Peas, beans, and clover are part of a big plant family with around 18,000 types. Many plants in this family, including these three, are called nitrogen fixers. They help add more nitrogen to the soil. Other plants use nitrogen to make proteins for growth and chlorophyll for photosynthesis.
In your garden, a smart move is to grow nitrogen-fixing plants alongside those that really like nitrogen. Another option is to plant a cover crop, like clover, which fixes nitrogen and makes the soil rich for the next year.
You might wonder, since nitrogen fertilizer does a similar job easily, why bother with nitrogen fixation? To get this, we need to know how nitrogen fixation works, and to get that, let’s look at the bigger picture – the nitrogen cycle:
Nitrogen is a big part of Earth’s air, about 78% of it. But most of this nitrogen is two atoms strongly bonded together, not useful to plants because it’s not very reactive. Plants cannot absorb this Nitrogen in its current form.
To make nitrogen useful for plants, we need the help of bacteria. Different bacteria chomp on atmospheric nitrogen and produce ammonium as waste. Another set of bacteria eats this ammonium and produces nitrite, which is then consumed by another type of bacteria, giving us nitrate. All of these forms of nitrogen are available for plants. Especially plants love nitrate, and while they can absorb it directly if it’s close to their roots, they usually team up with fungi. These fungi connect to the plant roots, trading nutrients for sugars and carbohydrates released by the plant roots. Dead plant material is a nitrogen-rich resource that worms help bring down into the soil. The poop of these worms is a treat for nitrifying bacteria.
Nitrogen can leave the soil in several ways: when crops are harvested, when water carries it away, or when it turns into gas and goes back into the atmosphere. If the soil lacks oxygen, specific bacteria grow and transform nitrates back into atmospheric nitrogen. It’s essential to note that these processes only occur with nitrogen that is free in the soil, not with nitrogen inside living organisms.
Now that we know the nitrogen cycle depends heavily on life in the soil, without them the plants would be quite sad, let’s clear up something interesting. While bacteria play a big role in fixing nitrogen, nitrogen-fixing plants, surprisingly, don’t do the fixing themselves. Instead, they provide a home for the bacteria that do the job. For example, clover plant roots have small nodules that shelter lots of nitrogen-fixing bacteria. These bacteria create ammonium, which gradually seeps into the soil for nearby plants and tiny organisms to benefit from. When the plant dies, the bacteria spread throughout the soil, creating a surplus of helpful bacteria for the nitrogen needs of future plants.
Remember when I mentioned that water can carry away soil nitrogen? Well, this nitrogen ends up in rivers and can cause trouble in ecosystems by letting algae take over. But it’s crucial to note that this only occurs with loose nitrogen in the soil, not with nitrogen inside living organisms.
When you use fertilizers in the soil, it adds pure nitrogen without the helpful organisms. When it rains, a lot of this nitrogen washes away and pollutes water. Loose nitrogen molecules are also more likely to turn into nitrous oxide, a powerful greenhouse gas, and go into the atmosphere. But that’s not the end of it – large amounts of pure nitrogen can harm earthworms, making them die or move away. It also disturbs the beneficial fungi on plant roots and alters the soil’s pH, making it unfriendly for bacteria. In a nutshell, it harms and weakens the soil.
When all the nitrogen is used up or washed away, the helpful organisms are no longer there to assist the plants in getting more. This means you have to add more fertilizer, which actually makes the problem worse. These organisms did more than just provide nitrogen – the root fungi also brought up essential minerals for the plants. Now that they’re gone, we have to use mineral fertilizer too. Instead of relying on a natural system where organisms freely share nutrients, we end up spending more money to add a lot of fertilizer to lifeless soil. This not only poisons the water but also contributes to climate change.
