Magazine: WHY HAS MY TANK MIX TURNED TO ****?

By Jerry Spencer in Consultancy on 18th Apr 2009 6:00

Tank mixing pesticides, liquid fertilisers or combinations of both can save time, labour, energy, equipment costs and in the case of pesticides be part of a resistance management strategy. Pesticide combinations can alter plant absorption and translocation with an example of this being azoxystrobin and what are termed activator adjuvants (Akzon Nobel, 2005)

Activator adjuvants modify biological effects of the pesticide being applied via the improved wetting characteristic of the solution, greater coverage of the target, rainfastness, or improved permeability of the cuticle. Uptake of pesticide into the leaf is restricted by the plant cuticle. The cuticle is a complex lipophilic membrane that protects the outer, above ground, surfaces of a plant. Surfactants can improve pesticide transfer across the cuticle, enabling the pesticide to enter the leaf, translocate to cellular targets, and manifest the desired effect in the plant.

However, not all changes are for the better. Negative effects can occur such as reduced pest control, increased damage to non-target plants (phytotoxicity) and the focus of this article, incompatibility problems between materials.
When tank mixing occurs there are four types of positive interactions that can occur:
1. Additive effects occur when mixing two pesticides provide the same response as the combined effects of each material when applied alone. The products neither hurt nor enhance each other. Such mixes save time, labour and equipment use with an example of this being 2,4-D and Dicamba.
2. Synergistic responses are often confused with additive effects and occur when two pesticides provide a greater response than the added effects of each material when applied separately. In some way they interact and the resultant effect maybe an increased control or efficacy. An example of this would be Atrazine and Clomazone.
3. Antagonism. When two pesticides applied together produce less control than if you applied each material separately is called antagonism. In addition to reducing control, antagonistic responses also may increase phytotoxicity to plants. Applying soil wetting agents and liquid fertilizers or SU herbicides with the herbicide families that only control grasses including diclofop methyl and fusillade both result in antagonistic reactions. Antagonism can also occur for example when mixing potassium nitrate and urea and affects the amount that can be dissolved.

Table 1. Solubility of potassium nitrate–urea mixtures at 30°C
% Urea (by weight)	% Potassium nitrate (by weight)	Solubility (g/100 g H₂O)
100	0	114
80	20	161
60	40	99
40	60	63
20	80	50
0	100	39

4.Enhancement is another type of interaction, but not between two pesticides. Enhancement occurs when a pesticide is mixed with an additive to provide a greater response than if you applied the pesticide alone. A common example of enhancement is mixing an adjuvant with a pesticide such as the aforementioned activator adjuvant Adsee-650.

However of more concern and what this article is concerned with are tank incompatibilities which can be either of a chemical or physical nature.

If you can mix two or more fertilisers, pesticides or combinations of these and no adverse effects occur then these can be regarded as being compatible. If the effectiveness of a pesticide's active ingredient is reduced or rendered ineffective, then that is considered a chemical incompatibility. In areas where pesticides, particularly certain insecticides, are mixed with alkaline water, inactivation has been known to occur, especially when held in suspension overnight or extended periods (Fishel, 2007). Physical incompatibilities usually involve the inert ingredients of a formulation and mixtures may form flakes, gels, layers, crystals, or other precipitates that are not suitable for application through spraying equipment
For herbicides, incompatibility most often occurs when you mix an emulsifiable concentrate (EC) formulation with wettable powders (WP). Similarly, you should not mix EC insecticides with fungicides or herbicides. Liquid fertilizers can also cause compatibility problems, mainly due to their strong electrochemical nature.
Fertiliser

One of the keys to successfully using tank mixes of fertilisers is using the right fertiliser. Generally liquids or technical grade materials are preferable as agricultural grade materials do not stay in suspension and can contain impurities that can cause blockages. Also be aware of anti caking materials used on some fertilisers that are actually not especially soluble and can in fact cause nozzle blockages.
When dissolving a fertiliser, it is recommended to fill half of the tank with water and slowly add dry fertiliser with constant agitation. Continue to add fertiliser and to fill the tank with water.

When most fertiliser are added to water there will be an immediate and considerable decrease in water temperature as dissolving of the salts needs energy. This is known as heat of solution and directly affects the amount of fertiliser that can be dissolved. To overcome this it is best to add small quantities of fertiliser at a time until the required amount is dissolved.

