Replace Nutrient or Top Off? Is One Better? - A U. Florida Study Says Yes

One of the most frequently asked questions is how often to service nutrient in a hydroponic system and just how to do it. The primary choice is between completely changing nutrient every two weeks and adding nutrient without changing what it left in the reservoir. One argument against “topping up” with standard nutrient is speculation on the state of the nutrient solution before and after adding additional nutrient.

 

A research report in the journal HortTechnology of 14 October 2020 address some of these issues. Investigators Elisa Solis-Toapanta, Paul R. Fisher and Celina Gómez of the University of Florida studied “Bush Goliath” tomato plants in deep-water culture systems in a greenhouse, and in an indoor grow room. The plants were grown for 12 weeks in a 6.5-gallon system.

Two nutrient maintenance routines were used:

·         - Biweekly total replacement of nutrient

·         - Biweekly topping up with the same nutrient.

Tomatoes were chosen because of their challenging daily light interval needs and their responses to seasons when grown outdoors, making any difference in production according to different nutrient maintenance significant. Commercial growers manage their nutrients by applying considerable sophisticated expertise and automated monitoring, things that are rarely available to home growers. Thus, the study examines one of the most commonly contemplated home growing tasks.

In fact, in studying social media posts, they found that nutrient solution management was the third most common topic among subscribers interested in hydroponics. Their study compared final growth and nutrient uptake with the two alternative routines. I will pass over the technicalities of environment and other factors, all of which were held constant between the two management alternatives.

The fertilizer used was a 10-3.4-13.3 complete nutrient set with micronutrients. It was prepared using tap water. pH was adjusted to between 5.5 and 6.5 . In systems under both nutrient routines, water level was monitored daily, and systems were refilled with tap water whenever the volume dropped to ¾ of the total (4.8 gallons).

Multiple harvest were taken. Solution E.C. and pH were monitored and recorded along with the volume of refill water required to keep the volume up. (Unless controlled, volume becomes an unwanted variable.) The number of leaves (>1 cm) were counted and total leaf area measured. Stems, leaves and flowers of shoots were taken and weighed after drying. Tissue and nutrient samples were taken and analyzed. The number of mature, immature and blossom end rot affected fruits were counted and fresh weight recorded at week 12.

E.C. levels were measured after every two-week complete nutrient replacement and continuously for the plants getting the nutrient topping up routine. E.C. levels remained within adequate limits for tomatoes, 1.6 to 5.0 . More water was required to maintain volume in systems getting the complete nutrient changes, probably because of plants getting only nutrient topping up had lower nutrient concentrations, the remaining nutrient having been partly consumed, while plants getting complete replacements had greater leaf area and more transpiration.

The results included the fact that the routine of only topping up with nutrient resulted in less leaf area, more shoot growth, less water uptake, similar fruit number – but greater incidence of blossom end rot, delayed fruit ripening and lower fruit weight. By the 12^th week, nutrients under the complete replacement program were being almost totally depleted, suggesting that increasing nutrient concentration at the fruiting stage was desirable.

It is thought that the higher shoot growth and shoot weight in the group getting only nutrient top-up was a result of accumulation of nitrogen in the solution, since, as we know, high nitrogen favors foliage growth. Fruits were not evaluated for flavor, but higher E.C. levels are known to increase quality parameters such as sugar, lycopene and more solid fruit, and plants getting only nutrient top-up had lower E.C.

The routine of changing nutrient solution every two weeks resulted in great nutrient uptake. The tap water used was of high quality, but it is thought that water with higher dissolved ionic compounds would result in nutrient imbalances over time.

So much of what we surmise was confirmed. Regular total replacement of nutrient, which insures more or less constant proper balance produced better production. But it is interesting to note that it seems that many home growers report using a hybrid of these two routines by topping up with plain water once or twice and then replacing the whole volume of nutrient. This study suggests that such a routine is by no means disastrous, although total replacement is naturally better.

The original report can be read here:

 https://journals.ashs.org/horttech/view/journals/horttech/30/6/article-p697.xml