Many different methods exist to fertilize adult trees. Trees are fertilized by either using granules, liquid foliar spray--or liquid fertilizer that you inject into the soil. Injecting liquid fertilizer into the soil around trees allows the fertilizer to reach the tree roots before it can run off or be used by turfgrass under the tree.
Calculate how many injection points you will have in the root zone. Measure the area under the tree canopy as this is where the majority of the tree roots will be found. Aim to have an injection every 3 feet under the canopy. Say your measured area is 9 feet by 9 feet. This area will have nine injection sites to maintain the 3-foot spacing.
Calculate how long your probe needs to be kept in each injection site. Use the probe and fill a 1-gallon jug to test the flow rate.
Determine how much nitrogen you need for your entire injection area. A good rule of thumb is to use 2 to 6 lbs. of nitrogen per 1,000 square feet. If your fertilizer is 25 percent nitrogen, use .64 to 1.96 lbs of fertilizer.
Dilute your fertilizer with water. A good mixing ration for fertilizer to water is 20 lbs. of fertilizer for every 100 gallons of water. Using the above calculated amounts, dilute .64 to 1.96 lbs of fertilizer with 3.2 to 6.96 gallons of water (maintaining the ratio by multiplying by 5).
Calculate how long the probe needs to remain in each injection hole. Say your probe puts out 2 gallons per minute. To deliver 2 lbs. of nitrogen per 1,000 square feet, each injection hole need .017 lbs of nitrogen delivered, which means .356 gallons of mixture should be delivered to each hole. This will take approximately 10 seconds per hole. To deliver 6 lbs. of nitrogen per 1,000 square feet, each injection hole need .054 lbs of nitrogen delivered, which means .773 gallons of mixture should be delivered to each hole. This will take approximately 23 seconds per hole.
Examine the injection site and ensure it is free and clear of objects and debris. Visualize a grid system under the tree and where each injection point will be.
Insert the fertilizer probe into the soil and inject the fertilizer for the above calculated time. Remove the probe and move to the next spot. Inject at this location. Continue until all spots have had fertilizer injected for the calculated times.
Examine the injection site after two to four weeks. Look for green tufts of grass at the injection location. Look to see if the grid system is uniformly laid out. Check the health of the tree and see that it is doing well.