For their development and well-being, trees and bushes need 17 key nutrients, which are primarily divided into macronutrients and micronutrients. Macronutrients consist of elements such as nitrogen, phosphorus, and potassium. In contrast, micronutrients encompass substances like Iron, Manganese, and Zinc, with crucial components such as water, oxygen, and light enhancing their growth.
Photosynthesis1 is a vital process carried out in the leaves, and it involves using sunlight to convert carbon dioxide and water into oxygen and energy-rich sugars. The roots absorb water and essential nutrients from the soil to support this life-sustaining process.
Naturally, trees acquire these nutrients from the rich organic matter and minerals in the soil, which are continuously replenished by decomposing plant and animal matter, ensuring a sustainable cycle of growth and nourishment.
Determining whether a tree requires fertilization hinges on 3 factors: its type, age, and visible health indicators.
For instance, fruit trees often need fertilizing if they are under 8 years old and make under 12 inches3 of new shoot growth yearly. New trees, once planted, should receive minimal fertilizer until they are fully established.
Signs of nutrient deficiencies, such as yellowing leaves, stunted growth, or poor flowering, are telltale indicators that a tree may benefit from fertilization.
However, it’s crucial to recognize when not to fertilize; for example, during drought or in late fall when trees are preparing for dormancy. The decision to fertilize should be based on a careful assessment of the tree’s condition, soil quality, and specific nutritional needs, ensuring that the intervention supports the tree’s health without causing undue stress or growth at inopportune times.
How to apply fertilizer to trees
The method of applying fertilizer to trees varies significantly depending on the fertilizer chosen.
1. Granular fertilizer: Calculate the required amount by weight per square foot around the tree for granular fertilizer.
To calculate the granular fertilizer needed for a tree, determine the recommended fertilizer rate, often in pounds per 1000 square feet. Measure the radius of the area around the tree (usually up to the dripline) and calculate the formula A = πr2. Adjust the fertilizer amount to the specific area.
For example, for a recommended rate of 5 pounds per 1000 square feet and an area of 314 square feet (with a radius of 10 feet), you would need (5 pounds/1000 square feet) * 314 square feet = 1.57 pounds of fertilizer. Spread this evenly around the tree.
2. Liquid fertilizer: Liquid fertilizers should be mixed with water as per bottle instructions and then applied to soak into the ground.
3. Spikes: Placing fertilizer spikes2 for plants around 2 feet from the root ball is generally recommended. In most cases, one spike is sufficient for each houseplant. For garden spaces, a common guideline is to use one spike for every 3 feet of area. This spacing helps ensure that plants receive adequate nutrients without over-fertilization.
4. Foliar sprays: When using foliar sprays4 for plants, opting for a premixed solution for convenience and accuracy is typically more effective, though postmix solutions can be used if precise control over the concentration is needed.
The application should be made either early in the morning or late in the afternoon to avoid rapid evaporation and to ensure maximum absorption. The amount to apply usually varies based on the product’s instructions, but generally, 1 teaspoonful/gallon for each 1 Ib/100 gallons of fertilizer material is recommended.
5. Organic manure: Chicken manure is generally considered the best option for fruit trees and general garden use because of its high nitrogen content, which is essential for plant growth. However, it must be composted well before application to avoid damaging the plant.
Horse manure is also a good choice, offering richer nutrients than cow manure but less than chicken manure.
To apply, spread the manure5 in a layer approximately 5-8 cm thick on the surface of bare soil. It’s important to break up any large lumps and ensure that the manure does not contact plant stems, as this might cause rot.
Calculate how much fertilizer is needed & Surface area to cover
The formula to calculate the amount of fertilizer needed for a root zone area typically involves knowing the recommended fertilizer rate per square foot and the total area of the root zone. The general formula is:
Amount of Fertilizer Required = Area of Root Zone (in square feet) × Recommended Rate of Fertilizer per Square Foot
To calculate the root zone area, you can use the formula for the area of a circle (πr2), where r is the radius of the root zone.
Fertilizing Mature vs New Trees
For mature trees, fertilizer should be applied less frequently, focusing on maintaining the tree’s overall health rather than promoting rapid growth. The application should be spread evenly over the soil’s surface, extending from near the trunk to the drip line, where the roots are most active.
