Mobility within tree: High (symptoms in older leaves first)

Uptake Form: NO₃^- (nitrate), NH₄⁺ (ammonium)

Function: Amino acid/protein, protoplasts, enzymes

Deficiency/Toxicity Symptoms:  Nitrogen (N) is a primary mineral needed in tree fruit nutrition.  It is essential for tree growth, fruit sizing and fruit set.  Nitrogen deficiency is seen by reduced top growth with short spindly shoots and pale yellowish green leaves.  Since N is mobile in plants, N can easily move from leaves to where it is needed.  Leaves therefore often exhibit symptoms of deficiency.  Fruit are often small and mature early.  Peaches can be astringent and stringy.  N deficiency can cause biennial bearing and increased susceptibility to winter injury as well.

Excess nitrogen can lead to poor fruit colour, storage rot, storage breakdown, bitter pit, softening, and increased crown and root rot of apples.  Over fertilization with N can lead to excess tree growth.  This excess growth can increase pruning costs; mildew; populations of aphids, leafhoppers and pear psylla; alfalfa greening; Anjou pit of pears; and susceptibility to fire blight.

When trees do not respond to increasing amounts of N, the cause might be a deficiency of other nutrients, most often Zn.  Nitrogen application can be wasted with excessive irrigation that drives nitrate (NO₃^-), the main available source of N for tree roots, down past the rooting zone.  Excess N applications combined with over irrigation can increase the risk of environmental pollution due to a greater contribution to greenhouse gas emissions (i.e. nitrous oxide) and leaching of NO₃^- into ground or surface water.

Nitrogen Sources

All N fertilizers must be converted to nitrate in the soil for the uptake of N by a tree.  However, some ammonium can be taken up directly.   Ammonium will readily convert to nitrate under optimum pH and aerobic conditions. Thus, the choice of fertilizer will determine how long it takes until the application is available to a tree.  The most commonly recommended ground nitrogen fertilizers are urea (46-0-0) and ammonium sulphate (21-0-0) that can take up to 3 weeks to convert, especially in cool springs.  Ammonium sulphate is more acidifying than other fertilizers.

Ammonium nitrate (34-0-0) is the old standard, but is only available through limited controlled locations, requires ID and is an expensive form of N.  Urea ammonium sulphate (34.5-0-0) is now a common replacement, but acts differently than ammonium nitrate (34-0-0) due to the absence of nitrate.  Calcium nitrate (15.5-0-0) though expensive has quick uptake, may be used with no acidifying effect and can be used with drip emitters on sandy soils.  It is also good for use when manganese toxicity is a problem. Calcium ammonium nitrate (CAN) (27-0-0) is half calcium nitrate (quick uptake) and half ammonium (can take 3 weeks to convert) with no acidifying effect.  Nitrate and urea forms of N are more easily leached out of the soil than ammonium forms.  Table 10 summarizes the properties of common N fertilizers used.

Due to the various options, it is helpful to record applications as “actual N” applied by acre or hectare so comparable rates are used.  For example, 100 kg of 21-0-0 would be 100 kg x 21% = 21 kg actual N; and 100 kg of 46-0-0 would be 100 kg x 46% = 46 kg actual N.  Therefore, for the same quantity of each fertilizer applied, ammonium sulfate would only provide about half the amount of N as urea.

Nitrogen Application

Nitrogen is best applied as a soil application very early in the spring. The N is then available very early to help the tree during critical growth and fruit development stages.  Nitrogen applications to the soil should not exceed the end of May in all fruit bearing blocks.  It is especially important in apples so that soil N levels are low by the time the fruit is colouring in late summer.  In young nurseries and new plantings, soil applied N should not be applied after mid-July (clay) or mid-August (sand).

