pH is a measurement of the concentration of hydrogen ions (H+) in solution. The pH shows how acidic or alkaline water is, water with a pH below 7 is acidic and water with a pH above 7 is alkaline. Most natural waters are between pH 5 and 8. The critical thing to remember about the pH scale is that it is logarithmic. Since this represents a logarithmic expression, H+ concentration at pH 6.0 is 10 times greater than at pH 7.0 and 100 times greater than at pH 8.0. So, what appear to be small changes in the water chemistry to us are much larger in chemical terms and can have a dramatic effect upon the nutrients in the water.

The generally accepted pH for irrigation water is between 5.5 and 6.5. This pH range optimize the solubility of the nutrients in irrigation water enhancing the nutrient uptake by the plants. In addition, these pH levels minimize the precipitation in the irrigations system. The ideal pH range for nutrient uptake for the plants is illustrated in Figure 1.

Figure 1. The nutrients availability in water at different pH levels.

The acidity or alkalinity of a water supply can affect plant growth, the water composition and irrigation equipment. The acidic water can have a detrimental effect on the plant growth causing nutritional problems while pH less than 4 could damage the plant roots due to acidification. In addition, lower pH could contribute to corrosiveness, which could lead to damage to metal pipes, tanks and fittings.

Alkaline water (hard water) can contain high concentration of bicarbonate and carbonate which induce a pH above 8 and 9 respectively. This could affect the macro nutrients availability such as calcium and magnesium and micron nutrients such as zinc, boron, iron and copper. The relation of carbonate concentration and pH also called carbonate speciation is shown in Figure 2.

Figure 2. Speciation of bicarbonate and carbonate at different pH levels.

The main function of the bicarbonate in the water is to act as buffer and minimize the pH fluctuation. Bicarbonate act as acid when alkaline is added in the water as shown in the equation below:

When acid is added in the water → H3O+ + HCO3- = H2O + CO2

In the same way, bicarbonate act as base when acid is added in the water as shown in the equation below:

When base is added in the water → OH- + H2CO3 = H2O + HCO3-/em

In general, bicarbonate helps to stabilize the pH in the water and make it easier to control pH levels.

The high pH levels also affect the phosphate form in the water. The phosphate undergoes several transformations as the pH levels continues to increase. Figure 3 shows the speciation diagram of various phosphorus species at different pH.

Figure 3. Speciation diagram of various phosphorus species at different pH levels.

The available phosphorus in the irrigation water is Di hydroxide phosphate (H2PO4- ) and most of the cation in the water including calcium and magnesium and this phosphate form have a higher solubility. However, the hydroxide phosphate (HPO42- ) has a low solubility with calcium and magnesium. The hydroxide phosphate (HPO42- ) availability increase at pH levels higher than 6,5. This can cause precipitation of calcium and magnesium.

The pH control depends mainly on the composition of supply water and the fertilizers. Various supply water source is used for irrigation water such as Tap water, surface water, ground water, rain water and reverse osmosis (RO) purified water. Surface water and ground water usually have higher bicarbonate levels which result in significant pH fluctuations. An unstable pH-level in the irrigation water, created by high levels of bicarbonate, has a negative impact on the uptake of nutrients by the plant. The Priva Neutralizer makes sure you get a stable pH by removing bicarbonate from all kinds of water. This system combines acidification and aeration to reduce the bicarbonate concentration to a desired maximum value of 0.5 mmol/l.

Rainwater and RO purified water contains no bicarbonate which also ensures pH fluctuations when acid or base is added in the water. The pH could be stabilized by adding 0.5 mmol/l bicarbonate in the rain and RO water which increase the pH to 8.5 in case the pH of the water is neutral. The pH could then be controlled to the desired value by dosing acid via dosing units usually in form of Nitric acid 38% or phosphoric acid. The parameter that determines the flow rate of the bicarbonate and acid dosage is listed in the table below.