In contrast, new trees require more frequent fertilization to support their developing structures and root systems. Here, the fertilizers should be applied in smaller amounts, closer to the root ball, to encourage root growth and establishment.
Regardless of the tree’s age, it is vital to avoid over-fertilization, which can lead to excessive foliage growth at the expense of root development, and to ensure that the application is made during the tree’s active growing season, typically in spring or early fall.
Fertilizing Deciduous vs. Evergreen Trees
Deciduous and evergreen trees differ in their leaf retention and energy requirements, impacting their fertilizing schedules. Deciduous trees lose their foliage seasonally in harsh conditions to conserve energy, resulting in lower nutrient needs during winter and autumn.
They require more nutrients after harsh weather for foliage renewal. Growing in less harsh climates, Evergreen trees retain their foliage, necessitating higher nutrient levels continuously, especially in adverse weather, to maintain their leaves.
This difference in nutrient requirements and leaf retention patterns may lead to variations in fertilizing schedules for these trees.
Nutrient deficiencies and their signs in trees
Nutrient deficiencies in trees can be identified by specific signs and addressed with targeted fertilization. For instance, a nitrogen deficiency is indicated by yellowing leaves, premature leaf fall, and small fruits, which can be corrected with deep-root fertilization to supply nitrogen directly to the roots.
Iron deficiency, leading to chlorosis (yellow leaves with green veins), often results from high soil pH; it can be treated with foliar sprays of iron sulfate or chelated iron. Similarly, manganese deficiency presents with yellow leaves and green veins, treatable with foliar sprays and soil applications, noting that high soil pH can limit manganese availability.
Problems excess fertilizing can cause
Excess fertilizing can lead to several problems for trees and shrubs. Over-application of water-soluble fertilizers, which often contain salts, can burn or injure plants. These salts can draw water out of roots, leaves, and needles, killing cells and leading to plant injury.
Additionally, excessive fertilizer use can make trees and shrubs more susceptible to certain pests or even harm or kill the plants.
Excess fertilizer not absorbed by the roots can leach through the soil, impacting groundwater, or move across the soil surface as runoff, contaminating surface water. Signs of over-fertilization include brown leaf edges or leaf tips, yellow, limp leaves, and leaf drops.
Best time to fertilize trees
The best time to fertilize trees is generally in the fall and spring. Fertilizing in the fall helps recover nutrients the soil lost during the summertime.
In the spring, fertilization supports a new flush of growth, greens up tree leaves to stay vibrant through summer and into fall, and supplies essential nutrients that keep the tree healthy and help it fight infection.
How often do trees need to be fertilized?
Newly planted trees typically do not require immediate fertilization and should be fertilized lightly only after their first growing season. It is recommended to fertilize young, rapidly growing trees annually, but not more than once a year, as over-fertilization can damage the roots and affect the tree’s health.
Generally, it’s advisable to fertilize a new tree once per year with granular fertilizer until established or twice a year with a natural fertilizer, depending on the tree’s growth and health.
- James Alan Bassham, Hans Lambers, (2024) Photosynthesis. <https://www.britannica.com/science/photosynthesis> Accessed: 18-03-2024
- Sandy Baker, (2023) How Fertilizer Spikes Work And When You Should Use Them. <https://www.housedigest.com/1308607/how-fertilizer-spikes-work-when-use-garden/> Accessed: 18-03-2024
- Steve Renquist, (2018) Fertilizing fruit trees. <https://extension.oregonstate.edu/gardening/berries-fruit/fertilizing-fruit-trees#:~:text=Another%20way%20to%20gauge%20if,you%20should%20apply%20more%20nitrogen.> Accessed: 18-03-2024
- Wade W. McCall, (1980) Foliar Application of Fertilizers. <https://www.ctahr.hawaii.edu/oc/freepubs/pdf/GHGS-24.pdf> Accessed: 18-03-2024
- Boninie L. Grant, (2021) Best Manure For Gardens – What Are Different Types Of Manure. <https://www.gardeningknowhow.com/composting/manures/manure-pros-cons.htm> Accessed: 18-03-2024
