Alkaline pH and hard water can strip performance from herbicides and insecticides, but disciplined mixing and the right adjuvants restore control. A strict W‑A‑L‑E‑S sequence keeps tank mixes homogeneous and on‑label.
Industry: Agriculture | Process: Pesticide_Application
In a year when agricultural pesticide use reached ~3.73 million tonnes of active ingredients worldwide (~2.40 kg/ha), even small efficacy losses matter (www.fao.org). The quiet spoiler in many spray tanks is the carrier water: its pH and hardness can accelerate breakdown, bind actives, and force repeat applications.
Alkaline mixes are a repeat offender. Mississippi State University reports that weak‑acid herbicides such as 2,4‑D and glyphosate undergo rapid hydrolysis at pH 8–9, slashing available active ingredient (extension.msstate.edu). Carbamate and organophosphate insecticides are also rapidly deactivated by alkaline water (extension.missouri.edu). A vivid example: carbaryl’s half‑life plummets from ~24 days at neutral pH to ~1 day at pH 9 (extension.msstate.edu).
Other chemistries invert the rule. Some weak‑base herbicides (e.g., metsulfuron) degrade in very acidic mixes, underscoring the value of measuring spray mix pH and targeting the manufacturer’s range, often pH 4.5–7.
Hardness chemistry and efficacy loss
Calcium and magnesium in hard water can chelate or precipitate many pesticides. A recent review found that increasing hardness from 0 to 1000 mg/L CaCO₃–equivalent reduced control by ~18–28% on weeds for herbicides like mesotrione, horseweed, and Palmer amaranth (Fig. 1) (www.cambridge.org). The antagonism is especially acute with weak‑acid herbicides such as glyphosate and 2,4‑D, which can bind to tank walls or nozzle deposits.
Conditioning the water mitigates the damage. Extension guides recommend ammonium sulfate (AMS) as a buffer/hardness fix; rates of 8.5–17 lb per 100 gal (≈1–2% w/v) are advised when hardness exceeds ∼100 ppm (extension.msstate.edu). Field data back the practice: under very hard water, including AMS with glufosinate improved weed control and delivered ~11% higher corn yield than glufosinate without AMS (www.researchgate.net).
In short, testing source water with a pH meter or test strips and a hardness test, then adjusting before or during filling, prevents large efficacy losses (often tens of percent) and potential yield penalties (extension.msstate.edu; extension.msstate.edu).
Adjuvants and buffers: function and rates
Adjuvants—spray additives used to modify solution properties or droplet behavior—are central to mix performance. Common categories include:
- Surfactants (nonionic or crop‑oil) to improve spray spread and leaf uptake, especially on waxy or hairy leaves; many labels specify a nonionic surfactant (NIS) at 0.25–0.5% v/v with soluble herbicides such as glyphosate or some fungicides.
- Stickers or stickermix agents (latex or polymer films) to make residues rainfast, often at ≈0.1–0.25% to guard against wash‑off.
- Oil concentrates—crop oils (COC) or methylated seed oils (MSO)—that enhance cuticle penetration for systemic herbicides (e.g., 2,4‑D, ALS inhibitors), typically at 1–2% v/v per label.
- Foaming agents—antifoams (“defoamers”)—in small amounts (<0.1%) to prevent pump foaming under vigorous agitation.
- Drift retardants that thicken droplets to reduce drift.
- Buffers/acidifiers (e.g., ammonium sulfate, ammonium phosphate, proprietary acidifiers) that lower spray pH and/or sequester hard‑water ions. AMS provides ammonium and sulfate ions that lower pH and bind Ca/Mg, protecting glyphosate and similar acids (extension.msstate.edu; www.cambridge.org).
Labels often call explicitly for AMS or a pH adjuster under hard‑water conditions. Industry sources cite performance gains of up to ~40% higher weed control or >1 ton/ha better yield in tough conditions with optimal adjuvant blends (www.nichino.uk). Where products do not include built‑in adjuvants, a “spreader‑sticker” nonionic surfactant plus the recommended buffer (e.g., 8–10 lb AMS per 100 gal) is a commonly used safeguard (extension.msstate.edu). Accurate addition of these materials is often supported by dosing pump equipment for controlled chemical dosing.
Mixing order and WALES sequence
The classic W‑A‑L‑E‑S order helps minimize incompatibilities (www.pubs.ext.vt.edu):
1) Water (50% fill). Clean water (≈½ tank volume) precedes any agrochemicals. If needed, add liquid acidifiers or dry AMS at this point to pre‑adjust water pH/hardness (extension.msstate.edu). For operators focused on solids‑free fills, “clean water” is often ensured via cartridge filters.
