Planting The Seeds Of Fertilization

By Ronald C. Smith, Horticulturist and Turfgrass Specialist; David DeCock, Cass County, N.D. Horticulturist; and Rich Hill, Grand Forks County, N.D. Horticulturist

One method of fertilizer calculation is to measure the trunk diameter of the tree at about 4.5 feet above ground level. Using this method, the amount to use is one pound of fertilizer for each inch of trunk diameter, if the analysis is 20-20-12, or something close to that. If the analysis for nitrogen is 10 percent or lower, then use two pounds per inch spread over the area under the tree?EUR??,,????'?????<photo: nrcs.usda.gov.

Fertilizer applications are used during the growing season to improve the health and appearance of trees. Most deciduous trees should be fertilized once every one to two years. Evergreens may be fertilized in the spring, but less often than other trees.

Trees growing in naturalized areas where little or no mowing takes place and leaves are collected may not need regular fertilizing.

The area to be fertilized under a tree canopy forms a circle. To calculate the square footage, take the radius squared X 3.14. Suppose this pin oak tree has a spread of 30 feet. This would equal 706 square feet that would be considered for fertilizer application photo: nrcs.usda.gov.

Methods

There are two main methods of applying fertilizer to trees: applying it on top of the soil surface, or applying directly into the soil. While these two basic approaches are pretty straightforward, there are a number of techniques that can be used to carry them out effectively.

Spreading

Spreading the fertilizer on the soil surface under the tree is the easiest and least-expensive method.

Soil Injection

Injecting the fertilizer down into the soil is more difficult but gets phosphorus and potassium into the root zone and provides the benefit of aeration. This can be accomplished by using a root feeder, drilling or punching holes in the soil, or by driving fertilizer spikes into the soil.

Root Feeder

Using a hose-attached root feeder will get the material into the root zone in a liquid form. Water flows past pre-measured tablets in an enclosed chamber and passes through a hollow needle inserted into the soil about 8 to 12 inches deep. Make sure to equally distribute the material to each of the holes.

Drilling

Another method of application is by making holes approximately 2 to 3 feet apart at and beyond the drip line of the tree. Holes are punched or drilled into the soil 8 to 12 inches deep, slanting toward the center of the tree. The calculated amount of fertilizer for the tree should be distributed equally to the available holes. Water thoroughly to dissolve the fertilizer. This is the most effective method of applying an iron supplement to trees suffering from iron chlorosis.

Spikes

Fertilizer spikes or stakes are a popular and convenient method for getting nutrients into the root zone, even though the distribution of nutrients is very limited. The cost is high for the nutrient content provided. Be sure to apply the correct amount of material to prevent damage from excessive amounts of fertilizer.

Using a hose-attached root feeder will get the material into the root zone in a liquid form. Water flows past pre-measured tablets in an enclosed chamber and passes through a hollow needle inserted into the soil about 8 to 12 inches deep. photo: www.adairtreecare.com.

Foliar Feeding

Foliar feeding fertilization is often used to correct any micronutrient deficiencies. Nutrients can be added anytime during the growing season, as it does not stimulate excessive growth but corrects a chlorotic (leaf yellowing symptom) condition typically common on some silver maples.

Foliar spray material is also used to help get young trees established in the landscape and help recently transplanted trees overcome the shock of being moved. As with other methods, be sure to avoid fertilizer salt damage.

Amount of Fertilizer

The amount is of fertilizer is usually determined by the nitrogen content of the material. For example, a standard recommendation is 1 pound of actual nitrogen per 1,000 square feet. This can easily be calculated by taking an example of 20-10-10 fertilizer. Divide the weight, for example 100 pounds, by 20, the percent nitrogen (100/20 = 5) to get the amount of material needed, 5 pounds. This means that by applying 5 pounds of 20-10-10 over 1,000 square feet, 1 pound of actual nitrogen would be applied (5 pounds of fertilizer X 20 percent nitrogen = 1 pound of actual nitrogen).

When looking at a bag of fertilizer for nutrient information, nitrogen is available from different sources?EUR??,,????'?????<

Application Calculations

The area to be fertilized under a tree canopy forms a circle. To calculate the square footage, take the radius squared X 3.14. Suppose a tree has a spread of 30 feet. This would equal 706 square feet that would be considered for fertilizer application. To get 2 pounds of actual nitrogen per 1,000 square feet over that area, 1.4 pounds would be needed (706/1000 x X/2 = 0.7×2X = 1.4). If the percent nitrogen in the fertilizer is 10, then 20 pounds of that material would be needed; if the percentage is 5, then 40 pounds; if the percentage of nitrogen is 20, then only 10 pounds, etc.

Another method of fertilizer calculation is to measure the trunk diameter of the tree at 4.5 feet above ground level. With this method, about 2 pounds of a complete fertilizer containing 10 to 20 percent nitrogen should be used for each inch of trunk diameter.

Whether using the broadcast, root feeder, soil auger or fertilizer spike method of application, spread the nutrients as uniformly and evenly possible for uniform nutrient uptake. Most of the fibrous roots are near or beyond the drip line of the tree and absorb the plant nutrients from the soil. Avoid applying fertilizer to within 2 to 3 feet of the trunk.

Permission to use information from ?EUR??,,????'?????< www.ag.ndsu.edu/pubs/plantsci/trees/h1035w.htmfor the educational, non-commercial purpose granted by North Dakota State University.