Answer: Fertilizer spikes can deliver needed nutrients to tree roots, but in a limited way. Because the fertilizer is concentrated in a single area, not all of the roots of the tree will benefit from the feeding. I wouldn't say they are no good, but I would say that they are not as efficient as other methods of feeding trees.
There are two main methods of applying fertilizer to trees. The fertilizer can be applied directly to the soil surface or it can be applied down into the soil.
Spreading the fertilizer on the soil surface under the tree is the easiest and least expensive method.
Putting the fertilizer down into the soil is more difficult but gets phosphorus and potassium down 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.
Using a hose-attached root feeder (example: Ross 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. Follow label directions to get the calculated amount of material equally distrubted to each of the holes.
Another method of application is by making holes approximately 2 to 3 feet apart at and beyond the drip line of the tree (see sketch). 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.
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 follow label directions to determine the amount of material to apply and to prevent damage from excessive amounts of fertilizer.
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