Increased Leaching Requirements Allow the Use of Source Water High in Salts for Plant Growth

Growers have different capabilities to alleviate salt stress in the growing substrate. One method to reduce substrate salt levels is to increase the volume of water applied during irrigation. This increases the leaching fraction (LF) which is the volume of water that drains from the growing substrate divided by the volume applied during irrigation. We can determine the leaching requirement (the minimum LF to maintain a desired substrate salt level) using the formula LF = ECw/5(ECe − ECw), where ECw is the electrical conductivity (EC) of the water and ECe is the desired EC of the substrate. We tested this formula to see if we could maintain an acceptable substrate EC of 4 dS⋅m−1 by modifying the LF for ‘Hope’ philodendron (Philodendron selloum) and ‘Tineke’ ficus (Ficus elastica) irrigated with tap water (EC 0.17 dS⋅m−1) or reclaimed wastewater (RWW) from Davie, FL, USA (EC 1.66 dS⋅m−1) and RWW from Hollywood, FL, USA (EC 2.93 dS⋅m−1). Shoot and root dry weight was greatest for both species with the tap water applied with a 5% LF. Increasing the LF to 15% for Davie RWW and a 55% for Hollywood RWW, produced acceptable growth for ‘Hope’ philodendron and ‘Tineke’ ficus. In our second experiment, we monitored the growth of ‘Looking Glass’ begonia (Begonia fibrous), ‘Freddie’ calathea (Calathea concinna), and ‘Déjà vu’ philodendron (Philodendron selloum) irrigated with tap water (EC 0.15 dS⋅m−1), salt water (EC 3.49 dS⋅m−1), or RWW (EC 3.48 dS⋅m−1) with LFs of 28%, 50%, or 65%. ‘Looking Glass’ begonia and ‘Freddie’ calathea growth was greater with 65% LF than 28% LF, respectively, for all three water sources. Philodendron growth was not different due to LF. However, philodendron, calathea, and begonia growth was greater with tap water and RWW than with saltwater. Although final leachate EC with saltwater and RWW was around 2 dS⋅m−1 using 50% LF, leachate sodium (Na) levels from salt watered plants was higher than for RWW or tap watered plants. We suspect that high Na levels in combination with lower potassium (K) and calcium (Ca) levels in the saltwater solution resulted in poor plant growth. Although the Na levels in leachate from RWW substrates was higher than tap watered substrates, Ca and K levels also were greater. Although we were able to use the salt equation to maintain substrate EC levels ranging from 2 to 4 dS⋅m−1, volumes of solution applied were two to three times higher when using RWW or salt water compared with tap water. We suspect that a balance between Na, Ca, and K supported better plant growth with RWW than salt water. However, additional work needs to be done on the benefits of supplemental Ca and K when using water high in salts or Na. This works suggests that in addition to monitoring EC, it also is important to monitor Na, Ca, and K concentrations.