Written by Bernardita Sallato, WSU Extension, IAREC Prosser, September 2020
Plants need light, carbon dioxide (CO2), water (H2O), and minerals for their development, growth, and producing quality fruit (Marschner 2002). Most nutrient uptake for tree fruit occurs through the roots, between bloom and the rapid vegetative growth phase. In most perennial tree fruit, however, initial spring growth and early fruit development rely mainly on reserves accumulated the previous season (Weinbaum et al. 1984). Fall nutrient management strategies have had positive effects on tree growth in apples (Nielsen et al. 1996) and cherries (Lang 2005), however, the response will vary depending on the site conditions, tree health, tree demand, and others. 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 demanded in much larger amounts than micronutrients. In most intensive cropping systems, the demand for 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.
While micronutrients, required in small quantities (parts per million), can effectively be managed with foliar sprays, especially when there are some underground limitations; any root growth-limiting factor; anoxia, nematodes, diseases, etc. extreme soil pH; alkaline soils (pH above 7.5) where zinc (Zn), copper (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 takes place. Despite the effectiveness of foliar sprays of micronutrients, 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, to manage vigor and return bloom, or for disease control. So, before making a decision, is important to define your needs and purpose. In the following report, I provide a few examples and things to consider.
Nitrogen is a highly mobile nutrient in the plant and in the soils, required in large quantities in tree fruit production systems (Table 1). In plants, nitrogen is part of many compounds such as chlorophyll, amino acids, proteins, and nucleic acids, and is also part of many metabolic processes.
Table 1. 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 will depend on cultivar, rootstock, tree density, vigor, tree shoot to fruit balance, among others. For example, a ‘Gala’ over Malling 26 (M.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 M.9 and G.41 respectively (Sallato, unpublished). Thus, the importance of determining the demand needs to be addressed at a block level.
During the fall, nitrogen is mobilized to the roots where it remains stored until the following season (Nielsen and Nielsen 2003). 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 the pollination and fertilization process happens 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 the following season’s shoots, 90% of N in spur leaves, and 60% of fruit N.
Thus, nitrogen fall application can be beneficial to build up the reserves for the following crop season. Fall N sprays should be done when growth has ceased to reduce the risk of winter freeze damage when promoting new growth (Righetti et al. 1998), but before natural leaf fall (green leaf), to ensure absorption and remobilization to the roots. But, before spraying, one needs to decide whether the block needs to support those reserves, because when trees have adequate nutrients, foliar sprays are ineffective (Wojcik and Morgas 2013), can lead to toxicity, or create excessive vigor.
Conditions that indicate the need to build up reserves.
Nitrogen deficiency: nutrient deficiency can be diagnosed with leaf tissue analyses, visual symptoms (Figure 1), and shoot growth or vigor assessment (Righetti et al. 1998). Nitrogen-deficient concentration in recently mature leaves are below 1.5% (Nielsen and Nielsen 2003). Nitrogen deficiency has also been associated with inducing biennial bearing. If you are in your “off-year” and you expect higher cropping the following year, fall nitrogen can increase vegetative to fruit relation, and vice versa; when expecting low crop, avoid fall nitrogen spray.
Heavily cropped trees: when trees have been cropped heavily, the demand for N increases and trees will show signs of stress after harvest. Table 1 can be used as a guide to estimate N demand based on yield.
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 range between 1.7 – 2.5% (for more information visit http://treefruit.wsu.edu/orchard-management/soils-nutrition/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.
Calcium is a key element for the tree’s stress response, growth and build of the cell wall, and development of plant tissue. Deficiencies in Ca are known to lead to several disorders that affect fruit quality and storability in many species: bitter pit in apples (Ferguson and Watkins 1989), cork spot in pears, and firmness and cracking in cherries (Shear and Faust 1980).
In most Washington soils, Ca availability is sufficient for tree and fruit demand, with few exceptions: excessively drained soils, presence of high levels of potassium (K) in the soil, and cold soil conditions. The main mechanism for Ca uptake is from the soils by mass flow throughout the root tips. New root growth depends mostly on phenological stage, soil moisture, and temperature. For example, in the Yakima Valley, WA, in well-irrigated orchards, root growth did not occur until soil temperature reached 59 F (15 C) in both apples and cherries, which occurred between 3 to 4 weeks after bloom in both species (Sallato, unpublished).