2) W – Water‑soluble powders (WP, WDG, WSP) and dry flowables. These dry formulations go first and should disperse fully before moving to the next step (www.pubs.ext.vt.edu).
3) A – Agitation. Continuous agitation is maintained from the start; medium mixing—not violent aeration—keeps the solution uniform (www.pubs.ext.vt.edu).
4) L – Liquids/soluble concentrates (SC, SL). Add aliphatic liquid herbicides, soluble concentrates, or liquid nutrients at this stage (www.pubs.ext.vt.edu).
5) E – Emulsifiable concentrates. Oil‑based EC products and microencapsulated formulations follow; when combining multiple ECs, each is introduced slowly under agitation (www.pubs.ext.vt.edu).
6) S – Surfactants and adjuvants. Remaining additives (nonionic surfactants, oils, stickers, etc.) go last. Since adjuvants often cling to surfaces or fo , adding them last ensures they disperse fully (www.pubs.ext.vt.edu).
After all components are added, the tank is filled to volume and agitated for several minutes before spraying. Several operators also refer to the similar D‑A‑L‑E‑S sequence when starting with dry products.
Compatibility checks and timing
Jar tests flag problems before they scale. Using the same tank water and products in a clear jar (proportional amounts), operators look for precipitates or gels; clumping, heat, or odor indicates incompatibility (www.pubs.ext.vt.edu). Timely application matters as well—many herbicides degrade faster after 6–12 hours in solution.
Checklist for sprayer operators
- Testing of source water pH and hardness; adjustment with acidifiers or AMS if above optimal ranges (extension.msstate.edu; extension.msstate.edu).
- Sprayer calibration; tank filling begins with 30–50% water volume ahead of additions.
- Adherence to WALES order (WP → agitation → liquids → ECs → surfactants/adjuvants) (www.pubs.ext.vt.edu).
- Agitation between each addition at medium speed to prevent clumping or foaming (www.pubs.ext.vt.edu).
- Post‑fill agitation and visual inspection; jar testing of new product combinations as needed (www.pubs.ext.vt.edu).
- Prompt field application to avoid in‑tank degradation beyond 6–12 hours.
Documented outcomes and economics
Results are quantifiable. Adding AMS with glyphosate essentially nullified hardness antagonism in trials—no yield loss (www.researchgate.net; www.cambridge.org). Conversely, allowing hardness to climb from 0→1000 ppm squeezed 2,4‑D choline control by >20% in research trials (www.cambridge.org).
A simple jar test or pH adjustment can pay for itself by avoiding re‑sprays or lost yield. Across programs, documented yield gains of ~10–20% have been reported when mixes are optimized (www.researchgate.net).
Sources and references
Authoritative extension and research publications underlie the practices described. FAO pesticide statistics provide usage context (www.fao.org); Mississippi State University details pH/hardness effects and AMS buffer rates (extension.msstate.edu; extension.msstate.edu); peer‑reviewed trials quantify yield and efficacy impacts (www.researchgate.net; www.cambridge.org; www.cambridge.org; www.cambridge.org); and Virginia Tech documents the WALES mixing procedure and jar testing (www.pubs.ext.vt.edu; www.pubs.ext.vt.edu; www.pubs.ext.vt.edu; www.pubs.ext.vt.edu; www.pubs.ext.vt.edu; www.pubs.ext.vt.edu). Nichino UK provides industry examples of adjuvant‑driven gains (www.nichino.uk). Detailed metadata for each source is listed below.
- FAO, “Pesticides use and trade 1990–2023” (FAOSTAT highlights, 15 Jul 2025) (www.fao.org).
- Mississippi State Univ. Ext. (P3896), “Water Quality and Herbicide Efficacy” (extension.msstate.edu; extension.msstate.edu).
- Missouri Univ. Ext., Fred Fishel, “Effects of Water pH on the Stability of Pesticides” (2017) (extension.missouri.edu).
- Daramola et al., “Spray Water Quality and Herbicide Performance: a review”, Weed Tech. 36:1186–1197 (2022) (www.cambridge.org; www.cambridge.org).
- Soltani et al., “Effect of ammonium sulfate and water hardness on glyphosate/glufosinate in corn”, Can. J. Plant Sci. 91:1053–1059 (2011) (www.researchgate.net).
- Virginia Tech Ext., Erin Ling et al., “Understanding Spray Tank Mixing Practices” (VT BSE‑351P, 2018) (www.pubs.ext.vt.edu; www.pubs.ext.vt.edu; www.pubs.ext.vt.edu; www.pubs.ext.vt.edu; www.pubs.ext.vt.edu; www.pubs.ext.vt.edu).
- Nichino UK (Jan 2023), “Master tank mix adjuvants...” (included for illustration of efficacy gains) (www.nichino.uk).
