THE IMPORTANCE OF PH IN AGRICULTURE AND FERTILIZATION: TREATMENTS AND IRRIGATION

pH and its control are of enormous importance in the application of phytosanitary treatments: foliar fertilization, copper treatment, insecticides, fungicides, herbicides, water use for irrigation, and fertilization. In agriculture, water is essential. Its origin is diverse and can vary depending on the time of year. Knowing its pH is important because it can affect our harvest. It is a parameter that must be taken into account, especially when applying phytosanitary treatments (herbicides, copper, etc.) and in localized irrigation. A pH above six can cause products to react with each other or with the salts in the water, leading to the creation of insoluble substances or reduced effectiveness of the active ingredient in the phytosanitary product.

Therefore, measuring and controlling pH can improve treatment efficiency and save costs. It is essential to consider that the pH of water can vary in a well within the same year and, in surface waters (canals, reservoirs, ditches), it also exhibits variations. It is necessary to emphasize that, in case of having a very basic or alkaline pH, it is advisable to use specific products to lower it (acids) before mixing phytosanitary products. Additionally, nitric acid can be used, which also provides nitrogen.

The market offers what they call pH correctors or regulators, which are actually common acids with fancy names and high prices. We recommend that if you want to lower the pH, a simple strong acid like nitric acid (€0.4/kg in 2020, 63% concentration, provides 15% nitrogen) is sufficient to regulate pH at a very low cost.

pH of the phytosanitary treatment solution

The pH determines the shelf life of the active ingredient used. Therefore, we recommend applying it gradually and noting the quantities used for subsequent additions during the same day. This is indicative for both raising and lowering the pH. Thus, spending five minutes regulating the pH the first time the phytosanitary solution is prepared can make the same treatment highly effective or significantly reduce its efficacy. Additionally, it can be applied later in the same day, indicating the same dosage; only the pH needs to be tested. As mentioned earlier, a good pH regulator is nitric acid. In the following tables, we can observe how pH influences treatment efficacy. Although there are many variations, in general, we find that insecticides are more severely affected than herbicides and fungicides. Furthermore, we observe that organophosphates and carbamates decompose much faster than chlorinated compounds.

Optimal pH and half-life of phytosanitary treatment solutions.

Solubility of copper compounds used in phytosanitary treatments (Ref.: Inía - Franco Bologna and colleagues)

Copper-based compounds, in their various formulations, have been and will continue to be one of the most commonly used tools in controlling fungal diseases of olive trees (such as peacock spot, olive sooty mold, and warts). The content of dissolved copper in the solution is affected by the pH (Fig-1). The concentration of dissolved copper varies with the copper source and the accompanying compounds in the solution. Thus, it is important to emphasize that the lower the pH of the solution, the higher the concentration of free copper in it. The amount of free copper at pH 4 is 50 times greater than at pH 7 for the oxychloride solution and 5.3 times greater for the cuprous oxide solution.

Conclusions

pH is an important factor to consider when preparing copper solutions. At pH 6 or lower, the amount of copper provided is significantly higher compared to the amount of soluble copper at pH 7. Variations in free copper in the phytosanitary solution are crucial from the perspective of treatment efficiency. The pH of the solution affects the amount of solubilized copper, whether using copper oxychloride (Fanavid 85) or cuprous oxide (Nordox 75). The lower the pH, the higher the solubility of Cu.

With all this data, we observe the importance of pH control in treatment solutions. Therefore, measuring pH can save a lot of costs.

How is pH measured?

There are several ways to measure the pH of water or a solution (treatment solution) for farmers. The simplest, easiest, and most cost-effective method is using pH indicator strips, also known as 'Litmus paper'. It is a quick and easy measurement system, although its accuracy may not be comparable to a laboratory pH meter, it still provides a good low-cost solution for farmers.

Using pH strips is straightforward. Simply dip the indicator paper strips into the treatment solution and observe the color change after a minute to determine whether the pH is within the desired range or if adjustments are needed. These pH strips can be obtained for a cost of 3-5 €, and a single package contains samples for numerous analyses.

The correct usage of the pH strip is crucial. It should be done with clean and dry hands to avoid contamination. Spending 5 minutes to regulate the pH during the first preparation of the phytosanitary solution can significantly enhance treatment effectiveness or reduce its efficacy. Furthermore, the solution can be applied later in the same day with the same dosage; only the pH needs to be tested.

What is soil pH?

It is well-known that the choice of crop depends on soil pH. Hence, we often say that "lupines are typical of acidic soils" or that "alfalfa and olive prefer alkaline soils". Therefore, it is of utmost importance to know the soil pH. The solubility of numerous phosphorus-containing compounds in the soil is mainly determined by pH. As a result, the pH of the nutrient solution in contact with the roots can affect plant growth in two main ways:

• Soil pH can affect nutrient availability. For the root system to absorb different nutrients, they must be dissolved. Extreme pH values can lead to the precipitation of certain nutrients, rendering them unavailable to plants.
• pH can affect the physiological process of nutrient absorption by the roots. All plant species have characteristic pH parameters in which nutrient absorption is optimal.

What is the pH of localized irrigation water (drip irrigation)?

We know that some waters can have a high content of carbonate or bicarbonate, known as alkaline waters. Using these waters under certain irrigation methods (such as sprinkler irrigation) can cause significant problems if not properly acidified beforehand (clogging of drip emitters). The pH of the water used should be between five and six (acidic pH). Therefore, if necessary, pH correctors (acids) should be used.

What is the pH of bottom fertilizers?

There are fertilizers that, when dissolved in water and used as foliar fertilizers or in fertigation (MAP, MKP, phosphoric acid, nitric acid, and ammonium sulfate), tend to acidify the treatment or irrigation water. On the other hand, others (potassium nitrate, calcium nitrate, magnesium nitrate, and potassium sulfate) tend to alkalize it.

From a practical point of view, and concerning pH, nutrient solutions for fertigation can be classified into three categories:

• Optimal (5.5 < pH ≤ 6.5)
• Suboptimal (6.5 < pH ≤ 7.5)
• Inadequate (pH > 7.5).

Just as the pH of water influences the preparation of the solution, the pH of the soil affects the degradation of the products applied. This same criterion can be used for the pH of the phytosanitary or foliar fertilizer treatment solution.