There was evidence of excess SO4 in the control solution
in Period I and in the substrate in Period II. This was likely
due to the manual addition of extra (NH4)2SO4, and also of
Na and Cl especially in the SUBBOR solution (both periods,
Table 2) and in the substrate (Period II, Table 4). Salt
buildup can be mitigated by periodic flushing of the substrate
in situ every 4 weeks (Resh, 1989) and/or by periodic
emptying of the tanks (Zekki et al., 1996). In between the
two test periods of this study, the tanks were emptied
and washed. As Period I was approximately 4 weeks, this
would have been insufficient time for significant salt accumulation
in the tanks and in the substrate. Fortunately,
any excess in salts was moderate and the tanks were emptied
and replenished with fresh solutions at the start of Period
II. Throughout the study, there were no signs of
nutrient toxicity or deficiency.