Answer: Arborvitae could thin for a number of reasons, including too much shade, a pest problem or possibly a disease. If one of these is the cause, then additional fertilizer would not correct the problem. However, good growing conditions including adequate fertility can help a plant resist infection, so it is worthwhile making sure that the soil is providing what the plants need. I would suggest you consult with your county extension as to diagnosing the thinning. They can then make more specific suggestions as to what to do as well as recommend a suitable fertilizer or other soil amendments based on the results of some basic soil tests.
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