Horticulture projects Irrigation design considerations

The purpose of this article is helping new horticulture project initiators choosing the best irrigation deployment and understanding the terms used by irrigation designers. In many cases I find that while relaying on supplier’s recommendations is likely to be sufficient for water distribution, when it comes to Fertigation the case is not that good. The focus in this article is on high-precision projects such as Nurseries, Propagation, Low-volume Hydroponics and Cannabis, all based on clear criteria. Terms glossary first:

Injection methods:

Client will require the ability to inject one or more Recipes of different nutrients coming from 4-6 stock tanks, and then to balance the pH to the level best for the plant (mostly 5.8-6.5). Those stocks might chemically react if ‘meet’ at high concentration, therefor a proper dilution must be obtained. The common machines for this purpose are:

Venturi Bypass array

The CFU takes a portion of the mainline water through the Venturi bridge where the different stocks are injected. The Venturi tube itself ensures dilution rate of 1:3 to 1:4. I strongly recommend to carefully review the dilution rate as more than once I found reaction occurred creating ceramic sediment that is difficult to remove.

Venturi In-Line array

Similar to the Bypass arrays, but in this case the entire main-line flow goes through the Venturi bridge. Those CFU’s are easier to install, and the dilution rate is much safer, but the main-line flow range is limited to narrower range.

Mixer (always In-Line)

Here the main-line water is spilled into a small tank, and a pump is repumping it to the field. Fertilizers are added to that tank ither by a Venturi bridge or other devices (metering pumps etc.). The down sides are energy waste of double pumping and delay on recipe interchanges. There are some advantages that will be discussed farther on.

Injection devices:

Metering pumps and Waterpower driven pumps (Dosatron, Amiad, TEFEN) are here while ago, but seldom used on Horticulture projects due to their low accuracy and high service demands. The Horticulture fertigation is dominated by the Venturi type. Venturi injector provides 3 important benefits: a) There are no moving parts or any mechanism at all, b) it’s the only type that is not affected by air intrusion, c) inherent dilution of the injected media. However, the volume to inject must be controlled in the most possible accurate way. There are 2 options:

Pulsing Electric valve (Dosing valve):

Suction power is present at the injector C port all through the irrigation. This suction power is obviously stronger than needed and therefor must be controlled. A special Electro-valve is installed at the suction inlet (port C) who can be opened intermittently at pulses as short as ½ a second.  The governing controller, being aware of the suction velocity (parameter that set by the operator) can calculate the total seconds ON per 1 m3 assuming off-course that the main-line flow is measured or calculated.  This total ON time per m3 time is optimized to short pulses achieving the best possible distribution of the dose over the m3.

The distribution importance is clear: pH & EC levels are on-line measured, so any OFF time longer than 3 seconds is reflected on its reading and consequently cause imbalance. This fact is a reason for a significant limitation of this method: Each Venturi suction velocity must be optimized to the main-line planned velocity so that total ON time is 50% to 75% of the M3 time.

For example: at main-line flow of 36 M3/H each M3 duration is 100 sec. For a recipe of 3.5 L/M3, 50% is 3.5L in 50 sec  , and 75% is . In such case if one of the valves will have lower main-line flow the controller will have to open larger gap between pulses. The pH & EC sensors will read the ‘gaps’ and the controller algorithm will amplify it by Overshoot – Undershoot responds.

Another problem of pulsing devices is mechanical challenges with measuring the fertilizer quantities. Standard fertilizer meters become wildly inaccurate (up to 40%!) hence most suppliers prefer to relay on calculation by ON time and avoid metering altogether.

The alternative: Analog valves.

This type of CFU uses actuated valve and the Venturi set’s suction flow is now a range from 0 to some maximum value (500 to 1,500 ). This set does not have the ability to get the user parameter for its flow as this is constantly changing, hence must have a reliable flow sensor (magnetic, Ultrasonic or Vortex) and becoming an independent control cycle that receives target flow from the main algorithm and keeps it. Some attempts were made to govern the control by EC or pH alone but the result of those CFU’s were not sufficiently balanced. Galcon’s product does the job, but the Belimo actuator is slow and it is more suitable for mainline flow > 100 m3/h.

The Mixer:

Fertilizer injection into the mixer tank uses pulsing devices as well, but here the distribution is natural. Even 10 second gaps will not pose a problem in some cases. For this reasons Mixers assigned when mainline flow ranges are wider. However, this range is limited too, and the rule is that the mixer’s tank volume is replaced from 30 seconds to 150 seconds.

