Guidelines for tank mixing

Methods to minimise spray failures through tank mixing

• Check the pesticide label for tank-mix recommendations and note any restraints, for example, water quality, incompatible mixes.
• If the mix is not well known and no label recommendations exist, do a jar test prior to spraying.
• Mix all pesticides properly and according to labels.
• Make sure all components of your spray rig have the correct filters.
• If you have a spray blockage, try to retrieve the mix, before disposing of your tank mix.

Check the label

First, check the labels of each product to be mixed. If the recommendations state that those products should not be mixed, don't mix them. If no label recommendation exists, seek competent advice and do a jar test (see How to do a jar test below) if you wish to proceed.

Some mixes can not be sprayed at all, for example, 2, 4-D amine and copper sulphate form a very insoluble green precipitate which is difficult to clean from tanks, filters, nozzles and control valves.

Other mixes are synergistic and if sprayed onto crops may cause crop damage. When controlling weeds prior to sowing, such mixes may be useful as they can cause the weeds to die faster.

Also, be aware of the water requirements for chemicals. For instance some pesticides such as glyphosate are deactivated by calcium and other salts so adding fertilisers such as calcium nitrate or trace elements particularly soluble salts of zinc, copper, manganese or magnesium will reduce the effectiveness of the chemicals. Similarly, some insecticides including dimethoate are rapidly decomposed in alkaline water.

How to do a jar test

If you are considering using a complex mix for which there seems to be little available information and no label recommendations exist, carry out a jar test first to check if the mix is sprayable. The jar test will give the same concentration of products as a tank-mix that is to be applied at a spray volume of 50 litres per hectare (L/ha). If the mixture remains stable, that is, it is free from formulation and chemical incompatibilities, then it should not cause blockages and can be sprayed. However, biological incompatibilities are not revealed by this test.

1. In a screw top jar add 500 millilitres (mL) of water.
2. For emulsifiable concentrate (EC) or ultra low volume (ULV) formulations, for every 1L/ha of product to be applied in the field, add 10mL to the jar, for example, if you are spraying 1L/ha of glyphosate and 200mL/ha bifenthrin, place 10mL of glyphosate and 2mL of bifenthrin into the jar.
3. Disposable syringes are very useful for measuring out small quantities of liquids. Some ECs will dissolve plastics very rapidly so the life expectancy of the syringe can be as short as 30 minutes.
4. For granular formulations add ¼ teaspoon of granules to the jar for each 100 grams per hectare (g/ha) to be applied in the field.
5. Cap the jar and shake it well.
6. Let the jar stand undisturbed for at least two hours, but preferably overnight.
7. Look for any obvious incompatibility such as flocculation or precipitation. Some settling of flowable or powder products is normal. But if you have difficulty in re-suspending sediment then extra agitation during spraying is needed or it may cause blockages and uneven application rates or it may be a reaction that seriously affects the pesticide.
8. Dispose of the jar and its contents safely after this test.

Boom sprayer loading guidelines – standard order of mixing

When loading the sprayer, it is critical to follow standard mixing order. Only deviate from this mixing order if there is a specific directive on the label of a key component of the mix.

1. Fill your tank at least two thirds with water which is suitable for all of the components of the proposed mix, that is, no pH, hardness, clay or other limitations.
2. Add water conditioners, for example, pH adjusters, ammonium sulphate.
3. Add granules/flowables/powders and mix well. It is best to apply these to the top of tank so that they disperse with plenty of water. Granules require time to separate and disperse, for this to happen good agitation is needed. Granulated products which disperse when added to the tank can cause the most problems if products such as undispersed ECs, for example, trifluralin or crop oils are mixed with undispersed granules. The granules may not disperse and so will block the filters. Allow 5-10 minutes for the granules to disperse before adding other types of products.
4. Add ECs and crop oils and mix well. If loading with a pre-mix pot, ensure good water movement so that the chemical is rapidly dispersed.
5. Add water solubles, for example, glyphosate, amines. If you are using liquid fertilisers as carriers, very good agitation is essential and this will stop products like trifluralin from separating from Flexi-N®.
6. Add wetters.
7. Top up with water, mix well and apply the mixture as soon as possible.

Increase spray volumes for complex mixes

For most situations 50L/ha is quite satisfactory. For complex mixes, that is, those of three products or more, additional water, may be needed to help stabilise the mix. A rough rule of thumb is to increase the sprayed volume by 10L/ha for each additional product after three or four products have been added. This includes non pesticide items such as trace elements.

Even with adding extra water, blockages may still occur if you have self cleaning filters. These filters can return any unfiltered material, for example, undissolved granules, into the tank where it can build up causing blockages. If you know that your tank mix has some problems with precipitation or sludge/scum formation, or you are using granulated pesticides, consider cleaning out the tank after every third load.

Setting up your sprayer

Simple attention to the set up of the boom sprayer will reduce the chances of blockages occurring. All products including water should be filtered into the tank and even if you have correctly followed the guidelines for mixing pesticides, in some cases, the combinations of wetters, oils, and dispersants of a complex mix can still result in filter blockages. Chances of getting blockages due to spray mixes can be reduced by ensuring correct filter sizes in the boom rig.

Correct filter sizes in your boom rig

Schematic of ground rig spraying showing filter sizes

• General tank filling: 50-100 mesh filter- to remove foreign material and lumps of wettable powders. Filling filters need to be able to keep out fine sand and can be finer than the main filter.
• Suction line to pump in-line filter: 30-50 mesh filter - to keep foreign matter out of the pump. Partly dispersed granules should be able to pass through this filter and be returned to the tank after travelling through the pump and hopefully broken up.
• Main filter: approximately 80-100 mesh filters - all filters downstream in the boom lines should be coarser than this filter. It should be a self cleaning large-capacity filter.
• Boom section filters: usually 50-80 mesh - are safety filters only and should not be finer than the main filter (otherwise, they will block up first). These filters are fitted to provide secondary protection for nozzles from any material escaping from the main filter due to breakage or when the main filter is cleaned. Note that boom sections should be cleaned occasionally to remove any sediment, for example, settled powders to stop the boom sections progressively blocking up from the end furthest from the inlet as powders settle out.
• Nozzle filters: usually 50-80 mesh - but this filter size depends on the nozzle size and the type of product being used. These can be removed if they keep blocking. The only role of nozzle filters is to keep nozzles from blocking from scale or dirt originating in the boom lines.

What to do with a loaded blocked boom sprayer

Some mixes result in apparently blocked filters, that is, high back pressure at the filters and reduced flow, but when cleaned, the filters seem to contain little more than white or clear slime. This is caused by marginal mixes, when passing through a filter or cavitation in a pump, separating into a layer of water in oil (like mayonnaise) blocking the filter. In these situations, cleaning the tank between loads reduces the build up on the filters as it is often by the second or third tank mix when problems are noticed.

A tank full of an un-sprayable mix should not be dumped until all avenues of advice have been exhausted. Seek advice from chemical company representatives, agronomists or Department of Agriculture and Food, Western Australia as soon as possible. Please have all of the details for all components in the mix at hand including the water source, any fertilisers and trace elements, wetter, oils and the order of mixing to assist trouble shooting.

It is often possible to retrieve a problematic mix. For instance, adding some extra wetter or using a fire fighter pump to cause vigorous agitation will break up lumps caused by granules which have been added after crop oils and are difficult to disperse.

Faulty products causing problems are fairly unusual but if no other causes seem apparent, contact the manufacturers as soon as possible with the batch numbers to sort out the problem to minimise any inconvenience.