The tank is a chemistry flask
When a herbicide "doesn't work", the water it was mixed in is a prime suspect. Spray water makes up more than 95% of what leaves the nozzle, and three of its properties — hardness, pH and dirt — react with crop-protection chemistry before the spray ever reaches a leaf. One rule stands above everything in this article: the registered product label is the law; where a label speaks about water quality, conditioners or mixing order, it overrides any general guidance here.
Hard water ties up the active
Calcium and magnesium in hard water are positively charged; many herbicide molecules — glyphosate above all, but also 2,4-D amine, dicamba and glufosinate — are negatively charged weak acids. They pair up in the tank, and the calcium-bound herbicide penetrates leaves poorly. The working numbers: caution starts at about 150 mg/L hardness (as CaCO₃), especially with pH above 7; registrant guidance for glyphosate keeps hardness under about 350 mg/L at low label rates and under 700 mg/L at high rates; and 2,4-D amine can be effectively neutralised outright in water above 600 mg/L. Many South African boreholes sit comfortably past these lines.
The standard remedy is spray-grade ammonium sulphate, dissolved before the herbicide at 1–2 kg per 100 L of water: the sulphate captures the calcium before the herbicide can, and the ammonium actually improves glyphosate uptake. Two caveats — ammonium sulphate does not rescue 2,4-D amine, and some labels prohibit it, so read first. The structural fix is better water: a small softener, or reverse osmosis for the mixing supply, turns every future tank into the easy case.
pH: the clock starts at mixing
Most insecticides and many herbicides are stable in slightly acidic water (pH 4–6.5) and fall apart in alkaline water — a process called alkaline hydrolysis, and it is brutally fast for some products. Dimethoate's half-life is about 12 hours at pH 6 and 48 minutes at pH 9; carbaryl lasts months at pH 6 and about a day at pH 9. Municipal water typically runs pH 7.8–8.5, so "it came from the tap" is no defence. The working rules: mix and spray the same day, never leave a filled tank overnight, and where your water runs alkaline, use a buffering adjuvant to bring the tank to pH 5–6.5 — checked with a strip or pocket meter, not guessed.
The exception that proves you must read the label: sulfonylurea herbicides prefer pH 7–8 and degrade in acidified water. Never blanket-acidify every tank.
Bicarbonate and mud: the two hidden tests
Two water problems don't show up on a hardness test. Bicarbonate — ask the lab for alkalinity explicitly — blunts the "dim" grass herbicides (sethoxydim, clethodim) and 2,4-D amine from about 400–500 mg/L as CaCO₃, which is how South African laboratories normally report alkalinity. If your certificate gives bicarbonate as HCO₃ instead, multiply by 0.82 before comparing: 500 mg/L as HCO₃ is about 410 as CaCO₃, and reading one for the other moves the threshold by a fifth. And suspended clay binds and deactivates glyphosate and paraquat on contact; the field test is famously simple: fill a 20 L bucket and drop a coin in — if you cannot see the coin, the water is too dirty to spray with. Muddy water also grinds pumps and blocks nozzle screens, so dam water used for spraying should pass through proper filtration first — the same media-and-cartridge train the drip system already justifies.
What good spray water looks like
Clear enough to see the coin; hardness under 150 mg/L (never above the label's line); alkalinity under 400 mg/L; pH brought to 5–6.5 for most products, left near neutral for sulfonylureas; and low overall salts (an EC under about 50 mS/m rarely causes trouble). Test spray water once a year like irrigation water — a single laboratory analysis is trivial against one failed spray pass over a full block, in chemical wasted and in the weeds that got a sub-lethal dose and hardened.
Rule of thumb
Fix the water before doubting the chemical: soften or condition hard water (ammonium sulphate in first, herbicide second), buffer alkaline water, and never spray from the muddy end of the dam. Mix and spray the same day. And whatever this page says, the registered label says it louder.