RTW/DTW Hydroponic Growing Systems V. Recycling Systems
Preferential Nutrient Uptake – nutrient depletion of some ions and buildup of others
Plants require a wide range of nutrient ions to support growth. Each of these specific nutrient ions has an ionic sufficiency range in which growth is optimized. The uptake and utilization of nutrients depends not only on the quantities but also on the ratios among nutrient types. If a particular nutrient is deficient, yields can be negatively impacted. A similar reduction in plant growth can arise when a particular nutrient is present at a concentration that is too high. All the nutrient ion types need to be within their respective ranges if plant productivity is to be optimized. Departure from these optimal levels in any of the nutrient ion types will have an influence on all the others as well.
One significant negative of the closed/recycling hydroponic system is that plants preferentially remove nutrient ions from solution at differing levels. For example, plants remove the elements nitrogen (N), phosphorous (P), potassium (K), and manganese (Mn) at high rates and these elements can be removed from solution in a few hours. Other elements such as magnesium (Mg), sulphur (S), iron (Fe), zinc (Zn), copper (Cu), molybdenum (Mo) and chloride (Cl) have lower uptake rates and are usually removed from solution slightly faster than water is removed. Lastly, calcium (Ca) and boron (B) are passively absorbed from solution at low rates and often accumulate in solution and substrates.
Therefore, in closed hydroponic systems because some nutrients such as N, P, K are preferentially removed by plants at high rates while others such as. Ca, S and Mg are taken at lower levels the nutrient that was originally placed in the system can become greatly altered over a relatively short period of time. Put simply, preferred nutrients get depleted while less needed nutrients accumulate in the working solution. This can quickly lead to deficiencies in some nutrients and excesses of others. As Savvas et al. (2009) puts it, “an imbalance in the nutrient solution is generated by excesses of the ions least consumed by the plant, normally SO4–, Ca2+ and Mg2, which disrupts the balance of the nutrients.” 
This situation creates a problem for novice hydroponic gardeners who have limited understanding of monitoring and correcting the nutrient solution in closed/recycling hydroponic systems. For example, in agricultural settings experienced hydroponic growers may use specific nutrient ion monitoring equipment or run lab analysis on the nutrient solution and/or plant tissue in order to ensure their crops are receiving ideal nutrition. Additionally, based on analysis, experienced professionals might dilute the nutrient solution to reduce elements such as calcium and magnesium building up while also topping up depleted nutrients through adding specific fertilizers to solution. For example, if N, P and K are lacking they can add small quantities of N, P and K containing fertilisers, such as potassium nitrate and monopotassium phosphate, to correct the situation. Under these circumstances, open/recycling systems are shown to perform extremely well (on par to RTW/DTW systems) where plant health, growth rates and yields are concerned.
However, novice gardeners, who have limited understanding of plant nutrition and who purchase full spectrum liquid fertilisers from hydroponic stores, do not typically apply the same level of expertise as their agricultural counterparts, and as deficiencies and excesses occur they go unnoticed and/or uncorrected. As a result, the nutrient becomes imbalanced and growth rates are negatively impacted.
For example, many novice growers are under the impression that an EC reading tells them if their plants are receiving adequate plant nutrition; i.e. If optimal EC is 2.5 and my EC is 2.5 all is good right? Not really! This comes back to the point we covered when discussing EC being somewhat of an imperfect measurement earlier in the book. That is, no EC meter has the ability to distinguish between different types of nutrient ions. This means the use of EC measurement is only helpful in checking total salt concentrations in the solution, but the concentrations of individual nutrient ions may vary considerably from the desired concentration. This is because EC only tells us the relative amount of total salts (ions) and nothing about each specific nutrient concentration in solution. So, for example, the true concentration of N, P and K may be lacking, while nutrient ions such as Ca, S and Mg become too high, even though the EC is within ideal range.
