University horticulturalists making recommendations to growers and gardeners have different ways of calculating how much to fertilize pear trees. The central issue is how much nitrogen to apply. Some advice for fertilizing is general; other suggestions are based on the age of a pear tree, the size of its trunk and how rapidly it grows.
A springtime application is usually recommended. Spread the fertilizer evenly on the soil in the irrigation basin under the tree and rake it lightly into the soil. After you apply the fertilizer irrigate the tree generously. Applying too much nitrogen can lead to over-production of fruit and excessive growth of leaves and the tree.
Horticulturists at Mississippi State University recommend a springtime application of fertilizer based on the age of young trees. Apply 1 lb. of complete fertilizer plus ¼ lb. of ammonium nitrate the first year.To this amount add 1lb. of complete fertilizer plus ¼ lb. of ammonium nitrate each year for the next three years. Apply 5 lbs. of complete fertilizer plus 1 ½ lbs. of ammonium nitrate the fifth year. After the fifth year apply 2 to 3 lbs. of complete fertilizer for each inch of tree diameter.
Horticulturalists at the University of Arizona recommend applying nitrogen fertilizer once each February or March following the second growing season after planting. The fertilizer is applied according to the diameter of the trunk of the pear tree measured at the ground. These are the recommended amounts of fertilizer: 4 oz. of ammonium sulfate per inch of the diameter of a pear tree trunk to a maximum of 2 ½ lbs.; 2 oz. of ammonium nitrate per inch to a maximum of 1 ½ lb.; 1 ½ oz. of urea per inch to a maximum of 1 lb.; 5 ½ oz. of blood meal per inch to a maximum of 3 ½ lbs.; and 11 oz. of cottonseed meal per inch to a maximum of 7 lbs.
Horticulturalists at Colorado State University say the correct amount of nitrogen for pear trees can be determined by measuring how fast the tree grew the previous year. When the bud grows each years, it leaves behind a scar; this is a growth ring. The bark will be a different color and texture between annual growth rings. To determine how much the tree grew the previous year, check the distance between the rings in several places. Trees that are not yet bearing pears should grow from 12 to 36 inches a year. Trees bearing pears should grow 12 to 16 inches a year. If the tree grew at the low end of recommended rate apply 1 ½ oz. of nitrogen per inch of trunk diameter as measured 1 foot above the ground. If the tree grew at the recommended rate, apply less nitrogen.
A General Recommendation
The British Columbia Ministry of Agriculture and Lands recommends not applying fertilizer to weed-free pear trees for the first two or three years. After that the Ministry recommends a fall application of 1 oz. of 12-16-12 or other complete fertilizer per square yard of root area.
A Cautious Recommendation
Horticulturalists at the University of New Hampshire says excessive vigor makes trees susceptible to fire blight. Fertilize young pear trees with 1 lb. of 10-10-10 or other general garden fertilizer in the spring. Fertilize mature trees with 2 lbs. of balanced fertilizer.