Estimated reading time: 6 minutes
During a recent Potatoes SA information day in the Sandveld, André Erasmus, a retired technical specialist at Inteligro, discussed certain aspects relating to water quality. Although water makes up more than 95% of a typical spray solution, many producers focus primarily on the chemicals being applied, even though water quality often determines a product’s efficacy. In an era where crop protection is becoming increasingly expensive and regulatory pressures are rising, poor water quality can lead to enormous financial losses.
Important definitions
- Solubility: How effectively a solid or liquid product disperses in water without forming precipitates.
- Compatibility: The ability of different chemicals to coexist in the same tank without precipitating or reacting.
- Stability: The preservation of the chemical structure of the active ingredient, from the moment it is mixed until it hits the target.
- Half-life: The speed at which a chemical breaks down and loses its potency due to environmental factors.
- Uptake by plants: The ease with which the plant leaf absorbs the active ingredient through its waxy layer and stomata.
- Efficacy of the active ingredient: The ability of the chemical to successfully control the specific pest, disease, or weed.
- Antagonism: Different components in the tank actively counteracting one another.
Water serves as the carrier for most spray applications. Since it is the medium in which products are dissolved or suspended, it plays an important role in the success of any tank mix. Poor water quality can lead to insufficient uptake, chemical degradation, nozzle blockages, incompatibility, and subsequent poor control. The problem, however, is that many of these consequences are not immediately visible.
The role of pH
One of the most important factors in water is pH. The pH scale is logarithmic, which means small changes represent major differences in acidity or alkalinity. For example, a pH of 5 is ten times more acidic than a pH of 6; this causes a small shift on the scale with a noticeable impact on sensitive chemicals.
Similarly, a pH of 5 is 100 times more acidic than a pH of 7. This shift represents the difference between neutral water and moderately acidic water, which drastically accelerates or decelerates chemical reactions. Likewise, a pH of 5 is 1 000 times more acidic than a pH of 8, representing an enormous chemical gap. Water that is alkaline (pH 8) will neutralise certain acid-attracted products within minutes.
These differences have a direct impact on chemical stability. Certain active ingredients degrade rapidly under acidic conditions, while others are affected by alkaline conditions. Using water with the incorrect pH can reduce half-life, lower efficiency, increase antagonism, and negatively impact uptake by the plant. Therefore, it is essential to adjust the water pH according to the label recommendations before mixing products.
Mineral content and hard water
Water often contains dissolved minerals (charged cations) that pose an invisible threat to chemicals. These cations can bind with certain active ingredients to form insoluble compounds. The key minerals involved include:
- Calcium, one of the major causes of hard water, which binds with herbicides such as glyphosate.
- Magnesium works hand in hand with calcium to increase water hardness and neutralise chemicals.
- Iron can cause precipitation, as well as discolouration or binding problems in the tank mix.
- Aluminium is a highly charged ion that, even in small quantities, can cause severe binding with negatively charged pesticides.
- Sodium increases the salinity of the water, which can be toxic to plants and compromise uptake.
These newly formed mineral-chemical bonds lead to poorer uptake, precipitates in the tank, and reduced efficiency. Producers should test their water regularly to manage these risks.
Beware of cold or insufficient water
Temperature affects the physical behaviour of chemicals, as well as the rate at which they react in water. Some products take longer to dissolve in cold water, for example, early morning water drawn from a dam or borehole. This increases the risk of clumping, incomplete dispersion, nozzle blockages, and uneven application. This means that the first section of the land being sprayed receives less product than the final part. To prevent this, sufficient mixing time is essential to allow the chemicals to dissolve completely.
Lower water volumes per hectare mean the same amount of chemicals is added to less water, which increases the overall product concentration in the tank. The higher the concentration, the greater the risk of product incompatibility, crystallisation, foaming, and crop damage from scorched leaves. Producers must therefore exercise caution when applying ultra-low volumes.
The importance of mixing order
One of the most common reasons for chemical incompatibility is adding them in the incorrect order. To reduce the risk of physical incompatibility, the following steps must be followed:
- Fill the tank with approximately 50% water to obtain the necessary water volume, ensuring that products are not too concentrated.
- Add buffers and pH regulators before adding the expensive active ingredients.
- Add water-soluble bags (WSBs). This requires clean water and time for the plastic-type pouch to dissolve completely.
- Add dry formulations (WG, WP). These require the maximum amount of water and sufficient mixing time to suspend the granules.
- Add dispersible liquid formulations (SC, SE). These thicker liquids are fluid but still require good dispersion in the water.
- Add liquid formulations (EC, SL). These are oil- or water-soluble liquids that mix more easily but can alter the water properties.
- Add additives (adjuvants, wetting agents, oils). These are designed to alter droplet behaviour and should only be added once the chemicals have been mixed.
- Add micro-elements and liquid fertilisers last. Salts with high charges that easily precipitate must be added as the very last component.
Legal aspects of tank mixes
Labels are the most important legal document in crop protection, and in terms of the Fertilizers, Farm Feeds, Agricultural Remedies and Stock Remedies Act, 1947 (Act 36 of 1947), it is an offence to act outside of the label recommendations.
Producers must follow the label instructions carefully and strictly adhere to the legal dosage, safety periods, and application methods. Only spray registered mixtures that have been officially tested and approved for that specific crop, as per the label. Keep complete records of the products applied and their specific purposes.
There is also increasing pressure from insurers regarding complex tank mixes. The more products placed in a tank, the greater the risk of claims and damage. If something goes wrong with an unauthorised ‘cocktail’ consisting of five or six products, insurers will refuse to pay out the damage.
Practical tips for producers
Regularly analyse the pH, hardness, and total dissolved salts (EC) of different water sources at an accredited laboratory. If necessary, use appropriate buffers to adjust the water’s pH and mineral levels before pouring any crop protection products into the tank.
When mixing, follow the correct order to avoid problems. In addition, ensure that the tank’s mixing system runs continuously, from the moment the first product is added until the last litre has been sprayed onto the land. Keep in mind that when too many different products (chemicals, nutrition, and repairers) are mixed together, it can exponentially increase the risk of chemical clashes.
Read labels carefully and pay special attention to water quality, compatibility, and withholding periods. Consult technical advisors before applying a new or complex tank mix.
Try not to skimp on wetting agents or oils; the right adjuvant can dramatically increase the spray material’s efficiency.
Lastly, avoid taking unnecessary risks such as leaving a mixture in the tank overnight or spraying during extreme daytime heat. Keep a record of successful mixtures and conditions to build up a reliable framework of references for your farm.
Water quality, formulations, and mixing order form the foundation of successful crop protection. Producers who understand and correctly apply these principles can prevent major losses and realise maximum value from their spraying programmes. – Annemarie van der Merwe and Francois Strauss, Potatoes SA