Do lightning-heavy years really mean a good rice harvest?
― The Japanese word for lightning hides some real chemistry that turns air into fertiliser
In Japan, lightning that flashes as the rice ripens has long been called "inazuma" — literally "rice-wife" — said to mean the thing that weds itself to the rice and makes it bear fruit. It sounds like pure folklore. But lightning really does make a form of fertiliser out of thin air. So is the bumper-harvest story true too? Once you run the numbers, the answer turns out to be "only half true."
A September evening. The rice in the paddies hangs heavy, and far off the sky flickers silently, dimly.
A while later, rain begins to fall. The next day, the grass beside the paddy somehow looks a shade greener.
Seeing this glow, people in the old days used to say, "We'll get a good rice crop this year." How much of that is actually true?
The short answer splits into two parts
About 78% of air is nitrogen. But in that raw form, plants can't use it at all. The intense heat along a lightning bolt's path converts that nitrogen into a form plants can take up. This isn't folklore — it's a real chemical reaction that actually happens.
Work out how much is produced, and it comes to roughly a thousandth of the nitrogen a paddy field needs in a year. That's nowhere near enough to explain a bumper harvest by "lightning spreads fertiliser."
In other words, the name "inazuma" gets the chemistry right but the scale badly wrong. Let's start by looking at why airborne nitrogen is no use to plants as it is.
Air is full of nitrogen — so why do plants run short of it?
Nitrogen is one of the three main plant nutrients. Plants can't make leaves or proteins without it. And nitrogen is all around us in huge quantities — about 78% of the air is nitrogen, so vast amounts drift above every paddy field too.
Yet plants can't touch it. Airborne nitrogen exists as pairs of atoms locked together by a triple bond — an extremely strong bond. Breaking it takes a huge amount of energy. Plants can only take in nitrogen once it's dissolved in water as nitrate or ammonium ions.
Something does break that stubborn bond by brute force: lightning. The air along a lightning bolt's path is thought to heat up to around 30,000°C — several times hotter than the surface of the sun. At that temperature, nitrogen and oxygen molecules fly apart, and as the air cools they recombine into a different substance: nitric oxide (Figure 1).
So how much nitrogen are we actually talking about?
This is where it gets interesting. Does lightning make enough nitrogen to meaningfully help a paddy field? Lightning worldwide is estimated to produce a few million tonnes of nitrogen a year. That sounds like an enormous amount.
But that's the total falling across the entire planet. Divide it by the Earth's surface area, and it comes to only about 10 kilograms per square kilometre. Scaled down to a 10-are paddy plot, that's a mere 10 grams or so. Rice, meanwhile, uses roughly 9 kilograms of nitrogen per 10 ares each year. The gap is about a thousandfold (Figure 2).
So where does a paddy field's nitrogen actually come from? One source is soil microbes: bacteria around the roots and in the water busily convert airborne nitrogen into a usable form. The other is fertiliser that people apply by hand. Next to these two, lightning's contribution is little more than a rounding error.
In old Japanese, the "zuma" ("wife") in inazuma could refer to either spouse. The name is thought to come from the idea that lightning "weds" the rice and makes it bear fruit. People long ago clearly noticed that lightning was frequent just before the rice harvest — their observation was accurate; only their explanation for it was off.
Grass looking greener after a storm is thought to owe less to any nitrogen delivered and more to the soil being thoroughly soaked and dust being washed away. The effect of the rain itself is far larger than any nitrogen boost.
Lightning reaches temperatures high enough to break apart airborne nitrogen — nowhere a person should be nearby. If you hear rumbling, leave the paddy or open field immediately and take shelter inside a building or a car. Stay away from tall objects such as trees and utility poles. If you find someone who has collapsed, call the emergency number right away.
Summary
Lightning really does break apart airborne nitrogen and convert it into a form plants can use. But the amount is only about a thousandth of what a paddy field needs. The name "inazuma" captures, in a single word, both a sharp eye for the phenomenon and a mistaken guess at its cause.
Lightning does make fertiliser out of thin air.
But it would take 1,000 years' worth to fill a paddy field.
For more on the shape of lightning itself, see Why does lightning strike in a jagged zigzag instead of a straight line?, and for how to stay safe, see Why shouldn't you shelter under a tree during a lightning storm?. For what happens inside a paddy field's soil, see Why do farmers deliberately drain paddy fields in summer? as well.
- From a safe spot indoors, count the seconds between seeing a flash and hearing the thunder. Multiply the seconds by 340 to get the rough distance to the lightning in metres.
- Photograph the same patch of grass or field before rain and again two days after, and compare the colour. If you spot a change, think about whether it's down to water or to nitrogen.
- If you have off-the-shelf nitrate test strips for soil, measure your garden soil's water before and after rain. A single thunderstorm won't produce a difference large enough to detect.
The third one is an observation designed to confirm "no detectable difference." Seeing an outcome that doesn't match your expectation is a perfectly good experiment too.
