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Nitrogen (N) is the most abundant element in our atmosphere. It promotes the vigor of the fruit and the overall health of the tree. Although there is an abundance of this element, nitrogen is not always in a form that’s readily available for plants in the soil.
Nitrogen is a macronutrient that needs to be frequently replenished for fruit trees. It can replenish naturally through soil bacteria and rainfall but not enough to support fruit trees. Nitrogen deficiency can be determined by soil testing, annual shoot growth, and leaf analysis.
Nitrogen can be supplied to the soil using synthetic or organic fertilizers, but the amount used and the timing of the application can impact the results.
In this article we will cover:
- The Role of Nitrogen in Fruit Trees
- Identifying Nitrogen Requirements of Fruit Trees
- How Much Nitrogen to Apply to Fruit Trees
- When to Apply Nitrogen to Fruit Trees
The Role of Nitrogen in Fruit Trees
Nitrogen is an essential nutrient that helps promote new shoot growth, flower bloom, and fruit development. However, too much nitrogen can promote lush growth and increase disease susceptibility of fruit trees.
If nitrogen is added too late in the season, it will encourage a flush of tender growth, making it vulnerable to frost damage (source).
On the other hand, too little nitrogen will deprive the tree of enough nutrients to produce a big, healthy fruit harvest. Therefore, it is vital to identify the needs of each fruit tree.
Identifying Nitrogen Requirements of Fruit Trees
There are three major ways to identify nitrogen deficiency in fruit trees: soil testing, annual shoot growth, and leaf analysis.
Soil testing is best for monitoring the pH of the soil and identifying the need for lime (raises pH levels) or elemental sulfur (lowers pH levels). As seen in the Alabama Cooperative Extension System chart (source), the pH of the soil is vital for a plant’s ability to absorb nitrogen and other nutrients.
Most fruit trees will thrive in a pH range of 6.0-7.0, however, be sure to check your fruit tree’s needs before amending the soil.
The University of Florida has a useful chart to help identify the average recommended pH range for various fruit trees (source). Consider testing your soil at least once every three years to maintain a healthy soil pH.
You can purchase a simple-to-use soil test online (link to Amazon).
Annual Shoot Growth
Farmers only need a chair and a keen eye to observe the need for nitrogen through shoot growth. Most shoot growth can be measured by the length of the branch between leaves or bud scale scars.
Fruit-bearing trees have a wide variance in the average annual growth rate, but the majority of them average around 12-18 inches per year. (source)
Trees that grow more than 18” pear year may be receiving too much nitrogen, while trees growing less than 12” per year may be receiving too little.
Colorado State University has a quick chart for specific growth rates of common fruit trees.
Leaf Analysis is considered the best way to determine the nutrients needed for perennial fruit trees.
Fruit trees recycle nutrients throughout the growing season. During the fall and winter months, nutrients are stored in the root and the trunk. In the spring and summer, nutrients are moved to new shoot production, flower bloom, and fruit development.
The leaf analysis indicates exactly which nutrients the tree is using and identifies its specific needs for the current growing season. In addition, leaf analysis can also help diagnose more severe nutrient or mineral problems.
Penn State Extension recommends the following process when collecting leaf samples (source).
- Collect leaf samples from mid-July to mid-August.
- Divide your orchard into thirds according to:
- Soil type
- Collect 50-60 leaves across the orchard block. Healthy leaves from the current growing season should be:
- Fully expanded
- From mid-way up the canopy
- From a non-bearing shoot or spur
- Avoid leaves that have been sprayed (wait at least 15 days from nutrient spray on the leaves and notify the laboratory for any other herbicide or chemical sprays).
- Place the leaves in a small paper bag.
- Label samples according to the laboratory’s requirements and submit them.
The North American Proficiency Testing Program provides a list of accredited laboratories for western states (source).
Consider performing a leaf analysis once every three years.
How Much Nitrogen to Apply to Fruit Trees
How much to apply depends on the results from the testing, the amount of nitrogen in the fertilizer, and the age of the tree. Proper testing sites will send a detailed report indicating the needs of your fruit tree. Amounts should be adjusted depending on the test results.
If the shoot growth is less than the average, or the leaf analysis returns with low levels of nitrogen, you may need to increase the amount or frequency of nitrogen applications. If the shoot growth or leaf analysis shows an above-average level of nitrogen, then you may need to apply a lower dose of nitrogen.
Below are general recommendations for the yearly application of nitrogen.
Newly Planted/First Year
The focus for newly planted fruit trees is to establish a healthy root system. Using nitrogen before planting can burn or weaken the roots. Application of nitrogen immediately after planting can stunt the growth of the roots by diverting the nutrients to leaf and fruit production.
Oregon State University recommends applying a light application (one cup of sulfate of ammonium, or 5-10lbs of aged manure for organic gardeners) around the drip line about six months after the roots have been established.
For the second to the seventh year, use approximately one-eighth of a pound of actual nitrogen per year of tree age (source). A simple equation is:
(1/8) x Tree Age = Amount of Nitrogen
For example, if your tree is four-years-old, you will use the following equation:
(1/8) x 4 = 0.5
In a relatively healthy growing environment, this four-year-old fruit tree would benefit from a half-pound of actual nitrogen. To calculate actual nitrogen, multiply the weight of the fertilizer bag by the percentage of nitrogen in the fertilizer. Fertilizers are categorized by the N-P-K ratio, with each number representing the percentage of actual nutrient in the mixture.
So, a 50lb bag of fertilizer that is 10-12-8 has 10% nitrogen, 12% phosphorous, and 8% potassium.
To find the actual nitrogen, multiply 50lbs x 10%, or 50 x .1, which is 5lbs of actual nitrogen. If you need to apply .5lbs of actual nitrogen, you would need 5lbs of fertilizer.
Do not apply all the fertilizer at one time. Apply it gradually throughout the season, and use a slow-release fertilizer for best results.
Organic gardeners can calculate actual nitrogen in manure to meet the annual nutrient needs of their fruit trees. Ohio State University Extension has nitrogen levels available for different types of manure and compost.
Eighth Year and Older
Trees eight years of age and older will only need 1lb of actual nitrogen per yea unless testing results state otherwise.
When to Apply Nitrogen to Fruit Trees
A good rule of thumb is to apply 50% of the required nitrogen in early spring (between bud-break and bloom), allow the fruit tree to be deficient in nitrogen mid-season (this will yield a higher fruit production), and then apply the remaining 50% of nitrogen after harvest (before the trees move into dormancy for the winter).
This gives the tree a nitrogen boost when it leafs out in the spring, and while it prepares for dormancy in the winter.
Whether you are using chemical or organic nitrogen fertilizers, remember to always read the instructions for proper application and safety.