When preparing fertiliser solutions containing two or more different fertiliser salts for injection into irrigation lines, it is possible that precipitate (sediment) may form. This occurs if reaction products form which are either insoluble or less soluble than the original products. The resultant precipitate may settle to the bottom of the mixing tank, or block filters and emitters. Reaction products may form when phosphate, sulphate, calcium, magnesium and trace element fertilisers are used in solution. So for example tank mixing potassium sulphate with calcium nitrate is not generally a recommended practice as this forms calcium phosphate an insoluble precipitate and potassium phosphate.

Precipitate may also form if the water is hard (high in calcium and magnesium), alkaline (high pH) or contains carbonate. If a precipitate is likely to form aim to use fertiliser solutions immediately and do not allow the solution to stand for an extended period of time, e.g. overnight, as this increases the risk or amount of precipitation, and the settling of sediment in the bottom of mixing tanks.
Regardless of the fertilizer source, rate selection and dilution is critical. To avoid fertilizer injury, aim for a fertilizer concentration of 1-2% of the total water volume. Do not exceed concentrations of more than 5%. So for example taking a 400L tank as being the industry norm this means:

Table 2. concentrations of tank mix partners assuming 400L tank
Fertiliser	1-2% Solution (kg/400L)	5% Solution (kg/400L tank)	Theoretical Maximum solubility in kg/400L @ water temperature of 25°C
Potassium sulphate	4-8	20	48
Potassium nitrate	4-8	20	142
Calcium nitrate	4-8	20	152

I cannot recall exactly how many times people have complained about their particular tank mix having issues with solubility. The table above

Solubility kg/L vs water temperature C for commonly used potassium salts

explains why this can occur with for example potassium sulphate. The maximum theoretical amount of this that can be dissolved in a 400L tank is 48kg. In reality the actual amount is much less due to water temperature and the presence of other ions in solution etc and in fact you are probably looking at a maximum of 35-40kg being soluble in the tank.

Chelates
Sulphate and oxide trace elements are not absorbed well into the plant with movement within the plant being limited. Chelates are protected from chemical reactions but if you really have to apply calcium you will be better using EDTA chelate as this is a much stronger chelate and so more stable in the tank and so do not cause so many issues with tank mixing in comparison to say the more commonly used sulphate forms. For example substituting EDTA magnesium or calcium in place of Epsom salts or calcium nitrate means that phosphorus can be added to the tank without a precipitate being formed as would be likely otherwise.

Glucoheptanates are weak chelates that are biodegradable and generally weaker than EDTA but useful as an alternative chelating agent for very high pH solutions. Metal cations like calcium, magnesium and barium can form low water-soluble salts with carbonates, sulphates and phosphates that precipitate out of aqueous systems. Glucoheptanate chelates are best used as foliars.

An important factor is the strength of the complex formed between the metal ion and the chelating agent. This determines whether the complex will be formed in the presence of competing anions. The stability or equilibrium constant (K), expressed as log K, has been determined for many metals and chelating agents. The
higher the log K values, the more tightly the metal ion will be bound to the chelating agent and the more likely that the complex will be formed.

Table 4. Stability Constants Log K Values

Metal Ion	EDTA	DTPA	EDDHA	HEDTA	NTA	GLDA	EDG	PDTA	Glucoheptanate	Citrate
Al	16.4	18.6		14.4	11.4	12.2	7.7	16.3
Ba	7.9	8.7		6.2	4.8	3.5	3.4	3.9
Ca	10.7	10.8	7.2	8.1	6.4	5.9	4.7	7.2	2.2
Cd	16.5	19		13.7	9.8	9.1	7.4	13.8
Co	16.5	18.8		14.5	10.4	10	8	15.5
Cu	18.8	21.2	8.0	17.4	13	13.1	11.8	18.8	38.9
Fe2+	14.3	16.2	35	12.2	8.9	8.7	6.8	13.4	1	3.2
Fe3+	25.1	28		19.7	15.9	11.7	11.6	21.6	37.2
Hg	21.5	26.4		20.1	14.3	14.3	5.5	19.8
Mg	8.8	9.3	8.0	7	5.5	5.2	3.4	6.2	0.7
Ni	18.4	20.1		17.1	11.5	10.5	9.4	13.6
Pb	18.0	18.8		15.6	11.5	10.5	9.4	13.6
Sr	8.7	9.8		6.8	5.0	4.1	3.8	5.2
Zn	16.5	18.2	7.7	14.6	10.7	10.0	8.4	15.2	1.7

As the table above shows calcium glucoheptanate is the weakest chelated calcium listed.