Fertilizer should be spread evenly within the clear “herbicide strip” around the tree trunks to target the main root zone.  It may be spread evenly over the orchard floor in orchards with closer-spaced smaller trees, however, fertilizer rates should be adjusted to match the total area of application.  Do not concentrate acidifying fertilizers such as ammonium sulfate in piles or narrow bands as this will cause “hot spots” of low pH.  Spread fertilizer a day or two after a full irrigation set, then irrigate for only 2 hours following application to help move the fertilizer just into the root zone of the trees.  It should be noted that most N fertilizers (especially nitrate forms) are highly soluble and are washed from the root zone with frequent and over irrigation.  Urea fertilizer left sitting on the surface of the ground in warm weather can lose N (in form of ammonia) to the air through volatilization.

Determining how much N to use is not always easy.   The N needs of an orchard will be higher with young trees filling space, large crop years and stone fruits.  There can be varietal differences in N requirements too.  For example, data from a N fertilizer trail in Honeycrisp apple with application rates of 30 to 120 pounds per acers in Washington State showed that yields and fruit size were improved with rates of up to 30 pounds, but no additional benefits was observed for rate over 30 pounds per acer. The fruit requires low levels of N, so N should not be applied as long as growth is adequate.  Pruning and crop load management can be used to encourage adequate growth instead of applying N as fruit size and quality is easily negatively affected.   Any suggested rates in this guide are just starting points.  They may need to be modified through observations and data collected from a particular orchard.  Possible application rates are discussed next for different maturity of trees.

Table 1.  Common available N fertilizers in BC 

Source	N (%)	Ca (%)	S (%)	Notes
Ammonium Sulfate 21-0-0	21.0	-	24.0	-
Ammonium Urea Sulfate 34.5-0-0	33.0–34.5	-	11.0	-
Calcium Nitrate 15.5-0-0a	15.0	34.0	-	Available in soil and foliar spray form
Urea 46-0-0*	45.0–46.0	-	-	-
Calcium Ammonium Nitrate 27-0-0	27.0	4.0	-	-
Liquid Blend 32-0-0	32.0	-	-	-
Liquid Blend 12-0-0-26S	12.0	-	26.0	-

Due to the various options, it is helpful to record applications as “actual N” applied by acre or hectare so comparable rates are used.  For example, 100 kg of 21-0-0 would be 100 kg x 21% = 21 kg actual N; and 100 kg of 46-0-0 would be 100 kg x 46% = 46 kg actual N.  Therefore, for the same quantity of each fertilizer applied, ammonium sulfate would only provide about half the amount of N as urea.

Nitrogen Application in Non-Cropping Trees

Young, non-cropping trees need to be fertilized more frequently than fruit-bearing trees, using smaller amounts of nutrient at each application (Carranca et al. 2018). The supply must also be targeted to the limited soil volume, where roots are present. Young non-bearing trees are highly dependent on new inputs of N, although the recovery of fertilizer N in the tree is relatively small during the first three years after transplant (Carranca et al. 2018). Tables 10 and 11 give rates to use for first-year non-cropping plantings for broadcast and fertigation applications, respectively.  The first application is recommended soon after planting. The fertilizer rate can be calculated to determine how much of a particular source of fertilizer is needed as:

Fertilizer rate = (Actual N rate) x 100 / %N in fertilizer.

For example, for urea (46-0-0) to be used on a sandy soil (Table 11), it will have a fertilizer rate of 120 kg N/ha x 100 / 46  = 261 kg urea/ha.

For fertigation (Table 2), applications are recommended to begin at post-bloom when P application is finished. These fertilizer rates compensate for the inefficiency of N uptake from sandy soils where irrigation can leach N below the root zone. It will be possible to reduce the amount of N applied by as much as 50% by avoiding excessive leaching loss of N, for example, by atmometer scheduled irrigation. Recent research has indicated in an orchard with a sandy loam soil and irrigation controlled by atmometer based scheduling that production and quality of apples could be sustained by applying a total of 12-15 g N per tree during any 4 week period listed below.

