Why Do Vegetables Stop Growing When You Plant Them in the Same Spot Year After Year?
― Something Is Quietly Carried Out of the Soil Every Year
Same planter as last year, same soil, same care. Yet this year's tomato plant stays short and its fruit is small. Farmers have long said, "Don't grow the same crop again and again," and have slipped in beans between crops. The reason why was worked out only about 130 years ago.
Last year, the cherry tomatoes in your balcony planter did wonderfully. It seemed a waste to throw out the soil, so you reused it and planted again this year.
But the leaves are pale, and the lower ones are turning yellow. Fruit does appear, but it stops at about half of last year's size.
The water and sunlight are the same as last year. The only thing that has changed is that the soil has been "used" for one more year.
There are two main reasons
Taking home fruit and leaves means taking what was in them out of the field. The thing most likely to run short is nitrogen, which is the raw material for the proteins in leaves and fruit.
Bacteria, fungi and tiny worms called nematodes gather around a crop's roots when they like that crop. If the same crop keeps coming, they get a fresh meal every year, and their numbers grow.
The poor growth caused by these two problems together is called continuous-cropping injury. What is interesting is how long it took humans to work out the cause of the first one.
1: How Much Nitrogen Leaves the Soil?
Leaves and fruit are made of water, carbon and a little nitrogen. Carbon comes in freely from the carbon dioxide in the air. Nitrogen, though, has to be taken up through the roots.
We work it out in the fold-out section below. If you harvest tomatoes from one square metre of field, about 20 grams of nitrogen leave the field with the whole plant. And this happens again every year.
Here is the odd part. Air is 78 per cent nitrogen. The air above one square metre of field holds about 8 tonnes of it. Yet right underneath, the plants go short of nitrogen.
Nitrogen in air is two atoms locked tightly together. The bond is so strong that ordinary plants cannot break it. There is a mountain of it overhead, yet out of reach. That is the trouble with nitrogen.
2: How We Learned the Culprit Was Bacteria Living in the Roots, Not the Plant
Crops grow well the year after beans. Even ancient Roman farming books already said so. So the effect itself has been known for 2,000 years.
Even so, until the mid-19th century most scholars believed that "plants eat the rotting organic matter in the soil itself." No one could explain why beans were special.
The turning point came in 1838. The Frenchman Boussingault kept carefully weighing crops and soil on his own farm. He is said to have found that only in plots where legumes were grown did the field's total nitrogen actually increase. Nitrogen was coming from somewhere.
But whether it came from the air, and who was doing the work, remained unknown. The answer came in 1886, from experiments by the Germans Hellriegel and Wilfarth.
They grew beans in sand stripped of nutrients and microbes. The beans did not grow. But in pots watered with a little water in which field soil had been steeped, small lumps formed on the roots, and the plants thrived.
So it was not the bean itself that could use nitrogen from the air, but bacteria living in the lumps on its roots. The plant offers a home and sugar; the bacteria break apart nitrogen from the air and hand it over. This trade was the secret behind the 2,000-year-old rule of thumb.
Much of the nitrogen the nodules take from the air moves into the beans and leaves. If you harvest all the soybeans and carry them home, the nitrogen left in the field barely increases. To enrich the soil, it suits better to cut the plants before they set seed and dig them in, a method called "green manure".
The enemies that build up under reason 2 are mostly shared by every crop in a plant "family". Even if you plant aubergine or peppers after tomatoes, they are all in the nightshade family, so the enemies in the soil stay the same. If you want to give the soil a rest, switch to a crop from a different family.
Summary
Crops do worse under continuous cropping not because the soil has grown old. Each year's harvest carries nitrogen and other things out, and at the same time enemies of that crop build up in the soil. Crop rotation is an old piece of wisdom that shifts both at once.
There are 8 tonnes of nitrogen above the field.
The problem isn't the amount, but whether it is in a form that can be broken apart.
Nitrogen from the air has one other natural route into the soil besides root-nodule bacteria. Is It True That Rice Does Better in Years With More Lightning? introduces the chemistry by which electrical discharge in the sky turns air into fertiliser. For the work of microbes in the soil, see also What's the Difference Between Fermentation and Rot?
- From spring to summer, sow edamame or green bean seeds in a planter. Use only a little fertiliser.
- When the flowers open, pull up just one plant and gently wash the roots with water.
- If you find round lumps about 2 to 4 millimetres across on the roots, those are nodules. Try splitting one in half with your fingernail.
If the inside is pale pink or reddish brown, that is said to show the nitrogen-fixing work is going strongly. Lumps that stay white or green either haven't started working yet or have already finished. It can also be fun to count and compare the nodules in a year with reused planter soil and a year with fresh soil.
For Those Who Want More ― Terms, Formulas and Links to TextbooksEach part is marked with its level, from junior high science to university specialist courses
- Junior highCovered in junior high school science
- High schoolCovered in high school "Basic Biology / Basic Chemistry"
- HS+Advanced high school material, or a textbook sidebar
- UniversityUniversity specialist courses (soil science, plant nutrition) not taught in high school
- ResearchTopics researchers are still investigating, not taught even at university as settled fact
Junior highTerms: This Phenomenon Has Names
- Continuous-cropping injury: poor growth when crops of the same family are grown in the same place repeatedly. It comes from a shortage of nutrients combined with a build-up of disease-causing microbes and nematodes in the soil.
- Nitrogen fixation: turning nitrogen molecules in the air into a form plants can use (such as ammonia). Living things do it, and so do lightning and industrial methods.
- Root-nodule bacteria (rhizobia): a group of bacteria that enter the roots of legumes, form lumps (nodules) and pass nitrogen from the air to the plant. In return they receive sugar from the plant.
