Effects of enhanced NH4+-N supply and concomitant changes in the concentrations of other nutrients needed for ion balance on the growth, yield, and nutrient status of eggplants grown on rockwool

SummaryEggplants (Solanum melongena L.) grown on rockwool in a glasshouse were supplied with a nutrient solution (NS) containing 1.0 mM NH4+-N, or three NS containing 4.0 mM NH4+-N. In one of the high NH4+-N treatments, the K, Ca, Mg, and NO3–-N concentrations were reduced to maintain the same electrical conductivity (EC; 2.0 dS m–1), total-N (14.25 mM), and K:Ca:Mg ratio as in the low NH4+-N treatment. In the other two high NH4+-N treatments, the K, Ca and Mg levels were not altered, thus the extra NH4+-N elevated the EC to 2.3 dS m–1, while the NO3–-N was either reduced to maintain the same total-N as in the low NH4+-N treatment, or not altered, in which case the total-N rose to 17.25 mM. The concentrations of P, Fe, Mn, Zn, Cu, B, and Mo were identical in all four treatments, while in the high NH4+-N treatments, ion balance was achieved by adjusting the concentration of SO42–. The increase in NH4+-N from 1.0 mM to 4.0 mM in the NS resulted in similar increases in NH4+-N in the root zone, as indicated by measurements of the drainage solution. High NH4+-N levels in the root zone restricted both the vegetative growth and fruit yields of eggplants by 17 – 30% and by 25 – 34%, respectively. The greater reduction in fruit yield was observed when the elevation in NH4+-N concentration was accompanied by an increase in EC, due to the additive effect of salinity. The adverse effects of an increased supply of NH4+-N may have been associated with a reduction in pH in the root zone and a fall in Mg levels in plant tissues. The increased supply of NH4+-N enhanced the concentrations of total-N, P, Mn, and Zn in fruit and leaf tissues, and those of Fe and Cu in leaf tissue. However, this effect was not beneficial to eggplant, in terms of growth and yield, because 1.0 mM NH4+-N proved sufficient to maintain adequate levels of these nutrients in leaves and fruit. We conclude that the increase in NH4+-N to 4.0 mM in the NS supplied was harmful to soilless eggplant crops. However, concomitant changes in the concentrations of other nutrients that are required to balance the extra NH4+-N supply may act synergistically if they increase the total salt concentration in the root zone.