Table 2. Possible broadcast N application rates and frequencies for first-year non-cropping plantings in different soil types

Rate	Sandy Soils	Loamy Soils	Heavy Soils
Actual N/ha	120 kg	90 kg	60 kg
Actual N/10 m²	120 g	75 g	60 g
Number of Applications	Split into 3 applications	Split into 2 applications	Single application

 

Table 3. Possible fertigation N application rates and timings for first-year non-cropping plantings 

Growth Period (Post-Bloom)	Sandy Soil (g/tree)	Loamy Soil (g/tree)	Heavy Soil (g/tree)
1–4 weeks	20	15	10
5–8 weeks	30	22	15
8–12 weeks	10	5	5
Total	60	42	30

 

For second-year trees transitioning to fruiting trees, application rates will depend on observations.  If high density planted trees have failed to adequately fruit after the first year, then continue with the first year schedule.  If vigour has been low, investigate the possibility of another nutrient deficiency by leaf analysis or inadequate first year irrigation. If vegetative vigour is excessive, reduce N application rates by half.

While low N can be a problem for most species of tree fruit, excessive N can be equally bad. Apple N programs vary according to many factors, so the grower must monitor tree growth, leaf nutrient levels, yield, and fruit quality in order to make annual adjustments to the N program (Spectrum Analytic Inc. 2006). A relationship between tree N requirements and the trunk cross-sectional area is reported in the young non-cropping trees. Therefore growth-based models can be used to predict N needs of deciduous trees and should be included in decision support systems for fertilizer N recommendations (Carranca et al. 2018).

Nitrogen Application in Mature Trees

For mature bearing trees, the broadcast application may need to be adjusted. Mature producing trees are considered to be those capable due to size of carrying 30 to 50 bins of fruit per acre. Reduce the suggested nutrient schedule if mature trees that are capable of carrying a full crop, have experienced a frost or event that results in a significantly lighter crop.  Smaller trees that must still grow to achieve a structure that fills the tree spacing and are older than 3 to 4 years may be carrying too heavy a crop for the tree size.  These weaker trees should receive the start of their nutrient schedule after bloom and should not have rates cut with the removal of crop load.

Factors such as fruit size, block colouring, crop load, susceptibility to bitter pit and maturity dates may influence the amount of N needed in a given block.  Growers should plan to make adjustments to their N rates based upon these and other influencing factors.  Higher crop loads have higher nutrient requirements, thus more total N is required to produce a 45 bin/acre crop compared to a 20 bin/acre crop.  Some of the rich silt soils may require very little, if any, N for mature producing trees for a number of years in a row.  Vegetative growth, leaf N analysis, fruit size and fruit colour development must be used to adjust rates of application.

Tables 4 and 5 give broadcast and fertigation N application rates for mature trees, respectively.  These N recommendations are guidelines only.  There is no substitute for careful observation.  In late dormant period, usually late March to early April, N applications can have a broad range of 30-140 kg actual N/ha.  As mentioned above, the rate will depend upon tree growth, commodity, and fruit quality.  The lower rates are for apples growing in good soil compared to the higher rates for peaches growing in sandy soil.

Table 4. Actual N application rates per tree calculated for various tree spacings and actual N application rates per hectare in established trees

Nitrogen Application Rates per Tree Based on Spacing (kg/acre)

Spacing (ft)	50 kg/acre	100 kg/acre	150 kg/acre	200 kg/acre	250 kg/acre	300 kg/acre
4.5 x 12	25 g/tree	50 g/tree	75 g/tree	100 g/tree	125 g/tree	150 g/tree
4 x 11	20 g/tree	40 g/tree	60 g/tree	80 g/tree	100 g/tree	120 g/tree
3 x 11	15 g/tree	30 g/tree	45 g/tree	60 g/tree	75 g/tree	90 g/tree
3 x 10.5	15 g/tree	30 g/tree	40 g/tree	55 g/tree	70 g/tree	85 g/tree
2.5 x 10.5	15 g/tree	25 g/tree	35 g/tree	50 g/tree	65 g/tree	75 g/tree
2.5 x 10	15 g/tree	25 g/tree	35 g/tree	50 g/tree	60 g/tree	70 g/tree
2 x 10	10 g/tree	15 g/tree	25 g/tree	35 g/tree	45 g/tree	55 g/tree
1.5 x 9	10 g/tree	15 g/tree	25 g/tree	30 g/tree	35 g/tree	40 g/tree

