We've had a pear tree growing for about five years now, and for the first three years the fruit was normal-sized, but over the last two seasons the pears have been getting smaller right before our eyes. I fertilize it like I always do, and the watering is fine too. Could it be soil depletion, or does it need some kind of special fertilizer?
It's most likely that the tree is overloaded with fruit - your pear simply can't feed that many ovaries! Try to thin the fruits at early stages, leaving no more than one or two ovaries per 15-20 cm of branch, and ruthlessly remove the rest. Plus check if you need a rejuvenation pruning - over five years the branches could have gotten dense and shading each other, which causes uneven nutrient distribution.
Of course, soil depletion makes sense too! But regular spring feeding and feeding during bloom usually does the trick. If you want to be extra safe - spread some compost under the tree before the season, it won't hurt and will improve soil structure. But potassium and phosphorus are especially important for fruit size, so make sure they're in your fertilizer, not just nitrogen.
Thinning the fruit definitely helps, but if the pears have been getting smaller for only the past two years, that's probably not the only issue. Over five years, a pear tree can actually deplete the soil, especially if it wasn't very rich to begin with. Check if maybe you're fertilizing "as usual" with some old habit, but the tree has grown and now needs more nutrients. Try adding a balanced mineral fertilizer in spring and nitrogen at the start of the season - pear trees especially love it.
But there's another thing to consider - the tree's age. From around five years old, pear trees start fruiting more actively, and if it sets too many fruits, there simply aren't enough nutrients to go around. That's when you need to thin the fruit set in June, while it's still small. Leave just one fruit per 15-20 centimeters of branch - that way each pear will get enough energy to develop properly.
Also pay attention to pruning. If you haven't thinned out the canopy in a while, it might be too dense, with leaves shading each other, and photosynthesis drops. The light conditions affect fruit size just as much as fertilizing does.
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