My recent article on the use of hard water in spray preparations generated a lot of questions. Most of these were around the order in which products should be added to a spray tank to minimise the interactions between products. This issue relates just as much to organic, plant nutritional and foliar fertilisers products as it does to synthetic pesticides.

Mixing products in a spray tank can cause serious problems. It is important to note that some products should never be mixed together and label directions must always be obeyed. As a young research agronomist I asked a farmer to mix a foliar fertiliser and a phenoxy herbicide in his spray tank for an on-farm trial. The whole mix turned into a green jelly. I spent the rest of the day with my head in the tank scooping out the mess with a bucket. Such invaluable lessons stay with us forever!

When products have to be mixed, following the correct order of addition is important to minimise the risk of products interacting in the tank.

Addition No. 1 – Water
Almost all interactions between chemicals are worsened by increasing concentration. Therefore, it is important to start by filling the tank with at least 70% of the total amount of water. This may seem obvious but I know of two cases where three or more products were added to a spray tank BEFORE any water was added resulting in severe interactions that would not have occurred under normal conditions. A similar issue can occur when mixing in inductions systems such as a Granni Pot.

Addition No. 2 – Water Conditioners
The purpose of conditioners is to modify the water in the spray tank to better suit or protect the products that are to be added. Therefore, where used, it essential that these are added first and are allowed to mix thoroughly throughout the tank before anything else is added.

Addition No. 3 – Wettable Powders (WP)
Solid chemicals react at their surface so the larger the surface area the greater the potential for reactions. Wettable powders have a huge surface area and are carefully formulated with matching wetters and dispersants to ensure that they wet-up and disperse uniformly in the tank to form a stable suspension. The ingredients in some other products can greatly interfere with this process. The ingredients in crop oils and emulsifiable concentrates are particularly problematic. For this reason, WPs are added BEFORE all of the following products. They must be given enough time in the tank to form a uniform suspension before other products are added.

Addition No. 4 – Wettable Granules/Water Dispersible Granules/Dry Flowables (WG/WDG/DF)
These types of products are essentially wettable powders that have been compressed into granules. Unlike WPs, however, the granules have to disintegrate before they form powders which then have to be dispersed throughout the tank to form a uniform suspension. They are added after WPs but otherwise are treated in the same way. As these granules must first disintegrate before suspending in water they take longer to form a uniform suspension than WPs. The time taken depends on the product, agitation in the tank and the temperature and quality of the water.  Many farmers are surprised to hear that our tests have shown that a minimum of 10 minutes is needed.

Addition No. 5 – Suspension Concentrates/Flowables
These products are powders that have already been mixed with water and so their surfaces have already been wet-up. This means that they will form a uniform suspension faster than the two proceeding product groups. However, they are also designed with a careful balance of dispersants and this balance can be upset by the presence of other, incompatible or competing, surfactants.

Once again the emulsifiers/surfactants in crop oils and emulsifiable concentrates pose the biggest risk.

Addition No. 6 – Emulsifiable Concentrates (EC)
Emulsifiable concentrates (ECs), including spray oils, are also designed with a balance of surfactants.  In this case the surfactants need to drive the active ingredient and solvent into stable emulsion droplets when added to water and so these surfactants are commonly called emulsifiers. A stable emulsion will only form if the balance of emulsifiers is correct. The ingredients in WPs, WDGs or SCs can be affected by these emulsifiers.  For this reason ECs are added after the proceeding product groups. It is also possible for the emulsifier system in one EC to be incompatible with those in another EC.

Addition No. 7 – Water Soluble Granules (SG)
These granules are made up of ingredient that dissolve in water rather than suspend and this process generally proceeds more readily than for WDGs. These products still contain dispersants and wetters to speed the process. The effect of these ingredients is not usually strong enough to break the emulsion formed by an EC but at Eureka! we have seen it tip the balance in some complicated mixes or where water quality is poor. SGs are best added after ECs.

Addition No. 8 – Soluble Liquids
When added to the tank these products undergo a simple dilution process and should be added after all the product groups above.

Addition No. 9 – Adjuvants
All remaining adjuvants are added last. This should be done after all the other products have had a chance to mix thoroughly through the tank. 

After sitting for just one hour the product combination on the left is separating into layers while that on the right is stable.

 If you have products that need improved compatibility or need their compatibility tested please contact Anthony Flynn

Written By: Anthony Flynn
© 2014 Eureka! AgResearch Pty Ltd