Table 5. Nitrogen fertigation schedule for established apple trees

Growth Period	Fertigation Guidelines
1–4 weeks before and during bloom	No nitrogen (N) applied by fertigation (assuming good N nutrition status in previous years).
Post-bloom (1st week of June)	Begin fertigating for crops of 40 bins/acre or heavier.
Mid-June	Start feeding crops of 20, 25, and 30 bins/acre.
Beginning of July	Start feeding crops of 10 and 15 bins/acre. Do not feed N to crops on mature trees with less than 10 bins/acre.
End of July	Finish feeding N for all trees.
Late N application (if needed)	For 40 bins/acre and larger crops, apply late N just before or after harvest, providing approximately 15–20% of the total N.

 

Nitrogen Foliar Application 

Table 6 gives times and rates to use for foliar application of urea.  To avoid delayed maturity of fruit, do not apply urea later than 45 days before harvest on apples or pears, and later than 21 days on stone fruit.  Use of foliar urea on pears may aggravate fire blight.  A fall spray (September) of 20 – 40 lbs /acre of urea on sweet cherries has been found to be very effective at helping with first leaf sizing the following season when N in the tree is low.

Ammonium thiosulphate (ATS) is a liquid fertilizer containing 12% N and 26% sulphur. A dilute spray, at rates ranging from 1.2 L per 100 L to 1.6 L per 100 L, applied to the point of run-off when 80% of the blossoms are open has resulted in larger fruit size and increased return bloom of several apple cultivars. Caution is necessary, however, as foliage injury and excessive fruit removal can occur when this material is applied.  Some growers concentrate spray more at the tops of trees where the nutrient is needed more.  It should be applied under good drying conditions. Cultivars differ in their sensitivity to ATS.  Application of ATS is not suggested for varieties such as Braeburn and Sunrise that have low fruit set tendencies as excessive fruit removal may occur.  Jonagold, Gala and Empire are sensitive to rates above 1.2 L per l00 L.  Spartan, Fuji, Delicious, Golden Delicious and McIntosh have benefited from rates up to 1.6 L per 100 L.

Table 6. Timing and rates of foliar urea (46-0-0) spray for fruit trees

Type of tree	Time of application	Rate of application (Airblast sprayer – Dilute)
		Per ha	Per acre	Per 100 L
Young non-bearing trees with poor growth - 2 sprays 2 weeks apart.	June - July	11 kg	4.4 kg	1.0 kg
Bearing trees - more than one spray may be required.	Petal-fall - June	13 kg	5.3 kg	0.5 kg

Nitrogen in Fruit and Vegetative Growth Balance 

A good balance between fruit and vegetative growth is desired in mature trees.  When a good balance is achieved, less N per kg of fruit is used.  Good consistently cropped trees have proportionately less vegetative growth, which leads to less demand for Ca and a lower leaf to fruit ratio.  Higher leaf to fruit ratio leads to more vegetative growth, a light crop particularly under low humidity and a higher leaf demand for Ca away from the fruit.  Large fruit size with a small crop will also contribute to calcium dilution in the fruit.

To determine if N application is good for mature tree fruit and vegetative growth balance, the current season’s terminal growth is measured.  Too much or too little growth, as well as leaf analysis data provides the information needed to help adjust the required N rates for the following season.  Measure the length of leading terminals growing outwards at an angle of 45 degrees around the outside of the tree and take an average of these measurements.  Increase, reduce or eliminate the quantity of N fertilizer applied to achieve the correct amount of terminal growth as shown in Table 16.  In cherries, studies from Michigan have suggested that a ratio of 5 leaves: 1 cherry is ideal for sizing quality of fruit as well.