pH control in fertigation
The goal of fertigation is to increase crop yield and reduce the costs of purchasing and applying fertiliser by controlling the chemical characteristics of the irrigation solution. By establishing a specific pH level in order to increase the system’s efficiency, we can boost nutrient uptake, prevent dissolved fertilisers and salts from precipitating in the irrigation water and help keep the irrigation system in good condition.
Proper automation of this control ensures a completely uniform process, as the chemical characteristics will not be affected by differences in the flow rates in different sections of the irrigation system or those caused by the heterogeneity of the water.
pH control
pH indicates the acidity of basicity of a solution. Its value ranges from 0 (acidic) to 14 (basic), with 7 being neutral.
The pH of a nutritional solution can affect the plants’ growth in two main ways:
- The availability of nutrients, as extremely high pH values result in some nutrients precipitating and taking on a form that cannot be absorbed by the plants.
- The ability of the roots to absorb the nutrients. Each type of plant absorbs nutrients optimally at a different pH level. Outside this range, the roots find it difficult to take on the goodness. If there is too large a deviation, the root system can become damaged or the soil may become toxic.
P, PI and PIQ adjustment
Proportional (P) algorithm approximation systems establish an injection flow rate that is proportional to the difference between the reading and the setpoint. These systems can be used to control closed circuits, such as tanks and swimming pools. When they are used in an irrigation line, we have to take into account the proportionality factor, alarms and hysteresis, and they frequently return a pH value that varies between two values.
Another system is PI (proportional-integral) approximation. Here, the algorithm doesn’t just include the degree of difference between the reading and the setpoint, but it also includes the errors that have previously been returned. In simpler terms, it remembers what the pH was when we injected a certain flow rate and it acts accordingly, obtaining readings with greater stability and accuracy.
The graph below shows the results of the use of these algorithms in adjusting the pH of a stable water flow:
The graph below shows the results of the use of these algorithms in adjusting the pH of a water flow, reflecting its behaviour when faced with a change in flow rate and, as a consequence, the amount of reagent needed:
Finally, we have the PIQ control method for adjusting the pH. This is a PI algorithm that has been conditioned to the flow rate used.
Using the pH parameter, it establishes an initial dose in proportion to the water flow rate, before correcting it using a PI approximation curve. This system guarantees pH stability even with highly variable flow rates, and it provides more agility in reaching and maintaining the setpoint.
Using the pH parameter (the initial pH setpoint), it establishes an initial dosage as a percentage of the water flow rate, before correcting it using a PI approximation curve. The dose of acid will be proportional to the water flow, adjusted to the setpoint using a PI algorithm. This system combines the benefits of proportional dosing with pH setpoint dosing, helping guarantee the stability of the pH reading even with highly variable flow rates.
The graph below shows the behaviour of these algorithms in a variable flow rate:
The proportion of reagent needed to reach a certain pH will depend not only on its type and concentration, but also on the type of water being treated and the other products applied to the water that may impact this parameter.
