Pear trees need moderate fertilizer to encourage a healthy growth rate that is not too vigorous. The nutritional status of the tree affects growth and fruit production. Tree and fruit health are the best indicator of deficiencies. Soil testing or leaf testing can help determine exactly which nutrients are appropriate for each tree. Pear trees that are over-fed are more susceptible to disease. Vigorous shoots are prone to fire blight, so avoid fertilizing when fire blight is a problem.
Test the pH of the soil before planting pear trees. Add lime as needed to raise the pH to 6.0.
Fertilize newly planted trees with 1/2 lb. of 10-10-10 or 13-13-13 fertilizer two to three weeks after planting. In the spring, before the tree blooms, and one month before harvest, spread 1/2 cup of fertilizer per year of the trees age, up to 6 cups maximum in each application. Spread the fertilizer in a wide circle beneath the tree, at least 1 foot from the trunk. Do not allow fertilizer to touch the trunk of the tree. Fertilize pear trees with ammonium sulfate (21-0-0) instead of 10-10-10 if the soil pH is above 7.5.
Apply sulfur in the form of calcium sulfate (gypsum) or other fertilizers containing sulfur to soils that are sulfur deficient. Anjou pear trees require more sulfur than other varieties.
Apply magnesium supplements, as needed, according to the soil or leaf test. Magnesium can be supplied in dolomitic lime when lime is also needed or as magnesium sulfate when the pH is above 6.0. Bosc and Comice pears need more magnesium than other varieties.
Apply sodium pentaborate by broadcasting or spraying when the soil is deficient in boron. Apply boron to fruit-producing trees only.
Apply zinc sulfate when soil tests indicate a deficiency. Apply zinc supplements late in the dormant season before budding or later in the season, after harvest.