweet cherries, fall application of Zn only improved bud and flower levels and didn’t affect overall Zn levels in leaves, suggesting the need for frequent applications when deficiency persist.
For leaf nutrient sprays, mixing micronutrients with urea have shown improved uptake (Fernandez et al 2013, Sanchez and Righetti, 2005). There are several formulations for each nutrient. The most common formulations are listed in Table 3. Whatever the source, always check the label recommendation. To calculate the amount of product based on the actual amount needed, divide the actual amount recommended by the percentage of the element indicated in the label.
Example: Urea (46% of N) = If you need to apply 8 lbs/acre, then 8 ÷ 46% = 17 lbs. of Urea.
Table 3. General fall recommendations for tree fruit under diagnosed deficiency.
Formulation or salt
Dose (lbs of actual element in 100 gallons of water per acre)
Urea (CO (NH2)2)
8 – 10 lbs. of N. When using Urea make sure it has less than 0.25% biuret.
Calcium nitrate (CaNO3)
3 – 5 lbs. of Ca
Sodium borate (Solubor) (Na2B8O13 * 4H2O)
1 to 1.6 lbs. of B Boron is not suggested to non-bearing trees.
Zinc sulfate (ZnSO4)
8 – 10 lbs. of Zn for apples and cherries. 3 lbs. of Zn for peach and nectarine. 0.5 lbs for non-bearing trees.
Note: For nutrients containing sulfate (example: Zn sulfate), wait until temperatures are below 80 º Fahrenheit.
The effectiveness of foliar sprays will depend on the environmental conditions at the time of application (Guak et al., 2004, Fernandez et al., 2013). Most important considerations are:
Don’t apply with temperatures below 68 F or above 85 F.
High wind reduces drying time of the droplet and reduce absorption.
Relative humidity. Low humidity influence droplet size and persistence in the leaf surface.
Other Potential Benefits
Fall sprays of urea have also been utilized to induce leaf drop when sprayed in higher concentrations. For example, urea at 5% has been utilized to reduce inoculum of Venturia inaequalis responsible for apple scab (Qazi et al., 2005, Beresford et al. 2015, Burchill, 1968, Carreño et al., 1982). Inducing early leaf drop during dry and warmer conditions can potentially reduce the risk of Bacterial Canker infection (Sallato et al, 2021). Urea in combination with ZnSO4 at 2% concentration has successfully been utilized to induce early leaf drop after harvest, augmenting cold hardiness (Fernandez et al., 2002, Sallato and Whiting, 2021, Ouzounis and Lang, 2005).
Fall sprays are beneficial and effective only when the trees are deficient for that particular nutrient.
Fall nitrogen applications can help in building up reserves for critical early growth the next season.
Fall zinc and boron spray can improve fruit set and early fruit development, if deficient.
Fall spray applications should be applied when growth has ceased but before natural leaf fall.
WSU Tree Fruit Extension Specialist
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