Fruit Calcium Is Influenced by Soil and Physiological Factors but Not by Fertilizer Applications in Floricane-fruiting Red Raspberry

Calcium (Ca), an important nutrient for cellular integrity, is often applied as a fertilizer to improve plant health and fruit quality in many fruit crops, including raspberry ( Rubus idaeus ). However, in many cases the benefits of applying Ca fertilizers are mixed and poorly understood. The objectives of this study were 2-fold: 1) evaluate the effects of strategically timed soil- and foliar-applied Ca fertilizers on floricane-fruiting raspberry Ca tissue concentrations, yield, and fruit quality and 2) explore soil and plant variables that may impact Ca nutrition in floricane-fruiting raspberry. Three floricane-fruiting raspberry cultivars were used in the study (Kulshan, Meeker, and WakeHaven). Treatments were applied to mature commercial fields in northwest Washington, USA, and included 1) soil application of Ca fertilizer (calcium sulfate) applied annually before budbreak, 2) foliar application of Ca fertilizer (calcium hydroxide) applied weekly during immature green to white stages of fruit development to align with fruit uptake, and 3) an untreated control with no Ca fertilizer. Neither method of Ca application affected yield, firmness, total soluble solids, pH, titratable acidity, or the concentration of Ca in the soil, leaves, or the harvested portion of the fruit (drupelets) in any of the cultivars. However, foliar applications increased the concentration of Ca in the receptacles of ‘WakeHaven’. In each cultivar, the concentration of Ca was at least 10 times higher in the receptacles than in the drupelets, suggesting the presence of a physiological barrier limiting Ca movement into the harvested fruits. Soil nutrient dynamics, particularly nitrogen (N), magnesium (Mg), potassium (K), and boron (B), were either positively or negatively associated with tissue Ca concentrations, highlighting how other nutrients may influence Ca uptake and translocation in the plants. Overall, results from this study show that standard fertilizer practices for applying Ca have little to no impact on fruit Ca, yield, or fruit quality in floricane-fruiting raspberry.