As per any nutrient demand for early development, it is expected that Ca source for initial growth for bud break, flowers, and fruit cell division, must also rely on available Ca reserves. Opposed to N, Ca movement inside the plant is slow and restricted to the xylem. Movement of Ca from roots to shoots is driven mainly by the transpiration flow. However, during bud break and right after bloom, when leaves are not fully developed, Ca movement has been attributed to a concentration gradient, from high concentration in the roots to low concentration in the young growing shoots (Faust 1989). It has been hypothesized that in the fall, the concentration gradient is reversed; thus a fall application of Ca (CaCl or CaNO3) has a greater chance to enter the plant and translocate to buds and wood, increasing its availability for the next season’s growth. According to Faust (1989), Ca spray during the fall was effective, although he indicated that this only occurred under experimental conditions. Fall application of Ca should be further investigated in orchards with known deficiencies, susceptible cultivars in modern rootstocks where Ca related disorders are present (Figure 2).
Boron is a micronutrient fundamental to meristematic growth (shoots and roots new growth), pollen germination and pollen tube growth, fruit set, xylem and phloem development, and consequently, fruit quality. 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 the mass flow, and similar to Ca, once in the plant, it has very low mobility.
Deficiencies have been reported widely in the PNW region due to extensive areas with high soil pH and excessively drained soils (Nielsen et al. 2004; Peryea et al. 2003). While there are accounts that fall boron applications can increase boron content in the following season, it is only effective when boron is deficient. Although adequate or high levels of boron in the leaf do not always correlate with fruit boron concentration, if boron is low in recently mature leaves during mid-summer, deficiency in the fruit is most likely (for more information visit http://treefruit.wsu.edu/orchard-management/soils-nutrition/fruit-tree-nutrition/).
Peryea et al. (2003) reported that boron maintenance sprays in apples and pears are more effective at the pink flowering stage, indicating that postharvest sprays haven’t been widely adopted in apples due to logistics and efficacy when sprayed in late harvest apples. 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 and also has low mobility in the plant. Deficiency symptoms are very distinctive with smaller leaf, yellowing, rosetting, and short internodes (Figure 3). Among different tree fruit crops, cherries and apples appear to be more susceptible to Zn deficiency (Chandler 1937, cited by Faust 1989). Levels in the leaf below 25 mg/kg in recently mature leaves are indicative of a deficiency.
Deficiency can be easily observed in younger leaves with interveinal chlorosis and shortened internodes, most frequently in cold, wet soils, or in soil with a high pH (above 7.5), where Zn becomes unavailable. These conditions are common in the PNW cherry and apple production. Under these conditions, soil application of Zn is inefficient unless utilizing chelated products or with soil acidification management. Alternatively, foliar sprays with Zn sulfate (ZnSO4) during the fall and throughout the season have shown to be effective in managing Zn deficiencies (Nielsen and Nielsen 1994).
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 2. 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 2. 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) Calcium chloride (CaCl)
3 – 5 lbs. of Ca
Sodium borate (BNa3O3)
1 to 1.6 lbs. of B Boron is not suggested to nonbearing 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.
Other Potential Benefits
Fall sprays of urea have also been utilized to induce leaf drop when sprayed at a higher concentration. For example, urea at 5% has been utilized to reduce inoculum of Venturia inaequalis responsible for apple scab (Qazi et al. 2005). Urea in combination with Zn at 2% concentration has also been utilized to induce early leaf drop after harvest to induce early dormancy, thereby reducing frost injury, pathogen pressure, and homogenizing bloom in low chilling areas. Ouzounis and Lang (2005) indicated that a urea spray for early defoliation improved cold acclimation in cherries.
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 the subsequent year´s critical early growth.
Fall zinc and boron spray can benefit reproduction and early fruit development.
Fall spray should be done when growth has ceased but before natural leaf fall.
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WSU Tree Fruit Extension Specialist