How to: Fertilize trees via liquid injection

When it comes to fertilizing trees, you'll find no shortage of options and decisions. You have choices of how and how much fertilizer to apply, what its nutrient content should be and much more. Examining all these options, for which plenty of differing opinions exist, could fill a book. So to provide a brief yet useful step-by-step guide to tree fertilization, this article will focus on the most common method: liquid injection.

Liquid injection involves placing fertilizer as a liquid suspension or solution into the soil around the tree. One advantage of liquid injection is that by applying fertilizer below the soil surface, you can make certain that nutrients, such as phosphorus, are more available to tree roots by placing them below the thick turf zone.

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Most managers' choice If you are like most grounds managers, once you've decided on performing liquid injection, you will use a fertilizer that is primarily slow-release but also includes some fast-release components. For a general rule of thumb in determining how much nitrogen to apply, use the rate suggested by the forthcoming American National Standards Institute (ANSI) A300 guidelines: 2 to 6 pounds of actual nitrogen per 1,000 square feet. The ANSI guidelines also suggest injection spacing and depth levels: 18 to 36 inches apart and 4 to 12 inches deep.

Of course, it goes without saying that you'll also need the proper equipment to perform liquid-fertilizer injections, which basically means a truck with a pressurized pumping system.

Step1: Determine how much fertilizer to inject Before you can begin the injection process, you must calibrate the probe. That is, you must determine the amount of fertilizer to deliver per injection point. To do so, you need to consider the following: *Injection spacing *Analysis of the fertilizer (specifically the amount of nitrogen) *Mixing rate of raw fertilizer per 100 gallons (described on the fertilizer container) *Rate of nitrogen per 1,000 square feet *Probe flow rate (how many gallons per minute flow through the probe). You can determine the probe flow rate by observing how fast the probe fills a 5-gallon bucket and dividing the number of minutes, or fraction thereof, into 5. Knowing this information, let's consider an example to get started. Imagine you have the following factors: *3-foot injection spacing *26-3-13 fertilizer (26 percent nitrogen) *Mixing rate of 20 pounds of fertilizer per 100 gallons *Desired nitrogen rate of 2 pounds actual nitrogen per 1,000 square feet *A 9- x 9-foot area to fertilize.

Now run the math: A 9- x 9-foot area is 81 square feet. With a 3-foot spacing, the area will contain nine injection sites.

You want 2 pounds of nitrogen for each 1,000 square feet. You know that 20 pounds of 26-3-13 fertilizer per 100 gallons equals about 5 pounds of nitrogen per 100 gallons (26 percent x 20 pounds = 5 pounds, with rounding).

But you want to use only 2 pounds of nitrogen per 1,000 square feet. So to apply 2 pounds per 1,000 square feet, you need 40 gallons of fertilizer.

Now that you understand how much fertilizer you need for 1,000 square feet, you can determine how much you need for 81 square feet. Figure the ratios again:

40 gallons / 1,000 square feet = X gallons / 81 square feet

To solve for the number of gallons per 81 square feet:

3,240 (40 x 81) / 1,000 = 3.25 gallons of fertilizer

Let's say it took the probe 90 seconds (1.5 minutes) to fill a 5-gallon bucket.

5 gallons / 1.5 minutes = 3.3 gallons per minute

If you need 3.25 gallons of fertilizer and the probe delivers 3.3 gallons per minute, you need roughly 1 minute of flow divided between the nine injection holes in the area to be fertilized. To determine how long each injection should last per hole:

60 seconds (the 1 minute of flow needed over the entire area) / 9 holes = 6.5 seconds per hole.

Thus, leave the probe in each injection hole for roughly 6.5 seconds to provide 2 pounds of nitrogen per 1,000 square feet.

(Bear in mind that these are rough guidelines. Factors such as soil density, the number of probes you are using and the pressure of the system all affect these values. If these variables change, you may have to re-calibrate the probe. Unlike injecting medication into humans, varying the injection amounts by as much as 25 percent is okay.)

Step 2: Prepare the injection site Before you begin the injections, you must assess and prepare the area around the tree. Check the ground. Are there irrigation heads? If so, try to determine where the irrigation lines are so you do not damage them. Check for gas lines, too. If you are at a residence and you see a gas grill without a propane cylinder, a gas line may be running underground from the house to the grill, so you'll need to watch for it too.

Are flower beds planted near the tree? If so, you will need to both step carefully and watch that your hose does not drag through the flowers.

Be aware of property lines as well. Unless you have obtained written consent from a neighbor, you can only fertilize the area of the tree that is on your clients' property. (In which case, you should fertilize at the pre-determined rate, not at an increased rate to compensate for partial fertilizing.)

Step 3: Establish the injection grid Most applicators base the injection grid on an imaginary box on the ground that encompasses the canopy edges. The injections will fall within the box at your pre-determined injection spacing.

Because the grid is something you'll likely want to simply "eyeball," it is helpful to find a visual reference point beyond the tree's dripline. To choose one, stand at the edge of the canopy. Pick a point that looks like it is at the edge of the canopy on the other side of your box. Then pick another point on a side of a building, or perhaps across the street, that is beyond the original point. By sighting both points as you move through your grid, you will be able to systematically and thoroughly cover the area to be fertilized. This technique is particularly helpful if the ground is uneven or if the area is not square.

Step 4: Begin the injections Now that you have oriented yourself in the grid box, you are ready to begin the injections: *Stand at your starting point at the edge of the canopy. *Insert the probe into the soil. *Inject the fertilizer for the time calibrated. (In the above example, that would be roughly 6 to 7 seconds.) *Remove the probe. *Walk 3 feet (again based on the above example) to your next grid point, keeping an eye on your visual references. *Insert the probe into the soil and repeat the process. *Continue until you have reached the other side of the canopy. Step over 3 feet to begin the next row. *Insert the probe and repeat the process until you have covered the entire grid area beneath the canopy.

Bear in mind that trees have many roots growing close to the trunk. So it is best to stay a foot or two away from it so that you can avoid driving the probe into the large roots close to the soil surface.

You don't always have to use a grid pattern to determine the injection area. For example, if you need to fertilize a conifer or other tree under whose canopy you cannot reach, the injection pattern should be circular, using about three rings. Perform the first ring of injections by reaching inside the canopy. The second ring should be at the edge of the canopy. The third injection ring should be about a foot or two outside the canopy.

Step 5: Inspect your injections Whether you use the box or circular pattern, uniformity is important. About 2 to 4 weeks after you have fertilized the tree, tufts of greener grass should appear at each injection site. Your client will be able to see if your injections varied from the grid. If they have, your work will have an unprofessional look to it.

Be sure to return to the site after you have fertilized a tree. Look for the tufts of grass to see whether you adhered to the grid. And observe the tree. Is it doing well? Does it look better or worse than before you fertilized it? As you observe, remember that fertilization is not a cure-all. Many other factors-such as soil moisture, amount of organic material, soil compaction, root damage and so on-may actually affect a tree more than fertilization.

Tree fertilization using the liquid-injection method takes some practice. Calibrating the probe may seem complicated at first, but it does eventually become easier. Likewise, following a grid pattern by sight does become more efficient over time. The benefits to the tree are well worth the learning curve.

Dr. Rex Bastian is director of technical services for The Care of Trees with headquarters in Wheeling, Ill., and offices serving metropolitan Washington, D.C., and New York City.

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