Calcareous soils are not suitable for blueberry cultivation. Our aim was to improve the performance of blueberry plants in calcareous soils by using pits filled with growth media in combination with high levels of RNH4+ (proportion of N-NH4+ among the total applied N). Rabbiteye blueberry (Vaccinium virgatum Ait. cv. Ochlockonee) plants were grown in pits filled with a tuff/peat mixture (TP), a sandy soil (S) or a calcareous (CC), in full factorial combination with three levels of RNH4+: 33%, 66% or 100%. The two higher RNH4+ treatments decreased the pH of the low-CaCO3 (S) and no-CaCO3 (TP) media to ≤6.0 over 250 days of fertilization, but did not affect the pH of the CC soil over 650 days. Plant performance was superior in the TP and S media, as compared to the CC soil. The type of growth medium was the dominant factor accounting for the improved plant performance. The plants were sensitive to Mn deficiency in leaves during the spring period. The current results suggest that growing blueberry in pits filled with good aeration and low pH buffering capacity medium in combination with a high level of RNH4+ is a positive approach for its cultivation in calcareous soils.

The appropriate soil pH for blueberry production is <5.5. However, the exact factors involved in the retardation of blueberry growth at higher pH levels have not yet been completely identified. In this study, we examined the short- and long-term combined effects of pH level and Ca concentrations on root morphology and development in relation to blueberry plant performance. Southern highbush blueberry (Vaccinium corymbosum, cv. Windsor) plants were grown in solutions of pH 4.5 or 7.5 that contained 0, 5, 10, 50 or 100 mg L-1 of Ca. The dimensions of the epidermal cell walls of the roots and the levels of pectin and hemicellulose 1 and 2 in those cells were determined after 1 and 3 days. Concentrations of essential macro and micro-elements in the plant organs, chlorophyll concentrations in the leaves and whole-plant biomass production were measured after 12 weeks. The results demonstrate a rapid (3 days) negative response of root tips at pH 7.5 expressed as reduced dimensions and deformation of the epidermal cell walls. This response was accompanied by reduced concentrations of pectin and hemicellulose 1 in those cell walls. These negative effects of pH 7.5 were amplified by a 100 mg L-1 of Ca over the long term (12 weeks), where the concentrations of P and Ca on the epidermal cell of the root were increased. Additionally, the translocation of Mn from the root to the shoot was retarded. The negative effect of the pH level on roots development and on the Mn translocation were associated with significant reductions in chlorophyll concentrations and biomass production. The results presented in this study demonstrate, for the first time, the negative effect of the combination of slightly alkaline pH and a high Ca concentration on root development, which associated with retarding blueberry plants performance.