Hydrogen potential (pH) is a measure of the acidity or alkalinity of a solution on a logarithmic scale ranging from 0 to 14. It refers to the relative concentration of hydrogen ions (H+) and hydroxyls (OH–). The lower the pH, the greater the acidity (lower alkalinity), that is, the greater the amount of H ions+ present in the solution.
In relation to agricultural pesticides, in general, each product has a pH at which its efficiency is greater. Glyphosate, for example, shows maximum efficiency when the pH of the medium is 3.5. When the pH of the solution is high, the molecules are degraded by alkaline hydrolysis, and the product loses its efficiency by reducing interaction with the surface of the leaves.
Below is a table with the main agricultural pesticides and their respective ideal pH for application:
Table 1. Effect of spray pH on the average life of phytosanitary products, growth regulators and fertilizers.
What happens when the pH of the syrup is different from that indicated?
Most phytosanitary products have an ideal pH close to 5 (Thiesen, et al. 2017), that is, in an acidic environment. Above this range, the active ingredients begin to undergo alkaline hydrolysis. Below the ideal pH range, acid hydrolysis begins to occur. In other words, when the pH of the syrup is different from that indicated, the product molecules break down due to the action of water, affecting the estabilidade do ingrediente ativo.
Most phytosanitary products reduce their hydrolytic half-life when mixed in alkaline water (Gassen), that is, the time to inactivate 50% of the product is reduced. Captan and Carbendazin, for example, both fungicides, have a half-life of 12 minutes at pH above 7 and 9, respectively. At pH levels between 5 and 5.5, they have a half-life of more than 30 hours. In other words, in certain products, it only takes a few minutes at an inappropriate pH for the product to lose its stability and efficiency.
Figure 1. Effect of water pH on the stability of the insecticide trichlorfon and the herbicide flumiclorac (Arbore). Source: Eng. Flávio R. Gassen.
In another study, carried out by Dinis Gomes Traghetta, it was shown that the herbicide atrazine, whose ideal application pH is 5, used to control broad-leaved plants in corn crops, in a very acidic environment, with a pH below 2, It is completely hydrolyzed within a few hours.
There are still few studies that quantify the losses resulting from the application of pesticides with non-indicated spray pH, but some authors say that there may be losses in efficiency of up to 25% for each pH point outside the ideal range (source: Aprosoja).
Where can I find information about the ideal pH of each product?
This information can be found in scientific papers or ask the manufacturer.
Do you need an adjuvant to correct pH?
Since most pesticides have an ideal pH for acidic application, many may think that it is necessary to add adjuvants to reduce the pH of the mixture and increase efficiency.
However, phytosanitary products themselves already have the capacity to lower the pH of the environment, due to the acidity of the active ingredient. According to Bianchi, 1995, in a study carried out with the herbicide glyphosate, when the water in the source is close to a neutral pH, control is practically not affected. The control becomes significantly affected when the pH of the water source is close to 10, as shown in the figure below:
Figure 2. Influence of water origin on glyphosate efficiency in black oats. Source: Bianchi, 1995.
We emphasize here that although the indication for glyphosate is that it is applied in a medium with a pH close to 3.5, the percentage of black oat control was quite high even in pH conditions above 4.5.
With this study, we can conclude that it will not always be necessary to use adjuvants to acidify the syrup water, at least with regard to the use of glyphosate, especially when the product formulation already contains buffers and acidifiers. However, it is always very important to check both the pH of the water being used and the pH of the syrup already mixed.
Studies like this should be carried out with more agricultural pesticides, so that producers do not make mistakes when deciding whether or not to use adjuvants to acidify the pH of the environment. We see that the effect of the pH of the solution on the application of phytosanitary products is poorly understood and there is still a lot of scientific research that must be carried out in this regard.
How to acidify the pH of the syrup?
Alkaline waters (pH above 7) can reduce the efficiency of herbicides, especially desiccants based on glyphosate, ALS inhibitors (Classic, Ally, Spider, etc.) and ACCase inhibitors (Fusilade, Verdict, Poast, etc.) – Mervosh & Balke, 1991; Nalejava & Matysiak, 1993. Souza & Velloso (1996) afirmam que os herbicidas do grupo químico das imidazolinonas (inibidores de ALS), especialmente imazethapyr e imazapyr, have their foliar absorption increased when the pH of the water used to prepare the syrup is in the range of 4.0 to 4.5.
In a study carried out by Sanchotene, it was concluded that the control of red rice with ALS inhibitors was much more effective using water with a pH in this range. In these cases, it may be interesting to apply acidic products so that herbicide inefficiencies do not occur. However, in a syrup there are salts with buffering power, which quickly increase the pH of the syrup after applying an acid (Kissmann, 1998). Therefore, in the past, in addition to the acid, a suitable salt (buffer) would have to be added so that the pH did not return to its original state.
Today, there are commercial products that correct the pH of the syrup and that already have the capacity to act as buffers, making the process of reducing the pH of a syrup easier.
What are the risks of acidifying the pH of the syrup?
In general, the lower pH limit is 3.5 (Gassen). Below this value, problems may occur due to phytotoxicity in the plants and poor homogenization in the mixture, therefore whenever acidifiers are added to the application mixture, the pH must be measured using digital peagometers or measuring tapes.
The Role of SaveFarm® in Optimizing Pesticide Application
Understanding the ideal pH for applying pesticides is crucial, but equally important is ensuring that these products are applied accurately and effectively.
This is where SaveFarm® comes into the picture. Our state-of-the-art sensor system allows for selective spraying, ensuring pesticides are only applied where needed. This not only optimizes the use of inputs, but also maximizes the effectiveness of pesticides, through the pulsed spraying that the system provides. Furthermore, by using SaveFarm®, you contribute to more sustainable and profitable agriculture.
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THIESEN, R., MOREIRA, C. R. Efficiency in the use of adjuvants in the application of fungicide in second harvest corn crops. Rev. Cultivating Knowledge, Special Edition, p. 144-154, 2017. USDA – UNITED STATES DEPARTMENT OF AGRICULTURE. World Agricultural Supply and Demand Estimates (WASDE 577). 2018. Available at: http://usda. mannlib.cornell.edu/usda/current/wasde/wasde-05-11-2018.pdf.;
MACIEL, S. R. What is the appropriate pH of water when applying pesticides. 07/14/2020 | Cultivate Machines. Available in: https://www.grupocultivar.com.br/noticias/qual-o-ph-adequado-da-agua-na-aplicacao-de-defensivos;
BETTEGA, M. Tank mixing. Available in: http://www.aprosoja.com.br/storage/comissoes/arquivos/apresentacao-mistura-em-tanque—mary-bettega.pdf;
GASSEN, F.R. Effect of water acidity on phytosanitary products. Direct Planting Farmers Cooperative, Information 061. Available at: http://www.terraaviacao.com.br/061AguapH.pdf;
KISSMANN, K.G. Adjuvants for phytosanitary product mixtures. Santa Maria Agronomy Society, 1998. p. 39-51. Available in: http://w3.ufsm.br/herb/Adjuvantes%20para%20caldas%20de%20produtos%20fitossanitarios%20-%20Kissmann.pdf;
Sanchotene, D.M. Influência de sais e do pH da água na eficiência de imazethapyr + imazapic no controle de arroz-vermelho. Planta daninha v.25 n.2 Viçosa 2007. Disponível em: https://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-83582007000200023&lng=pt&tlng=pt.