Want to know more? ― Terms, formulas, and where this fits in the curriculumEach item is labelled with its level, from lower-secondary science to university specialist courses
- JHSCovered in lower-secondary school science
- HSCovered in upper-secondary "Chemistry Basics / Chemistry"
- HS+Advanced upper-secondary content, or textbook sidebar material
- Univ.Not covered at upper-secondary level — university specialist content (atmospheric chemistry, soil science)
- ResearchNot yet settled even at university level — a question researchers are actively working on
JHSTerms: this phenomenon has names
- Nitrogen fixation: converting airborne nitrogen into a form living things can use. Besides lightning, this is also done by bacteria such as root-nodule bacteria, and in factories too.
- Triple bond: a bond in which atoms are held together with the strength of three bonds. Nitrogen molecules have this form, making them extremely hard to break apart.
- Nitrate ion: the form nitric acid takes once dissolved in water. Plant roots absorb nitrogen in this form.
JHSHSChecking with a formula: how much nitrogen does lightning deliver to a paddy field?
We'll scale the global total down, step by step, to a single paddy field. All figures are converted to kilograms, and we'll work through with plain numbers rather than symbols.
| Nitrogen lightning produces worldwide per year | About 5 billion kilograms (5 million tonnes) |
| Earth's surface area | About 510 million square kilometres |
| Nitrogen rice uses per year (per 10 ares) | About 9 kilograms |
| Convert to per square kilometre | 5000000000 ÷ 510000000 ≒ 9.8 |
| Scale to 10 ares (0.001 square kilometres) | 9.8 × 0.001 = 0.0098 |
| Compare with the amount rice uses | 0.0098 ÷ 9 ≒ 0.0011 |
That final figure, 0.0011, means roughly a thousandth. 0.0098 kilograms is about 10 grams — roughly the weight of two teaspoons of water. That's lightning's entire yearly contribution to a single paddy field.
HSHS+Why does it take such extreme heat?
HSThe energy needed to break the bond in a nitrogen molecule is thought to be about 940 kilojoules per mole — an exceptionally large value among everyday molecules. At the temperature of an ordinary flame, almost nothing happens.
HS+The reaction that forms nitric oxide from nitrogen and oxygen absorbs heat, so it proceeds more readily at higher temperatures. The nitric oxide formed along the lightning path survives because it cools too quickly to break down again, then slowly turns into nitrogen dioxide and, later, nitric acid.
Univ.From an atmospheric-chemistry view, lightning is "one source of nitrogen oxides"
In atmospheric chemistry, lightning is treated as a source of nitrogen oxides alongside cars and factories. Because it forms high above the surface, it also affects ozone formation. From soil science's perspective, though, lightning-derived nitrogen is just a tiny slice of "wet deposition" — nitrogen that dissolves in rainfall and reaches the ground. The same substance carries very different weight depending on which field you view it from.
ResearchWhat's still not fully understood
- Estimates vary widely Studies differ by several-fold on how much nitrogen lightning produces, because it's hard to directly count how many molecules a single discharge creates.
- How lightning frequency will change Some projections say rising temperatures will bring more lightning, but studies disagree on how much more.
- Few studies have tested the link to harvests Hardly any research has isolated the effect of lightning-heavy years on crop yield after accounting for weather. The old saying itself remains largely untested.
In other words, even this article reflects "the best explanation available for now." The global-total figure in particular could well be revised by future research.
Where this fits in the curriculum (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | Science: weather / plant structure and function | The composition of air, and why plants need nitrogen |
| HS | Chemistry Basics / Chemistry (bonding and thermochemistry) | The energy needed to break a triple bond |
| HS+ | Chemistry (equilibrium and temperature) | Nitric oxide forming at high heat and surviving rapid cooling |
| Univ. | Atmospheric chemistry / soil and fertiliser science | Nitrogen oxide sources, and treatment as wet deposition |
| Research | Atmospheric chemistry / agricultural meteorology | Estimate uncertainty, and testing the link to good harvests |
| ― | Everyday relevance | Stop outdoor work and take shelter when you hear thunder |
- Schumann, U. and Huntrieser, H. "The global lightning-induced nitrogen oxides source," Atmospheric Chemistry and Physics, 2007
- Japan Meteorological Agency, explanatory material on lightning (on how lightning forms and on staying safe)
- Ministry of Agriculture, Forestry and Fisheries (農林水産省) and prefectural fertilisation guidelines (typical nitrogen application rates for paddy rice)
- Japanese Society of Soil Science and Plant Nutrition (日本土壌肥料学会), ed., a soil and fertiliser science textbook (chapter on the nitrogen cycle and nitrogen fixation)
※This article is a general-interest science explainer. The figures given are approximations meant to help illustrate the underlying mechanism. When lightning approaches, please follow the guidance issued by the Japan Meteorological Agency and instructions from fire services and local authorities.