Table 5. Fertiliser options for tank mixing

Nutrient	Fertilisers	Analysis	Other	Solubility g/100 mL @ 10 ºC	Solubility g/100 mL @ 80 ºC	Time to dissolve (mins)	pH	Final conc ppm)*
Nitrogen	Ammonium thiosulphate	12-0-0	S = 26	Liquid				N = 1.2 S = 2.6
	Ammonium sulphate	21-0-0	S = 24	73	95	15***	4.5	N = 2.1 S = 2.4
	Ammonium nitrate	34-0-0		Liquid			5.6	N = 3.4
	urea	46-0-0		84		20	9.5	N = 4.6
Nitrogen and phosphorus	MAP	12-27-0		28		20	4.5	N = 1.2 P = 2.68
	Ammonium polyphosphate	20-13-0		Liquid				N = 2.0 P = 1.3
Phosphorus	Phosphoric acid	0-23-0		Liquid				P = 2.28
Phosphorus and Potassium	MKP	0-23-29		18			4.5	P = 2.27 K = 2.9
Potassium	Potassium chloride	0-0-51	Cl = 49	31	51	5	7-9	K = 5.8 Cl = 4.9
	Potassium sulphate	0-0-41.5	S = 18.4	9		5		K = 4.15 S = 1.8
	Potassium nitrate	13-0-38		21	169	3	10.8	N = 1.3 K = 3.8
	Potassium thiosulphate	0-0-30	S = 25	Liquid				K = 3 S = 2.5
Magnesium and calcium	Magnesium nitrate	10.8-0-0	Mg = 9.5	220				N = 1.1 Mg = 0.95
	Magnesium sulphate	9% Mg	S = 12.8	62				Mg = 0.96; S = 1.28
	Calcium nitrate	15.5-0-0	Ca = 19.6	113				N = 1.5 Ca = 1.96

* When dissolving 1kg of fertiliser in 100L of stock solution, injected at a dilution ratio of 1:1000
** Always check compatibilities.
*** Solution temperature drops to 0°C, hence it takes longer for all material to dissolve
**** Ammonium polyphosphate gives excellent performance in calcareous soils due to the resistance of the pyrophosphate ion to hydrolysis, leading to a steady supply of P to plants in the vital early stages of growth. Pyrophosphate ions appear to be able to persist in highly P fixing Australian soil types with approximately half of the P added as pyrophosphate remaining after three weeks of incubation. Sorption data suggests that pyrophosphate is rapidly sorbed in these soil types, a possible explanation for its relatively long-term survival.

Ammonium polyphosphate is also able to “chelate” micronutrients like Zn, Cu and Mn in the soil, protecting them from being converted to insoluble forms which are not plant-available. Chelated micronutrients are more compatible than sulphates with APP but they can be very expensive, depending on the form of chelate.
***** KTS is: water, pesticide, KTS and/or other fertiliser. Always perform a jar test before injecting blends. Potassium thiosulphate provides not only potassium, but the thiosulphate is oxidized by Thiobacillus bacteria to produce sulphuric acid. This acid reacts with calcium carbonate in the soil, which releases additional calcium for the plant. Thus, potassium thiosulphate use on calcareous soils not only supplies potassium and sulphur, but aids in increasing the availability of calcium to plants. This could also be used to increase the plant availability of calcium amendments such as Calcour® or Calrite®.

Nutrient-enriched irrigation water may also result in increased microbial activity, causing blockages in irrigation lines and emitters in trickle irrigation systems.
To overcome this aim to carry out a ‘jar test’. This involves putting some fertiliser solution into a jar of irrigation water and observing if any precipitate forms over a period of say 2 hours. If cloudiness is apparent there is a chance that it may block emitters or nozzles.

