The use of increasing proportions of N-NH4+ among the total applied inorganic N to improve acidification and the nutritional status and performance of blueberry plants in soilless culture

Abstract

The increased demand for blueberries and limited availability of low-pH soils have led to the increased use of soilless culture systems. The objective of this study was to evaluate the effect of a wide range of RNH4+ [RNH4+ = 100*N-NH4+/(N-NH4++N-NO3−)] values on the acidification of the growth medium and the nutrient status and performance of blueberry plants. RNH4+ treatments of 25% (AN25, control), 50% (AN50) and 100% (AN100) and a 25% N-NH4+ plus concentrated sulfuric acid (AN25-acid) treatment were applied by fertigation to southern highbush blueberry plants (Vaccinium corymbosum, cv. Sunshine Blue) grown in a soilless substrate (50% tuff, 25% peat, 25% coconut coir). The original solution for the AN25-acid treatment was adjusted to a constant pH of 4.5. The pH and mineral concentrations in the leachate, pH of the growth medium and mineral and chlorophyll concentrations in the leaves were monitored periodically. Plant water uptake was monitored periodically and foliage volumes were calculated at the end of the growing season. Application of high levels of RNH4+ decreased the pH of the leachate and medium to below 5.5 and increased the concentrations of Fe, Mn and Zn in the leachate, as compared to the control treatment. Those treatments also increased the chlorophyll concentrations, total plant water uptake, foliage volumes and leaf Mn concentrations relative to the control treatment. The effects of the high-RNH4+ treatment on acidification and plant performance were not significantly different from the effects of the commonly used AN25-acid treatment. It seems, therefore, that the increased RNH4 reduced the pH of the growth medium to the required level for blueberry production, providing a safer and more environmentally friendly alternative to the use of sulfuric acid.