Understanding Physical Compatibility vs Biological Compatibility in Broadacre Cropping
Tank mixing is becoming an increasingly common way to save time, fuel and labour across the paddock — but there's more to a successful mix than what you can see in the tank. This edition we're diving into the difference between physical and biological compatibility, and what it could mean for your bottom line.
As input costs continue to bite and machinery, labour and fuel costs climb, many growers are looking hard at reducing spray passes across the paddock. One obvious strategy is increasing the use of tank mixes — combining herbicides, fungicides, insecticides, foliar nutrition and adjuvants into fewer applications.
From a logistics and cost perspective, it makes perfect sense. But there is a catch!
Just because products physically mix in the spray tank does not mean they are biologically compatible in the paddock.
And this is where costly mistakes can occur.
The Two Types of Compatibility
1. Physical Compatibility
This is the easy one to see.
Physical compatibility simply means the products can be mixed in the spray tank without causing issues such as:
- Layering
- Separation
- Sedimentation
- Flocculation (“cottage cheese” effect)
- Gel formation
- Blocked filters and nozzles
- Excessive foaming
If the tank remains stable and sprayable, many people assume the mix is “compatible”.
But that is only half the story.
2. Biological Compatibility (Efficacy Compatibility)
This is the compatibility that really matters financially.
Biological compatibility refers to what happens after mixing:
- Does one product reduce the activity of another?
- Does the mix reduce crop safety?
- Are weeds harder to kill?
- Does the fungicide still penetrate properly?
- Does the insecticide still work effectively?
- Does the nutrition tie up the chemistry?
- Does the adjuvant alter uptake or translocation?
A mix may look perfect in the tank but still result in:
- Reduced weed control
- Lower fungicide performance
- Increased crop damage
- Reduced residual life
- Poor uptake
- Delayed activity
In short:
A spray mix can be physically compatible but biologically incompatible.
That distinction is becoming increasingly important as tank mixes become more complex.
Why Tank Mixing Will Increase
Several factors are driving the shift toward larger and more complex tank mixes:
- Higher fuel costs
- Labour shortages
- Larger farm sizes
- Narrower spray windows
- Pressure to reduce machinery operational costs
- Greater use of autonomous and high-capacity sprayers
- More products requiring timely application
Growers are increasingly asking:
“What else can we add while we are going over the paddock?”
That mindset is understandable — but every added product increases the complexity.
Common Compatibility Risks
Hard Water, Water Quality and Water volume
Some herbicides are highly sensitive to calcium, magnesium, bicarbonates or dirty water.
Products like glyphosate can become tied up before they even reach the plant. Research has shown that water hardness can reduce the efficacy of glyphosate by up to 90% - https://eurekaag.com.au/glyphosate-effect-hard-water/
Adding trace elements or liquid fertilisers can sometimes worsen antagonism.
Low water volumes and or cold water can also render a normally compatible mix into a disaster. Dilution can often be the solution. Mix in larger volumes of water.
Mixing Order
Poor mixing order is still one of the biggest causes of compatibility issues.
Dry flowables, suspension concentrates, EC formulations, adjuvants and liquid fertilisers all behave differently in solution.
A technically sound mix can fail simply because products were added in the wrong sequence. Follow the Crop Smart Mixing Order Guide. Click this link to find the guide
Suspension Concentrates (SC) – A look out
Suspension concentrates (SC formulations) contain finely milled solid active ingredients suspended in liquid, rather than fully dissolved in the solution. They generally provide good handling and mixing characteristics, but they do require adequate and constant agitation to keep particles evenly suspended in the tank, otherwise settling, uneven distribution and compatibility issues can occur — particularly in complex tank mixes or when added in the wrong mixing order.
Herbicide Antagonism
Certain grass herbicides can be negatively impacted when mixed with broadleaf herbicides.
This is especially relevant in tight ryegrass situations where every percentage point of control matters.
Reduced uptake or slower translocation can turn a good spray job into a resistance problem.
Clethodim and Atrazine are physically compatible but this combination in a tank mix will reduce grass control efficacy. Axial is physically compatible with Tigrex but you will experience some loss of grass control with this mix.
Adjuvant Conflicts
Not all adjuvants are “helpers”.
Some can:
- Increase burn
- Reduce uptake
- Alter droplet behaviour
- Destabilise formulations
- Increase crop damage
More adjuvant is not always better.
Sometimes the “hotter” mix is actually less effective. An example here are some oil adjuvants that can be quite good in summer weed mixes but may be too hot for in crop uses. This can cause leaf burn to the weed and crop.
Nutrition + Chemistry
Foliar nutrition is increasingly added into pesticide applications to save a pass.
Sometimes this works well.
Other times:
- Salt load becomes excessive
- Uptake pathways compete
- Crop stress increases
- Efficacy declines
The trace element zinc can help a crop overcome herbicide phytotoxic effects but it can also reduce herbicide efficacy. An example in this situation is zinc trace elements mixed with Clearfield (imidazolinone) herbicides.
Key Questions Before Building a Tank Mix
Before adding another product into the tank, growers and agronomists should ask:
- Has this mix actually been tested?
- Is the compatibility only physical, or also biological?
- What is the water quality?
- What is the crop stress level?
- What are the environmental conditions?
- Is the adjuvant appropriate for all products in the mix?
- Could one product antagonise another?
- What is the salt load?
- What happens if efficacy drops by 5–10%?
Because sometimes the cheapest pass is the one you never have to re-spray.
Final Thoughts
The future of broadacre spraying will almost certainly involve:
- More tank mixing
- Fewer passes
- More complex chemistry combinations
- Greater pressure on application timing
But successful tank mixing is no longer just about:
“Will it mix?”
The better question is:
“Will every product still perform as intended?”
That is the difference between physical compatibility and biological compatibility — and it may become an important agronomic discussion moving forward.
At the end of the day, the goal of every spray job is simple — make every drop count. With tank mixing on the rise, understanding both physical and biological compatibility has never been more important. If you'd like to talk through your spray program, get in touch with your local CropSmart agronomist — we're here to help you get it right. 🌾