Blueberry quality and plant nutrition

There is considerable interest in how to optimize blueberry quality through fertilization practices. Here are some thoughts on common questions.

Can calcium additions improve blueberry quality?

Calcium (Ca) has specific effects on physiology of some fruits. Calcium sprays reduce the severity of bitter pit in apples and cork spot in pears, disorders associated with low fruit Ca levels. Increasing fruit Ca levels generally reduces fruit respiration rates and delays post-harvest fruit softening. This has been demonstrated in apples and pears, as well as kiwifruit, raspberries and strawberries. Calcium treatments have also been shown to reduce the incidence of gray mold (Botrytis rot) in strawberries. A characteristic of the responses to Ca is inconsistency; treatments show benefits in some studies, but not others. Responses may be related to the Ca content of test plants or application methods and timing.

The response of blueberries to Ca additions has also been inconsistent. In the 1990s, we studied soil additions of calcium sulfate (gypsum) and pre-harvest foliar sprays of calcium chloride on Michigan blueberries, but did not see effects on berry size, firmness or shelf life. Gypsum soil treatments were also tested recently in Argentina on O’Neal and Bluecrop blueberries. Gypsum treatments during the previous season appeared to reduce berry respiration and delay post-harvest softening to some degree. Some interesting work is being done at the University of Georgia with rabbiteye blueberries. Foliar sprays of several commercial Ca products (Calexin, KeyPlex, Cell Force) were applied prior to harvest. Early results show no effects on berry firmness, but an indication that sprays may increase average berry size.  It seems that Ca fertilization has the potential to improve blueberry quality, but responses are inconsistent and may be affected by the Ca status of plants and growing conditions, as well of the rates, timing and Ca product used.

Does urea use soften blueberries?

This question has not been researched thoroughly, but there does not seem to be an intuitive reason to expect that the use of urea as the nitrogen source would affect berry firmness. We could speculate that nitrogen use might affect firmness indirectly. Larger berries tend to be softer, so any fertilization practice that increases average berry size might affect firmness. Nitrogen rates also affect canopy density, which might affect firmness and quality by altering spray deposition and rot incidence, or shading developing berries.

Do late applications of nitrogen result in softer berries?

Some early work in New Jersey compared standard granular fertilizer application to fertigation up until harvest. Fertigated plants that received the latest nitrogen actually produced firmer berries than those fertilized with granular materials earlier in the season. Again, the response may be related to plant nitrogen status and actual rates used.

Dr. Hanson’s work is funded in part by MSU’s AgBioResearch.

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