Open/ RTW/DTW Growing Systems Provide Better Nutrient Status without the Expertise
In open/RTW/DTW growing systems nutrient from the reservoir/tank is fed to the plants and the nutrient and water that isn’t taken up by the plants or absorbed by the substrate is not returned to the nutrient tank/reservoir, but instead runs off into a catchment tank/reservoir as waste. This waste is then disposed of and not run through the system again. What this means is that the nutrient being fed to the plants is not altered by selective plant uptake and then returned to the tank/reservoir. Therefore, nutrient deficiencies and/or excesses are unlikely to occur and any such nutrient excesses or deficiencies in a RTW/DTW system would be a result of adding an imbalanced nutrient or too little or too much nutrient to the tank/reservoir in the first instance. This is just one reason why I tend to promote RTW/DTW growing to novices. Quite simply, RTW/DTW growing, when handled correctly, promotes a better nutrient status, more easily, than recycling does.
Pathogen/Disease Transmission in Closed/Recycling Hydroponic Systems
Another concern related to closed/recycling hydroponic systems is the potential for the spread of root pathogens.
This is because the plants in a closed hydroponic system share the same nutrient solution and if a single plant becomes infected by root pathogens (e.g. Pythium) the disease can easily spread throughout the entire system, via the shared nutrient solution, and infect other plants. 
Risk of Root Disease Transmission is reduced in Open/RTW/DTW Systems
Growing RTW/DTW significantly reduces the risk of root zone pathogen cross contamination between plants when compared to growing in closed/recycling systems. This is because the plants aren’t being fed a recycled nutrient that can become contaminated by a single diseased plant and then the disease is spread throughout the crop as a shared nutrient comes into contact with all other plants in the system.
More Nutrient and Water Usage in Open/RTW/DTW Systems than Closed/Recycling Systems
Open hydroponic systems use more water and nutrients than closed systems. For example, one study that compared closed and open hydroponic systems showed the average water and nutrient saving when growing tomato in a closed/recycling system is 24 % water and 34 % nutrient. This is one of the few disadvantages to growing RTW/DTW. Ultimately, you will go through more nutrients and use more water over the course of the crop cycle.
Lower Nutrient Tank/Reservoir Maintenance Time in Open/RTW/DTW Systems
One benefit of growing RTW/DTW, when compared to closed/recycling systems is daily nutrient tank maintenance time is reduced. For instance, what I do is set up a 200L nutrient tank/reservoir and feed it onto another 200L tank through a float valve system (400Ls in total when a fresh batch of nutrient is made up). This means I can stock up on a lot of nutrient working solution. Because the solution isn’t recycled, EC checks, water and nutrient top ups, and nutrient dumps aren’t required (although I do check the solution pH every day).
Coming to Terms with Feeding Times can be More Difficult in Open/RTW/DTW Growing than in Recycling/Closed Systems
One of the key operational differences between recycling/closed and open/RTW/DTW systems is how you regulate the feed regime.
In a closed/recycling system you are giving the plants large feeds at set intervals which tend to remain static/set and unchanged throughout the entire course of the crop cycle. Whereas, in RTW/DTW you are typically giving the plants small feeds at interspersed intervals and increasing the level of feed to accommodate to crop demand. This demand starts off low (where plants are young and small) and steadily increases as the plants grow.
Thus, the feed times in closed/recycling systems are typically the same regardless of the stage of growth, or how large the plant is. Conversely, in an open/RTW/DTW system you are constantly changing the feed times to accommodate for the size and vigour of the plant/s, which ultimately dictates water and nutrient requirements.
In simple terms, the governing/measuring factor in open/RTW/DTW is not the feed time, but the amount of waste collected after the irrigation solution has passed through the substrate. Conversely, the key to feed times in open/recycling systems is following very simple instructions as to say e.g. feeding every two hours for 15 minutes.
This means that controlling ideal feeding in open/RTW/DTW growing tends to be a bit harder for novice growers to come to grips with than it is in recycling.
Finer Control of the Nutrient Supplied in Open/RTW/DTW Provides More Ideal Levels of Water and Nutrient than in Closed/Recycling Systems
Fertigating correctly in RTW/DTW growing comes down to measuring what is fed to the plant/s and then measuring the run off/waste that comes out of each pot/plant after passing through the substrate/root zone with a set percentage of waste in mind. So, for example 10 – 30% waste is typically stated as optimum, dependent on the quality of the source water supply. For example, if very pure RO water is being used as source water then 10 – 15% waste is sufficient while if mains water, which contains impurities such as sodium and chloride, is used as the source water supply it may be desirable to run at 25 – 30% waste. Therefore if say our desired amount of waste were 10% and we had 10 plants which we fed a total of 10 litres (1L per plant) of nutrient solution over the course of the day we would want to collect one litre of waste (i.e. 10 litres (amount fed) x 10% (desired waste) = 1 litre).
