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As with all plants, nitrogen plays a major role in fruit and tree growth. Fruit color development can be limited by the presence of too much nitrogen.

Fruit size increases with higher levels of nitrogen. Since flesh firmness decreases as fruit gets larger, growers need to effectively balance nitrogen so that both size and firmness are served.

Similarly, when growers reduce nitrogen to aid in fruit color development, they must be careful not to bring on biennial bearing problems with varieties such as McIntosh and Golden Delicious.

When adequate calcium is present in fruit, storage goes well and storage disorders — bitter pit and internal breakdowns, for example — are minimized. On the other hand, too much potassium interferes with calcium use and uptake, while too little creates problems with leaf development.

In earlier research, Dr. Gennaro Fazio, a plant breeder and research geneticist with the USDA-ARS Plant Genetic Resources Unit in Geneva, New York, found higher sulfur in fruit usually occurs with higher calcium.

That’s because calcium moves easily in acidic environments, the kind of environments with which sulfur is usually associated.

“It may be possible to formulate sulfur-based fertilizer applications that may increase calcium transport to the fruit,” he said.

Magnesium and manganese also seem to be associated with calcium. “Manganese seems to use the same transport system calcium does,” he said. “Magnesium seems to compete with calcium to get into the fruit but not the leaves.”

This is good news for tree health because of magnesium’s contribution to photosynthesis. When magnesium is not available, premature ripening and pre-harvest fruit drop are right around the corner. Blind wood and brittle spurs also result from magnesium deficiency.

Manganese, iron and copper are all involved in photosynthesis. When there is too little manganese present, it generally means photosynthesis is not occurring at optimal levels. Poor photosynthesis results in reduced tree and leaf growth.

Phosphorus’ value lies in its role in creating and stabilizing fruit cell walls. Too much phosphorus in leaves can indicate a zinc deficiency, while too little may be a sign of low soil pH.

Boron is necessary to develop shoot tips, flowers and roots. Low amounts of boron result in corking and cracking of fruit and poor root development. In addition, when boron is limited, so, too, is calcium.

Zinc helps to move calcium within trees. A zinc deficiency results in poor leaf and shoot growth as well as reduced flowering, fruit set, size and coloring.

Sodium can reduce soil microbial activity if too much is available. The best way to reduce its effect is to apply gypsum.

— by Dave Weinstock