Essential Nutrients

Nitrogen (N)

Form used by plants:

NO₃^- or NH₄⁺

Important functions:

• Primary building block for all plant parts – leaves, shoots, roots, fruit buds, etc.

• Amino acid synthesis and protein formation.

• Important in chlorophyll, nucleic acids, and enzymes

Ideal foliage range for apple leaves:

Age of tree	Leaf N (%)
Non-bearing	2.4-2.6
Young bearing	2.0-2.4
Mature	1.75-2.75

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Ideal fruit concentration:

Ideal soil range:

The nitrogen-supplying power of the soil is largely dependent upon soil texture and organic matter.  Mineralization rates will increase with an increase in organic matter content (also C:N ratios, temperature and moisture driven). This will supply the trees with some of the nitrogen they require. 

Mobility: when in the nitrate (NO₃-) form, it is very mobile and can easily be leached from soils.  The ammonium (NH₄+) form is not very mobile except in soils with very low CEC (cation-exchange capacity) and low organic matter.

Influence on soil pH: uptake of nitrate or ammonium creates increased and decreased root medium acidity:

3NO₃^- ----> 3 NH₂ + 2OH^- (increases soil pH)

3NH₄⁺ ---> 3R-NH₂ + 4H⁺ (decreases soil pH)

Best indicators:

Tree growth and performance are the best indicators for nitrogen management in orchards. To date, soil tests have not been useful in determining tree needs. Additionally, leaf samples may be useful.  In general, leaf nitrogen levels are higher in samples from trees carrying heavy crops. Biennial bearing trees in their off year or trees with a light crop typically have lower nitrogen levels. Table 2 shows leaf nitrogen standards for apple.

Mobility in plant:

Remobilized within plant

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Form of nitrogen fertilizer:

Fruit trees respond to any form of nitrogen fertilizer. Certain considerations, however, should be addressed.  For instance, ammonium and ammonium-forming fertilizers like ammonium sulfate, urea, and ammonium nitrate will decrease soil pH over time and are not recommended for soils with pH below 6.5.  Nitrogen when applied as urea may be lost from the soil via ammonia volatilization when this material is applied to the surface of warm, moist, alkaline soils.  Leaching of nitrates is another manner by which nitrogen is lost from the system.  This is especially true with coarse-textured soils that are irrigated frequently.

 

Urea sprays may be effective in increasing fruit set but are not sufficient to supply apple trees with the entire N they require.  Nitrate sprays are not recommended for apple trees because it is often associated with increased corking.

 

Nitrate may also be applied to orchard soils with the irrigation water.  It is therefore recommended that the irrigation water be tested periodically and credited against tree fertilizer requirements.

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Deficiency symptoms:

Because N is remobilized within the tree, older leaves will show deficiency symptoms first.  Leaves will be small and uniformly light green or yellowish (see middle photo above).  Tips and margins may show necrosis.  Bark is yellowish orange or will have a reddish tinge.  Shoots/spurs will be short, thin and spindly.  Fruit set may be reduced.  Current season's growth is <4".  Fruit color however, is enhanced.  

Toxicity symptoms:

Excessive shoot growth; large, succulent, dark green leaves; profuse flowering (see photo above); poor color development; prolonged growth into fall and higher susceptibility of winter injury and diseases such as fireblight.

Interactions with other elements:

• Heavy N fertilization intensifies copper and zinc deficiencies.

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