I HAVE A GRAPEFRUIT TREE NOW FOR 40 YEARS IN THE HOUSE DURING WINTER. WHEN I TRIM THE TREE WOULD IT BE GOOD TO GRING THE LEAVES IN A BLENDER WITH WATER AND POUR IN BACK IN THE PLANT SOIL.
Yes that's a good idea. In nature the fallen leaves provide soil nutrient replenishment, so you are following the same principle with the help of your blender. But it's not enough. Some nutrients are leached out of the soil with watering, and some are used by the plant metabolism, so a general purpose or citrus specific fertilizer is needed as well.