Practices for increasing calcium content and improving fruit quality and shelf life of blueberries

Calcium is a key component of fruit quality and is known to increase berry firmness and shelf life in blueberries. Usually, Ca is applied by adding gypsum or lime to the soil or by using foliar sprays. However, these materials tend to be ineffective for increasing Ca in the fruit. Previously, we determined that nearly all Ca in the fruit is taken up during the short period between the early green and late green stages of berry development. Therefore, maintaining a high concentration of Ca in the root zone during these stages may facilitate uptake of the nutrient into the fruit. One potential way of doing this is to inject Ca fertilizer into the drip irrigation system and apply it by fertigation. Currently, there are a number of Ca products that can be used for fertigation, including micronized solution grade gypsum (which is certified organic) and Ca thiosulfate. We are testing fertigation with both of these products in five cultivars of northern highbush blueberry (Vaccinium corymbosum L.), including ‘Duke’, ‘Earliblue’, ‘Bluecrop’, ‘Elliott’, and ‘Aurora’, and are comparing them to soil applications of gypsum, foliar applications of CaCl₂, and control treatments with no Ca fertilizer. So far, we found that fertigation increased Ca in the soil solution relative to the other treatments but had no effect on fruit Ca. In this case, we fertigated once a week from petal fall to fruit coloring using water-driven injectors. Fertigation rates were limited by low solubility of the micronized gypsum and the label rate for calcium thiosulfate. Foliar applications of CaCl₂, on the other hand, increased fruit Ca but had negative effects on yield and fruit quality in two of the cultivars. Specifically, it reduced average berry weight by 8% in ‘Bluecrop’ and resulted in 10% more fruit loss from heat damage in ‘Aurora’. Next, we plan to fertigate more frequently (e.g., two or three applications per week) in an effort to increase Ca in the fruit. We will also test a different type of injector for gypsum.