Getting chemistry in the spray tank right
Author: Alistair Lawson | Date: 28 Jan 2015
With many in the southern region getting their sprayers set to hit summer fallows following widespread rain, Dow AgroSciences formulation chemist Robert Buttimor is encouraging growers to take care when mixing chemicals, so their spray tank “party” is a success.
Fallow sprays are needed to keep weeds and pests at bay and to conserve nitrogen and water. In order to maximise the benefits of fallow sprays, Mr Buttimor says they need to be managed to the same degree as in-crop sprays so growers do not end up with a mess in the spray tank.
He says for growers to fully understand the “party” in the spray tank and how the chemicals get along, they need to understand how emulsifiable concentrates and soluble liquids are developed and where they come from.
Acid herbicides – reluctant party-goers
Common herbicides such as glyphosate, 2,4-D, picloram and clopyralid all start life as acids. However, acids are not natural party-goers. They don’t dissolve in water.
To dissolve fully with water there has to be partial positive charge and partial negative charge associated with the herbicide molecule. Mr Buttimor says amines are reacted with solid acid herbicide actives to create herbicide salts. This produces an ion that has a partial positive and an ion with a partial negative charge so it can dissolve in water, just like table salt.
Different salts include potassium, DMA, IPA and TIPA, which are all standard salts used in glyphosate, 2,4-D and clopyralid formulations. Each of these salts have different properties that relate to active loading, as well as physical and chemical compatibility.
“With clopyralid we notice when we use a DMA salt we can drastically increase the loading – i.e you can deliver more in a smaller drum, thus creating Lontrel Advanced,” Mr Buttimor says. “We wanted to make sure this formulation had the same compatibility and efficacy because we switched the salt, and it did.”
When it comes to spray tank compatibility the reactions that create the water soluble salts can go the other way, forming insoluble herbicide acids, or they can combine with other ions and form insoluble salts. Things such as hard water (the party venue), complexity in the spray tank, spray volume (crowd control) and pH (alcohol at the party) can all contribute to the problem and block spray filters and nozzles.
The party venue
In order to ensure the reaction does not go the other way, growers mixing chemicals need to look at their water – the party venue. Many do not have a choice with their water as it is either hard – 300 parts per million and above – or soft.
“Hard water are attractive ions in the party,” Mr Buttimor says. “You can get calcium, magnesium, iron and sodium ions which interact with water. The water can absorb a few of those ions, but if you start packing more attractive ions in the water, they start interacting with 2,4-D. Calcium and 2,4-D can create a half-salt which can crystallise. Sodium can do the same thing with glyphosate. It’s very insoluble and you’ll end up cleaning it out of your filters.”
Hard water typically comes from bores, soft water comes from rain water. Growers who suspect they are having problems with crystallisation should have their water tested.
Mr Buttimor likens ammonium sulphate (AMS) to glyphosate’s wing-man. The sulphate ion distracts the other ions at the party, such as magnesium and calcium, so glyphosate can ‘hook-up’ with the ammonium ion.
“When glyphosate hooks up with ammonium, it actually enters the leaf surface more readily and you get better efficacy, especially in hard water scenarios,” he says. “It’s very important you use AMS in hard water, but even if you have soft water you get this effect. Knowing the hardness of your water allows you to adjust this and tailor AMS inputs.”
As a general rule, if growers are using rain water (soft) they would have 1 percent AMS in the tank mix. If they are using bore water (hard) they would need to use 2 percent AMS.
“Growers using 2 percent AMS in soft water are effectively wasting their money,” Mr Buttimor says.
The trick to throwing a good party is to invite a group of people who will all get along, and not too many diverse groups which could clash. Mr Buttimor says growers need to be applying the same principle to their spray tank mixes.
“If you use just one product in your spray tank you can go at extremely high loads,” he says. “Once you add diversity to your party, especially fertilisers which are notorious, they break up into ions and attract 2,4-D, clopyralid and glyphosate and crash out.
“Control the crowd, keep it simple in the spray tank, don’t try to put too much in there and you should be alright.”
There also needs to be lots of room to party – the more room there is to party, the less chance of trouble. Mr Buttimor says higher spray volumes give better efficacy and better coverage on plants, so it is important always err on a higher spray volume.
“As soon as you reduce that spray volume, particularly with 2,4-D acids, glyphosate and AMS at higher concentrations, it has the propensity be less effective. We have developed charts which say if you’re going to use 2 percent AMS – the highest recommendation – in the softest type of water at the coldest temperature, these are the maximum rates you should be using (table 1).”
Table 1: Water rates required for mixing 2,4-D (Statesman 720) with glyphosate and 2 percent AMS
|Glyphosate 450 (L/ha)||Statesman 720 (L/ha)
Table provided by Dow AgroSciences
pH – the alcohol of the spray tank
Mr Buttimor says pH has a similar role to alcohol at a party – the right amount can make things run smoothly but too much can make things turn ugly.
“It’s very important you don’t over-acidify your spray tank. The optimum pH range for glyphosate is 4.5 to 5.8 to get proper efficacy, but you don’t need acidifiers to hit that. When you add glyphosate to your spray tank it naturally drops the pH within this range, as glyphosate is acidic in itself.”
Growers worried that their water is too alkaline can do a test by adding water to a jar, adding the glyphosate relative to that amount of water and use litmus paper or a pH meter to check the pH level. It is important to test the pH of the solution “after” the glyphosate goes in, as it will adjust the pH naturally.
“If the pH is in the 4.5-5.8 range, you’re fine and you don’t need to add any acidifiers to it as you can take the pH too low,” Mr Buttimor says. “You very rarely see water that needs any acidifying.”
Emulsifiable concentrates – the oily side of partying
Acids are esterified by taking the carboxylic acid group and adding a chain of carbons to it so it can be soluble with oil. But because oil and water do not mix, emulsifiers – the chaperones of oil – are added to it so it can enter the water.
The emulsifiers have a negative charge around them so when the oil droplets go into the water they bounce off one another and do not come together to form a bigger droplet. If they did not have that negative charge, they would come together and form bigger droplets which would either float to the top or sink to the bottom of the spray tank.
By adding lots of other ions to the spray tank in the form of hard water or soluble liquid herbicides, it neutralizes these charges and the oil droplets can come together to form large oil droplets, oily gels or general glug that will float to the top or sink to the bottom of the spray tank. This ties up the active and creates a big mess.
A smooth party
During spraying, the emulsifiers, wetters and spreaders take the droplet and spread it across the surface of the leaf, disrupting the waxy layer on the surface and driving the chemicals into the plant where they will act. Eventually the droplet will evaporate, forming crystals.
“The type of crystals that form on the leaf are important,” Mr Buttimor says. “The glyphosate-ammonium drives it into the leaf quicker. The oil spreads out onto the leaf surface and obviously oil and wax are more similar than water and wax so this penetrates the leaf surface and you get better efficacy.”
Below are general guidelines for tank mixing. It is always important to read labels and follow mixing instruction, because the exceptions to the rule can catch you out and cause problems.
- Fill spray tank 2/3 with clean water, turn on agitation (make sure there is lots of room to party)
- Add water conditioners (cover up those attractive ions at the party)
- Add granules/flowables/powders and mix well (these take time to mix in)
- Add emulsifiable concentrates and crop oils (let the chaperones set up first, less distraction)
- Add soluble liquids i.e glyphosate (the ions added at this stage will less likely interfere with emulsification chaperones. Also, the water conditioners have covered up the hard water)
- Add wetters (these are surfactants, add after ECs have set up)
- Top up with water
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