No matter how much we feed the plant/s, the waste is always our guide and as long as we collect our desired 10% waste for whatever volume we are feeding the plant/s, this tells us that we are meeting the requirements of the crop and feeding correctly.
Therefore, in open/RTW/DTW growing the plant tells us how much to feed through the amount of nutrient and water it doesn’t uptake/use. This unused nutrient becomes waste (runoff) which we collect and measure with a set percentage of waste in mind. What this means is that the plant/s tell us exactly how much nutrient solution they require. In turn, this means that we have a high level of control and are precisely able to meet the water and nutrient demand of the crop through measuring waste.
Comparatively, in some closed/recycling systems, such as satellite and ebb and flow systems, novice growers are typically given very simple instructions such as e.g. turn the pumps on for 15 minutes every hour or two, regardless of the nutrient and water demand of the crop. This can mean that at some points of the crop cycle plants are being ferigated too frequently while at other points too infrequently. Basically, because there is no accurate way to measure the nutrient and water demand of the crop through measuring waste, feed times are almost handled haphazardly by some open/recycling system growers.
Feed Requirements of RTW/DTW versus Recycling
Because critical nutrient ions can become quickly depleted in recycling hydroponic systems, studies have shown that plants grown in RTW/DTW systems require less nutrients in solution (lower EC of nutrient in the tank/reservoir) than the same crops that are grown in recycling systems.
For example, in research by Gül et al (2008) it was shown that optimal growth with tomato could be achieved in a RTW/DTW system with a half strength nutrient solution when compared to a recycling system. This particular study concluding this may be the result of the depletion of some elements in the recycling system.” A study by Ferrante el al (2000) came to similar conclusions. It was shown that nutrient concentrations in the substrate are lower in recycling systems than in RTW/DTW systems.
Therefore, the nutrient requirements of RTW/DTW systems differs from that of recycling systems and nutrients in RTW/DTW can be run at a lower EC than where recycling.
For example, I tend to recommend running an EC of about 2.8 in full bloom where growing in a recycling system. However, in RTW/DTW growing I tend to recommend optimum EC at about 1.8 where several irrigation events take place over the course of the day. Or, where only a single irrigation per day is applied, I tend to recommend an optimum EC of between 2.2 – 2.4 in RTW/DTW.
This brings us to our next point; how the fertigation frequency influences what would be considered the ideal EC of the nutrient solution in RTW/DTW growing.
 Bugbee, B. 2004. Nutrient Management in Recirculating Hydroponic Culture. In: Proceedings of the South Pacific Soilless Culture Conference. M. Nichols, (ed.). Acta Hort 648: 99-112.
 Savvas, D., Sigrimis, N., Chatzieustratiou, E., & Paschalidis, C. (2009). Impact of a progressive Na and Cl accumulation in the root zone on pepper grown in a closed-cycle hydroponic system. Acta Horticulturae, 807, 451-456.
 Postma, J., Van Os, E.A. & Bonants, P.J.M. (2008). Pathogen detection and management strategies in soilless plant growing systems. In: Raviv, M. & Lieth, J.H. (eds) Soilless culture – theory and practice. USA: Elsevier BV, pp. 425-458.
 Tuzel, I.H., Tuzel, Y., Gul, A. Meric, M.K. Yavuz, O. Eltez, R.Z. (2000) Comparison of Open and Closed Systems on Yield, Water and Nutrient Consumption and Their Environmental Impact
 A. Gül, F. Kıdog˘lu, Y. Tüzel1 and I. H. Tüzel (2008) Effects of nutrition and Bacillus amyloliquefaciens on tomato (Solanum lycopersicum L.) growing in perlite
 FERRANTE A., MALORGIO F., PARDOSSI A., SERRA G., TOGNONI F., 2000. Growth, flower production and mineral nutrition in gerbera (Gerbera jamesonii H. Bolus) plants grown in substrate culture with and without nutrient recycling. Adv Hortic Sci 14(3), 99-106.