Fertigation is the irrigation of plants with nutrient enriched (and probably pH corrected) water. A full fertigation controller is thus a combination of a multi-station horticultural irrigation controller and a fertiliser injection system.
Fertigation can be achieved in a number of ways including a batch dosed tank, In-line injection or In-line injection with small mix tank.
Each of these methods has its pros and cons as follows:-
Batch dosed tank. This method is simpler, has the potential for more buffering and manual intervention in the event of failure but is really only suited to a single feed recipe and common EC for the complete crop. It also requires more time for refilling and dosing the tank between irrigations and this will limit the number of valve zones (stations) that it can handle.
In-Line injection without a mix tank is only suited to situations where the raw water is of adequate quality. This is slightly simpler in operation as compared with an in-line mix system and less prone to faults with mix tank overflows etc. In-line injection can have multiple injection channels allowing recipe changes on the fly so that different valve zones receive different feeds to suit a variety of crops or else the same crop at different stages of growth. If the water is pretreated then this is a very good choice.
In-Line injection with mix tank is similar to the above but the mix tank allows a little time for chemical reactions to occur and for CO2 to gas off and precipitates to form so they are caught by the filter before reaching the drippers.
A modern fertigation controller like the MULTI will operate in any of the above modes and will normally have a variety of trigger sources to start irrigations including time-of-day programmed irrigations, radiation sum (solar integration) triggering with the ability to modify the integration rate to compensate for humidity, temperature and wind speed. The solar integrator (with temperature and humidity modifiers) will automatically increase the frequency of irrigations in sunny, hot dry weather and reduce it in dull, cool, damp weather. A rain override is useful for outdoor crops as it can be set to zero the integration counter if rain exceeds a specified level.
These controllers may generate a single irrigation trigger and then sequentially irrigate all stations in turn. The Autogrow MULTI controller has multiple trigger groups so that any combination of irrigation valves can be assigned to any trigger group. Each trigger group can have completely different settings for radiation sum, start and stop times etc. This allows crops with differing water requirements to be serviced by a common controller. In fact with the MULTI it is possible to mix inside with outside crops and crops requiring watering every half hour with those that need watering once per week. Of-course each group can be set to have a different EC and recipe.
It is important that the solar integrator, fertiliser injection and irrigation functions are all within the same controller as this provides the highest level of functionality. For example, if a separate solar integrator was used, the manual triggering of an irrigation will not be “seen” by the solar integrator and so its counter will continue counting. The solar integrator will not have its counter cleared and may trigger a second irrigation immediately after the manual one. In the case of an integrated controller this will automatically be taken care of.
Using an electronic controller employing feedback control allows a number of different methods to be used to actually get the fertiliser into the pipe. The first and most obvious method uses variable speed injection pumps to inject direct into the line. This becomes very expensive as soon as larger flow rates are required. The second method uses venturi injectors to entrain the nutrient and pH stock solutions into the flow of water through them. To adjust the rate of nutrient uptake it is common to use simple on/off solenoid valves that continuously pulse on and off. By varying the ratio of the on time to the off time, good proportional control can be achieved. This method is usually called “pulse width modulated” (PWM) control. To work reliably the venturis need a significant pressure drop across them and this may lead to an auxiliary pump being required to develop this. This is a simple robust method but may require high pressure sensors in the line and also the system design must ensure satisfactory mixing occurs before the concentrated stock solutions are allowed to mix with each other. If they meet in concentrated form a chemical reaction can take place which will change the formulation of the fertiliser. In addition, the resulting precipitation can cause solids that can later block drippers and be a general nuisance. The mixing is normally achieved by using a filter in the recirculating "cyclone" loop.
The last method uses an in-line turbulent mixing vessel into which the fertiliser stock solutions are dosed. This method provides excellent mixing and has the advantage that the dosing and sensing is done at ambient pressure. Typically a drum of one-to-two hundred litres capacity is used and is filled at the same rate as it is being emptied.