When carrying this out use the appropriate dilution rate which mirrors that which occurs in the tank. For example if using a 400L tank and you want to carry out a jar test in 1L, simply divide all the intended inputs going into the tank by 400. Then add the amounts to the litre.

Jar Test for Compatibility of Pesticide Mixtures
Always wear personal protective equipment (PPE) when pouring or mixing pesticides and carry this out
in a safe area away from food and sources of ignition.

Step 1. Measure 1 litre of water into a clear glass jar. Use the same water that you will use when making up the larger mixture.
Step 2. Add ingredients in the following order. Stir each time a formulation has been added.

• Compatibility agents and activators.
• Wettable Powders and Dry Flowables.
• Water soluble concentrates or solutions.
• Emulsifiable concentrates.
• Soluble powders.
• Remaining adjuvants and surfactants.
Step 3. After mixing, let the solution stand for 15 minutes. Stir well and observe the results monitoring such things as chemical reactions occurring (heat, foaming etc). If the mixture appears to be undergoing a chemical reaction then it is not compatible. Let the mixture stand for about 15 minutes and feel again for unusual heat.
• If scum forms on the surface, if the mixture clumps, or if any solids settle to the bottom (except for wettable powders), the mixture probably is not compatible. Finally, if no signs of incompatibility appear, test the mixture on a small area of the surface where it is to be applied.
Some “dos and don'ts”
1. Always follow the correct filling order for products, starting with 50 percent of the water in the tank prior to adding products.
2. Avoid mixing concentrated products together before mixing in the tank.
3. Don't mix non-chelated iron sulphate products with amine formulations of certain phenoxy herbicides. This could cause a cottage cheese like precipitate to form. Remember READ THE LABEL, which should make it clear if this is a potential problem.
4. Don't mix strongly acid materials with strongly alkaline materials. Again, the label should mention it if this is a potential problem.
5. You should pre-dissolve dry flowable herbicides in water prior to adding nitrogen
6. Don't mix potassium fertilizers or other fertilizers that have a high salt load (ionic strength) with emulsifiable concentrates and liquid flowables. This can cause compatibility problems and phytotoxicity.
7. Ensure that when you apply herbicide/fluid-fertilizer mixtures, you're applying the correct herbicide rate. Sprayer output may not be the same using large quantities of fluid fertilizer as when using water as the carrier. You should recalibrate sprayers for the fertilizer carrier.

Tank Mixing Guidelines

• Read the label. This is your first step when considering tank-mixes.
• Perform a jar test with any new mixes.
• Test pH. Many incompatibilities result from excessively alkaline (sometimes acidic) pH in the tank. The addition of buffering adjuvants can help.
• Make a test application to expose any phytotoxicity or antagonism before you make a large-scale application. If you overlap a few strips, this also can show you how much of a margin of safety you have. Wait a few days for symptoms to become visible.
• Take care with fertilizers. If you add fertilizers, be aware that they can have substantial effects on the chemistry of a tank mix, especially pH. Read the pesticide label for any fertilizer restrictions.
• Do not mix iron sulphate with phenoxy herbicides. Iron sulphate is incompatible with amine formulations of some phenoxy herbicides and can cause a precipitate to form, clogging spray equipment.
• Mix no more than one soluble or emulsifiable chemical with any insoluble products such as wettable powders or flowables.
• Avoid mixing strongly acid materials with strongly alkaline materials.
• Apply sprays soon after mixing. Mixes that sit for several hours or longer are prone to degrade, especially if the pH is alkaline.

Proper Mixing Procedures

• Mixing Order. I n general, follow the W-A-L-E-S plan when adding herbicides to a tank mix.

1. Wettable Powders (WP) then Flowables (F, DF)
2. Agitate then add adjuvants such as anti-foaming compounds, buffers
3. Liquid and Soluble products
4. Emulsifiable concentrates (EC)
5. Surfactants

References
Akzo Nobel, Adjuvants for Fungicides. Surface Chemistry Agro Bulletin, , 2005.

Fishel FM 2007http://edis.ifas.ufl.edu/PI182 - FOOTNOTE_2 Pesticide Interactions Publication #PI-145 University of Florida http://edis.ifas.ufl.edu/PI182

Smith RM and Martell AE Critical stability constants Plenum Press New York and London 3rd Edition

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