Why does the same vegetable stop growing well if you plant it in the same spot every year?
― Something is quietly leaving the soil every year

Same planter, same soil, same care as last year. And yet this year's tomatoes won't grow tall, and the fruit is small. Farmers have long said "don't grow the same crop over and over," and planted beans in between instead. It's only in the last 130 years or so that we actually found out why.

There are two main reasons

The poor growth that results when these two overlap is together called continuous cropping injury. What's interesting is how long it took humanity to pin down the true cause of reason 1.

Reason 1: how much nitrogen actually leaves the soil

Leaves and fruit are built mainly from water, carbon, and a little nitrogen. Carbon comes in freely from carbon dioxide in the air. But nitrogen can only be drawn up through the roots.

We'll work through the numbers in the fold-out section below, but harvesting tomatoes from one square meter of garden carries roughly 20 grams of nitrogen out of the plot, whole-plant included. And this repeats every year.

Here's the strange part: 78 percent of the air is nitrogen. The air sitting above one square meter of garden contains roughly 8 tonnes of nitrogen. And yet the plants right underneath it run short of nitrogen.

Nitrogen in the air comes as two atoms locked tightly together. That bond is so strong that ordinary plants can't break it. There's a mountain of it overhead, but it's out of reach. That's the awkward thing about nitrogen as an element.

Reason 2: how we learned the culprit was "not the plant, but bacteria living in its roots"

Crops grow well the year after beans are grown. Even ancient Roman farming manuals already recorded this. In other words, the effect itself has been known for 2000 years.

Even so, up until the mid-19th century, most scholars believed that plants fed directly on decayed organic matter in the soil. No one could explain what made beans special.

The turning point came in 1838. In France, Boussingault carefully weighed his crops and soil on his own farm, year after year. He is said to have found that only the plots where legumes had grown showed an overall increase in the field's nitrogen. Nitrogen was welling up from somewhere.

But whether it came from the air, and who or what was doing it, remained unknown. The answer came in 1886, from an experiment by Hellriegel and Wilfarth in Germany.

They grew beans in sand stripped of both nutrients and microbes. The plants failed to grow. But in pots watered with a little dissolved farm soil, small nodules formed on the roots, and the plants grew vigorously.

In other words, it wasn't the bean plant itself that could use nitrogen from the air — it was bacteria living in the root nodules. The plant provides housing and sugar; the bacteria break apart atmospheric nitrogen and hand it over. That trade turned out to be the truth behind 2000 years of folk wisdom.

Summary

Growth declines under continuous cropping not because the soil has "gotten old," but because each harvest carries nitrogen and other nutrients away, while organisms that prey on that crop build up in the soil at the same time. Crop rotation is old wisdom for shifting both problems out of sync.

There are 8 tonnes of nitrogen above the garden.
The problem isn't quantity — it's whether it's in a usable form.

Besides root-nodule bacteria, there's one other natural route by which atmospheric nitrogen enters the soil. See Is it true that rice harvests are good in years with lots of lightning? for the chemistry of how lightning turns air into fertilizer. For more on what soil microbes do, see What's the difference between fermentation and rotting?.

※This article is a general-audience science explainer. The figures given are approximations meant to help illustrate the underlying mechanism. Actual yields and fertilizer needs vary widely by crop variety, soil, and region. For growing decisions, please follow the guidance of your local agricultural extension center or municipal resources.