Answer: In my experience, astilbes do best with a rich, organic soil that is evenly moist. Since your soil is sandy, it would need a generous amount of organic matter such as rotted leaves, aged stable manure and bedding, or compost worked into it followed by annual top dressings of compost and a year round organic mulch to aid in a continuous feeding for the soil. The organic matter would also help the soil hold moisture better over a longer period of time. Keep in mind too that a sandy soil will leach nutrients relatively quickly, along with the moisture.
Since soil tests show that the nutrients seem to be sufficient, I would suspect that water has become the limiting factor in the blooming performance. If you still feel additional nutrients might be beneficial, you could add additional compost or well rotted well aged manure in early spring and/or late fall.
It is also possible that the tree is causing too dense a shade, since interestingly enough these plants will tolerate quite a bit of sun and bloom very well provided the soil is moist.
Finally, if the plants are very old and have grown into very large clumps, they might benefit from being divided to stimulate vigorous new growth. This would best be done in very early spring.
I hope this helps you trouble shoot.
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