Interactions between ammonium and pH of the nutrient solution supplied to gerbera ( Gerbera jamesonii ) grown in pumice

Abstract

The interactions between the NH4-N/total-N supply ratio (Nr) and the pH of the nutrient solution delivered to the plants regarding the nutrient availability and its influence on plant growth and yield were investigated in a gerbera crop grown in pumice. Four treatments obtained by combining a low and a high Nr (0.02 and 0.13 on a molar basis, respectively) with two different levels of nutrient solution pH (5.0 and 5.8, respectively) were compared in a long-term glasshouse experiment. The low Nr resulted in significantly higher pH levels in the rhizosphere, as indicated by the values measured in the drainage water. A high nutrient solution pH further increased pH in the root zone at low Nr but had no influence on it at the higher Nr level. The high pH in the root zone restricted significantly the Cu, Mn, and Zn uptake as indicated by both analytical results and visual symptoms, which were typical for Cu- and Mn-deficiencies. Mn-deficiency was induced only when a low Nr was combined with a high nutrient solution pH. The Cu-deficiency symptoms appeared earlier, were more severe and occurred also when the low Nr was combined with a low nutrient solution pH, but to a lesser extent. As a result of these nutrient deficiencies, both the growth and the flower yield were severely restricted when Nr was low. These results indicate that gerbera is prone to Cu- and Mn deficiencies at pH values above 6 in the rhizosphere. The maintenance of root zone pH below this level in soilless culture is most successfully attained by properly adjusting the NH4-N/total-N supply ratio rather than the pH of the nutrient solution delivered to the plants.