The interactions between the NH4-N/total-N supply ratio (N r) 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 N r (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 N r 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 N r but had no influence on it at the higher N r 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 N r was combined with a high nutrient solution pH. The Cu-deficiency symptoms appeared earlier, were more severe and occurred also when the low N r 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 N r 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.
