How an unlikely plant saved Denmark’s farms.


If you were trying to grow something in Denmark in the 1700s, you were in for a host of problems. Your field was prone to flooding, your yields decreased every year and you might wake up one day to find that your field is covered in a foot of sand.

By the 1850s, though, Denmark was a different story. It was a breadbasket and dairy for all of Europe, with rich and productive fields. I came across this reference in a book on Danish cooking and wondered how they had turned around the country in such short time. I was able to find a great book on Danish ecological history on Google Books and learn the answers.

How to screw up your agriculture: a Danish history.

Denmark utterly ruined its agriculture over a period of two hundred years. It was a combination of many things. Let’s set the stage, though. The year is 1500 and the Black Death has receded. It killed a great number of Danes, which is horrific – but also has the consequence of lowering the pressure on fields and forests for food. The forests regenerate and people enjoy a productive century. They enjoy it enough that they have a lot of babies. The population doubles between 1680 and 1780.

If you’re enjoying relative prosperity and you’re a king, you do the obvious thing and build a huge palace. This requires a colossal amount of lumber. You would also start building a navy. You’ll need lighthouses to dot the coast and protect your shipping. All of these things are constructed from bricks, which need to be fired with wood (nobody is using coal yet). Ergo, you cut down all the forests.

No forests and intensive agriculture mean that rain swells the streams and rivers. The fields don’t drain as well. The sand dunes, ever a fixture of Denmark, expand and sweep around the country. That’s where you might have your field covered in sand after a sandstorm.

Nitrogen is the key to all agriculture.

Plants need nitrogen to grow. It is a tricky element because it disperses quickly again into the water or the air. Modern fertilizers are made to release it slowly and provide concentrated amounts, but none of that was available in the 1700s in Denmark. Nobody knew what nitrogen was, but they were aware that manure and night soil (that is, human waste) would improve a field when it was applied.

The Danes used manure that accumulated in stables to fertilize their fields. In the cities, people engaged in a manure trade, selling it to outlying farms. For a capitalist, by the way, this is just about the best business you could be in – literally selling horse crap. All of this nitrogen distribution is just robbing Peter to pay Paul, though. The hay comes from fields and over time, those fields get depleted. No new nitrogen is being added. The effect was that the farms outlying the cities had better access to manure and those in the countryside had lower yields due to decreased access. The Danes even forbade selling manure to other towns at one point in an effort to keep nutrients in local fields.

To maintain yields, the Danes used a three-field system of crop rotation that had been standard in Europe since the 12th century. Divide a field into three sections. In one year, you plant a third with wheat or rye, a third with oats or peas, and you let the final third grow wild (going fallow). Those oats would be fed to draft horses capable of pulling more powerful plows. This increased productivity in the fields, but it still stressed the nitrogen.

The key was to get the abundant nitrogen from the atmosphere into the soil. Luckily, there was a plant that could do it.

How clover saved the Danish fields.

Clover can “fix” nitrogen from the air. It has nodules on its roots that contain a bacteria capable of transforming atmospheric nitrogen into a solid, water-soluble form. That means that clover is not zero-sum in the nitrogen game – it adds more into the soil. This is a trait it shares with legumes, vetch, lucerne and a few other plants.

The Danes didn’t understand why clover was helping their fields, but they knew it worked. We have a tendency to view pre-scientific societies as hopelessly ignorant, but humans are keen observers. We knew how fermentation worked for four thousand years before we figured out what was happening.

Clover is the unsung hero of the European agricultural revolution. The Danes domesticated Spanish clover in the 1740s and it is widespread by the 1780s. Remember that three-field system above? Instead of leaving the third field fallow, they planted it with clover. This was enormously useful: the clover’s nitrogen-fixing powers are in its roots and not in the aboveground plant. That means that a farmer can cut all the clover for hay or put his cattle on the field and they will not disturb the nitrogen in the soil below! The third field finally came into useful agricultural production. When the field was plowed under for wheat, rye and barley, the nitrogen fed those plants richly.

By the 1830s, cattle populations had gone from 450,000 to 600,000. More butter and beef for everyone. Fields were under less production pressure, which meant that the marginal fields bordering sand dunes could be converted back into forest or plains, preventing those ruinous sandstorms. By 1900, Denmark had some of the very best soils in Europe, thanks to clover’s nitrogen fixing and use as a feed crop to horses.

How to apply the Danish lesson to your home garden.

After studying this, I got to thinking about why it was significant to study the Danes on this. The answer, I think, is that they had mediocre soils to begin with (no chernozem like Ukraine has) and ruined what they had. The clover revolution was going across the rest of Europe, but they started from a better position because they had less flood-prone fields and richer soil to begin with.

The lesson here is simple: any soil an be made more productive if you stop doing bad things and start doing good things.

The chief bad thing you can do is leave the land bare. After harvest, people leave their plots exposed to the elements for months. Nitrogen dissipates in about six months from the soil, so you don’t have a great timeline to work with. The small-scale and organic approach is to sow a cover crop, and what better to use than clover?

Clover can be planted in the spring or the fall. You can toss the seeds onto a patch of dirt that you just harvested. It’s an easy plant to grow. You can buy seeds online or from your local rural supply store. When you’re ready to plant that square of ground with something again, just turn under the clover about three weeks before you’re ready to plant. Three weeks gives the soil bacteria some time to start breaking down the plant matter and releasing nitrogen.

The good thing you can do is to remember that everything that gets taken out of the garden must have its nutrient load replaced. Composting is a good way to recapture some of the loss. If you compost store-bought plant waste, you’re engaged in a zero-sum game – taking the nutrients from that field and putting them in yours. It’s better than tossing it all away, though. Some people go so far as to recapture their own human waste for agriculture, but I’m, uh, not there yet. Industrially, nitrogen is chemically fixed from the atmosphere by using chemical processes (invented by one of the most controversial Nobel Prize winners). I use industrial fertilizers at times (like with potted and container plants) but I am always mindful of that nutrient deficit and the need to repay it.

So: plant a cover crop on patches of ground that are bare and cut or plow them in when you want to plant. Rotate the crops through your plots so that you don’t exhaust the soil with growing the same thing. And finally, love that clover.