Junior highHigh schoolChecking with a Formula: How Much Nitrogen Leaves the Field in One Harvest?
We cannot measure the nitrogen carried away directly. So we work backwards from the amount of protein in the harvest, because protein is the only major nutrient that contains nitrogen.
| In symbols | N ＝ M × p × k |
| In words | Weight of nitrogen removed ＝ weight harvested × share that is protein × share of protein that is nitrogen |
| Where the formula comes from | It rests on the idea of conservation of mass: elements do not vanish along the way. The nitrogen share of protein is taken to be about 16% on average, and this figure is also used in food composition tables. |
| Tomatoes from 1 m² of field (rough guide) | About 6000 g, it is said |
| Protein per 100 g of tomato | About 0.7 g, it is said |
| Share of protein that is nitrogen | About 0.16 (16%) |
| Nitrogen in the whole plant, leaves and stems included | About 3 times that in the fruit, it is said |
| Convert the harvest into "units of 100 g" | 6000 ÷ 100 ＝ 60 |
| Protein in the fruit | 60 × 0.7 ＝ 42 |
| Nitrogen in that (g) | 42 × 0.16 ＝ 6.72 |
| Whole plant with leaves and stems (g) | 6.72 × 3 ＝ 20.16 |
So about 20 grams of nitrogen per square metre leave the field each year. Over 5 years that is 100 grams. If you don't add any back, the soil steadily loses it.
| Weight of air over 1 m² (kg) | 101325 ÷ 9.8 ≒ 10339 |
| Of that, nitrogen (about 75% by weight) | 10339 × 0.75 ≒ 7754 |
| The 20 g removed is 0.02 kg, so | 7754 ÷ 0.02 ≒ 387700 |
The nitrogen overhead is about 390,000 times the amount that leaves in a year. It is like starving next to a warehouse: what is lacking is not the amount, but a way to turn it into a form that can be broken apart.
High schoolHS+Why Can't Plants Use Nitrogen From the Air Directly?
High schoolA nitrogen molecule is two nitrogen atoms joined by a triple bond. The energy needed to break this bond is said to be about twice that of an oxygen molecule, among the largest of any molecule around us. That is why nitrogen in the air neither burns nor rusts, but simply stays as it is.
HS+Living things break this bond by biological nitrogen fixation, carried out by an enzyme called nitrogenase. This enzyme stops working when it meets oxygen, so inside a nodule a pigment called leghaemoglobin captures oxygen and keeps its level low. The pink you see when you split a nodule is the colour of this pigment.
UniversityThe Nitrogen Cycle and How Nodule Symbiosis Arises
Soil science and plant nutrition treat this whole flow as the nitrogen cycle. Nitrogen fixation, uptake by plants, conversion to ammonia as organic matter breaks down, nitrification to nitrate, and denitrification back to gas together form one loop. The relationship between legumes and nodule bacteria is called symbiotic nitrogen fixation. It is known that the partners recognise each other through signal molecules from the plant (flavonoids) and responses from the bacteria (Nod factors), and that the bacteria enter root cells through infection threads.
📖 Derivations and further reading: Nitrogen fixation (Japanese Wikipedia) ／ Root-nodule bacteria (Japanese Wikipedia)
ResearchWhat Is Still Not Clear
- Can rice and wheat be given root nodules? Research continues on moving the machinery of symbiotic nitrogen fixation into non-legume crops, but it needs a complex set of genes and is not yet ready for practical use.
- What is the main cause of continuous-cropping injury? In some cases a particular disease-causing microbe is responsible; in others an upset in the balance of the whole soil microbe community matters. The situation is thought to differ from crop to crop.
- What happens when too much nitrogen is applied? Some of the nitrogen put on a field turns, during denitrification, into nitrous oxide, a gas with a strong greenhouse effect. Estimates of how much is released, and under what conditions, still cover a wide range.
In other words, what this article says is also "an explanation of what is known so far". The inside of the soil is still one of the hard-to-observe worlds.
Links to Textbooks (by Level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science (links among living things, role of decomposers) | Microbes in the soil, and nutrients flowing in and out |
| High school | Basic Biology / Basic Chemistry (nitrogen assimilation, chemical bonds) | The triple bond of the nitrogen molecule, and working out nitrogen from protein |
| HS+ | Biology (nitrogen fixation, symbiosis) | Nitrogenase and leghaemoglobin |
| University | Soil science, plant nutrition | The nitrogen cycle, nitrification and denitrification, how partners recognise each other |
| Research | Plant molecular biology, agricultural environmental engineering | Transplanting symbiosis into non-legumes, release of nitrous oxide |
| ― | Links to daily life | Reusing planter soil, planting by shifting families, green manure |
- Ministry of Education, Culture, Sports, Science and Technology (文部科学省), "Standard Tables of Food Composition in Japan 2020 (8th revised edition)" (protein content, and the factor for converting nitrogen to protein)
- Japanese Society of Soil Science and Plant Nutrition (日本土壌肥料学会), ed., "Introduction to Soil Science" (土壌サイエンス入門) (basics of the nitrogen cycle and soil fertility)
- Historical accounts of J. B. Boussingault's field experiments on the nitrogen balance of legume crops (1838)
- H. Hellriegel and H. Wilfarth, "Studies on the nitrogen nutrition of legumes" (1888, an experimental report showing the role of root-nodule bacteria)
- Ministry of Agriculture, Forestry and Fisheries (農林水産省), "Materials on soil improvement and use of organic matter" (the thinking behind continuous-cropping injury and crop rotation)
*This article is a popular-science explanation for general readers. The figures given are rough guides to help you understand how things work. Actual yields and the amount of fertiliser needed vary greatly with crop variety, soil type and region. For growing decisions, follow the guidance of your local agricultural extension centre or municipal materials.