Mixer tank volumeMin mainline flowMax mainline flow
200 liters4.8 m3/h24 m3/h
500 liters12 m3/h60 m3/h
1,000 liters24 m3/h120 m3/h

Mixers used to be the default technology up until 1995. Electronic and computing levels were not advanced enough for accurate on-line injection back then. Nowadays unavoidable large mainline flow variety is the only excuse.

Irrigation lines deployment

There are 2 familiar methods for pipes deployment, and we will review here a third and special option.

Single line

The classic design: ‘tree’ like system where one pipe leaves the CFU and distributes the water to the different irrigation blocks based on the opening sifts requirement. Pipes diameter will mostly defined not to exceed 1.7 meter/second flow velocity.

This is the best economy deployment which fulfills the requirements on 90% of the cases. However, the total volume of the water in the pipe system is not negligible, so for cases where there are significant differences of fertilizer recipes, what happens is that blocks will get the wrong recipe at the first few minutes of irrigation. It becomes more acute problem if the irrigation cycle itself is short in time (Hydroponics).

Multi line

In non-homogenous projects, especially with pulse irrigation, separating the pipes right at the CFU outlet is recommended. Pipes in such project are of smaller diameter but it is still more expansive and much more complicated. Each valve (or small group of identical blocks) has a pipe stretched all the way from the CFU to the block. The valve itself can be ither near the CFU or near the block or both.

Circular system

On rear cases, mainly nurseries with beam irrigation where sometimes recipe will change every few meters of movement, we face a structural problem: The CFU is always far enough from the beam jets so users has to operate it for a while, static on neutral place, till the right recipe reaches.

A creative solution here is proposed, where every pipe is returning to the CFU (from the beam inlet) enabling the refreshment of the water constantly. But to avoid wasting the returning water the system must be based on Day Tanks deployment, so let’s discuss Day Tanks first.

Day tanks

Day tanks was a common deployment in Northern Europe as Horticulture started to develop and was driven from the limited and inaccurate control devices at that time. The need for safely balanced Fertigation mixture is satisfied by a set of large tanks that are prepared during the evening for being used on the following day. The delay allows corrections, sampling for analyses and pH balancing. Each tank must have its own irrigation pump what makes the whole setup both expansive on initial investment and energy and maintenance costly. On the other hand, users doesn’t have to compromise due to irrigation cycle limitations. With advanced controllers and injection devices this method turned to be redundant, but there are some rear cases where I still recommend it.

The first case is in projects characterized by low volume (500 liters or less) sessions, even if just part of them. Places where few pots or growing bags might need a separate treatment. There is absolutely no modern CFU that can provide a good accuracy in such small sessions. Among those cases you can find Cannabis projects, Nurseries, and mother-plants such Ivy, Strawberry etc.)

The other case is the above-mentioned circular method: The pipes volume that need to be evacuated can be very high and you want to let it go back to the tank it came from.

In both cases can work the tanks doesn’t have to be large: 2,000-5,000 black PE tanks are good and the pumps at their outlet not larger than 2.5 kW. In older full-size day-tank projects each tank was 100-250 m3.

But there is one case where this practice is required just like old days: When the source water content is high on bi-Carbonates or other Alkali agents. In some places the Alkali content of the water is so high that it is not possible to balance the pH on-line. You inject the acid, and it seems like you reached the required pH, but when you check down the line, or by the dripper, you find it to scale up again simply because the chemical reaction with the acid hasn’t completed at the CFU. The day-tank (it can be a single lagoon for the whole project) is circulated through the CFU all night with just a pH balancing program, so that by the morning it is well balanced and can be used.

Selection rules:

The following table helps to identify your private case and the recommended technology.

*if ‘Mixer’ is not selected in CFU type.

Blocks in gray are cases that this particular technology/deployment has no advantage but selecting it will not have negative effect on the performance.

The table above is a general guideline that cannot cover all the cases. The undersigned will be happy to consult farther if needed.

Over the years I have seen many projects that could have been done better or cheaper. Looking into the reasons I mostly find ither:

  1. Initiators or their consultants are tending to follow nearby / familiar technology without deep study of the circumstances.
  2. Irrigation equipment providers recommendation that are biased to what they have in hand or what leaves them better profit.

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