Storm rain carries nitrate and ammonium formed by lightning — arriving at pH 5.6, versus tap water’s 7.5–8. The sky has been running a fertilizer operation the whole time.
Lightning converts atmospheric nitrogen into plant-available nitrate and ammonium, which then fall with raindrops directly into the root zone. That chemistry is what separates a thunderstorm from a garden hose. The greening gardeners notice after a storm is biology responding to a genuine nitrogen delivery, not imagination.
Tap water does the hydrating part. It doesn’t do the rest.
How Lightning Turns Air Into Plant-Available Nitrogen
Lightning supplies enough energy to break apart nitrogen gas (N₂) and oxygen gas (O₂). The resulting nitrogen oxides — NO and NO₂ — react further to form nitric acid and nitrate. Falling raindrops scavenge those compounds from the atmosphere, delivering nitrate, nitrite, and ammonium to the soil.
This is a gentle supplement, not a substitute for fertilization. Atmospheric wet deposition adds reactive nitrogen on the order of several kilograms per hectare per year across temperate regions. A single storm contributes measurably, but the input remains modest compared with deliberate fertilizer application.
What Rain’s pH 5.6 Does to Nutrients Already in the Soil
Rainwater equilibrated with atmospheric CO₂ settles at around pH 5.6. Most municipal tap water sits closer to 7.5–8. That gap matters because micronutrient solubility is sensitive to pH.
Iron, manganese, zinc, and copper tend to bind tightly in alkaline soil conditions. When rain temporarily shifts soil pH toward neutral, those locked micronutrients become soluble and mobile again — accessible to roots that couldn’t reach them before.
Repeated use of alkaline, high-salt tap water pushes soils in the opposite direction over time.
What Tap Water Carries That Rain Doesn’t
Municipal water is treated for human health, not plant chemistry. It arrives with chlorine or chloramine, often fluoride, and frequently elevated sodium from water-softening processes. Prolonged exposure to high chloride concentrations can inhibit nitrate uptake by roots directly.
Rainwater, by contrast, is naturally soft — low in dissolved salts and hardness minerals. It also tends to arrive closer to ambient outdoor temperature, which reduces cold shock to roots that chilled tap water can cause.
Rain also soaks more uniformly and deeply than hose application. Gardeners rarely apply hose water long enough or evenly enough to replicate the depth of a natural rain.
After a thunderstorm, plants are responding to a specific chemical event — nitrogen chemistry in the water, an unlocking of bound soil nutrients, and the absence of the solutes that work against them daily. Every one of those factors separates storm rain from what comes out of the tap.
The storm was always also a meal.
Frequently Asked Questions
Why do plants look greener after rain than after watering?
Storm rain delivers reactive nitrogen from lightning, temporarily lowers soil pH to unlock bound micronutrients, and contains no chlorine or added salts that tap water carries.
Does tap water contain any nitrogen for plants?
Some tap water contains trace nitrate, but the concentration is low and inconsistent — not a meaningful fertilizer input compared with nitrogen delivered by storm rain.
Is rainwater’s pH of 5.6 considered acidic enough to harm plants?
No. A pH of 5.6 is the natural equilibrium of rainwater with atmospheric CO₂, and it falls within or near the optimal range for most garden plants.
Does lightning produce ammonium or only nitrate in rain?
Both. Nitrogen oxides formed by lightning ultimately yield nitrate and nitrite in rainwater; ammonium forms through additional atmospheric and soil microbial processes and is also present.