In fertigation, fertilizers are delivered to the plants through the irrigation water.
The pH of the irrigation water is of great importance, as it affects many chemical reactions.
In fertigation, the reasons for adjusting the pH to an optimal range are:
The pH affects the availability of many nutrients.
The plant can absorb only nutrients that are present in the nutrient solution or in the soil solution. When a nutrient “precipitates out of the solution”, it is no longer available to the plant.
Precipitation, in this context, refers to a formation of a mineral. Calcium and magnesium carbonates, iron minerals and other compounds will tend to precipitate at neutral to basic pH level (pH > 7.0). Once precipitated, the nutrients are no longer available to the plant.
Solubility of Calcium Phosphates
In soilless culture and in sandy soils it is very important to keep the pH of nutrient solution at an optimal range for nutrient availability. Normally, a pH of 5.5 to 6.5 is considered to be optimal for most crops.
When growing in soils, the pH of the irrigation water will not always affect the soil pH. In fact, in most of the times – it won’t, especially in soils with high carbonate content.
Precipitated minerals form deposits which might accumulate in emitters (e.g. drip system) and clog it. As we mentioned above, the pH of the nutrient solution will determine whether precipitation will occur, depending also on the composition of the nutrient solution and on the concentration of elements in the solution.
The ability of an acid to decrease water pH depends on the following:
Number of H+ (protons) in the acid.
HCO3 and CO3 concentrations in the water
Initial pH of the water.
When adding acid to the water, most of the protons will first react with the carbonates in the water and some will stay free in the water and lower the pH. As more acid is added, the carbonates concentration in the water will go down, while the pH will remain nearly constant, or will decrease slowly.
Once all carbonates in the water react with the acid, any additional amount of acid will sharply lower the pH.
The main equations that describe the relationship between carbonates and pH are:
pH = pKa + log(HCO3/H2CO3) pKa=6.37
pH = pKa + log (CO3/HCO3) pKa=10.33
The best practice is to bring the pH to its optimal range, while keeping a certain concentration of bicarbonates in the water, in order to avoid drastic drop in the pH. The accepted concentration of bicarbonates to be left unreacted is about 60 ppm (approximately 1 meq/l).
By definition, any application of acid to the irrigation water is done by fertigation.
The injection of acid to the irrigation water should be, as much as possible, uniform and continuous, throughout the entire duration of the irrigation.
In automated control fertigation systems – if the acid in the tank is too concentrated it will result in fluctuations in the pH of the nutrient solutions, i.e. the pH will go up and down sharply, while the controller tries to stabilize the pH to the desired level.
In fertigation systems, without control – uniformity in the acid injection can be achieved by adjusting the injector flow rate in correlation with the volume of the acid stock solution.
In the drawing above, we can see three different injection patterns. The average pH is the same in all three. However, pattern B will result in a more stable and uniform pH of the irrigation water.
Calculation of acid injection in SMART! software. Injection by time and injection by ratio.
150 ml acid per 1 cubic meter (m3) of irrigation water are required to lower the pH to the desired level (150 ml/m3 = 1.92 oz/100 gal)
The flow rate of the acid injector is 50 l/hr (=13.2 gal/hr).
Irrigation flow rate is 20 m3/hr (5283 gal/hr).
Irrigation duration: 30 minutes
In 30 minutes the grower applies 10 m3 of water to his field.
To lower the pH of the water 150 ml X 10 = 1500 ml = 1.5 Liters of acid are required.
The acid injector injects 25 liters in 30 minutes.
A good practice would be to dilute the 1.5 liters of acid in a 20-25 liters tank, as this will result in a uniform and continuous application of acid.
*m3= cubic meter
The most common acids for agricultural use are Sulfuric acid, Phosphoric acid and Nitric acid.
These acids contribute essential plant nutrients (sulfur, phosphorus and nitrate). These nutrients should be considered in the crop nutrition program.
Note that the water amount applied will affect the amount of nutrient applied with the acid.
Concentration X Volume = Amount
100 ml of 65% Nitric acid contain 18.5 grams of nitrogen.
Assume that bringing the pH of the irrigation water to the desired level requires 100 ml of nitric acid per cubic meter. With the application of 10 m3 of water we will apply 100 ml/m3X 10 m3X 18.5 g/100 ml = 185 grams.
With 30 m3 of water, the nitrogen amount applied will already become 555 grams.