All vegetable gardens require nitrogen, because it’s the nutrient that most directly causes plants to grow lots of green leaves and to be generally healthy. Buying and using nitrogen fertilizer every year costs a lot, and over the long term, chemically made nitrogen fertilizer can harm the life in your soil. Luckily, for millions of years nature has made its own nitrogen fertilizer, through an amazing relationship between plants in the legume family and certain soil bacteria, known as rhizobia. If gardeners can grasp how this natural system works, they have a good way to keep their soil productive without having to buy anything for it.
The Science Behind Biological Nitrogen Fixation
Peas, beans, lentils, clover, alfalfa, and soybeans are all legumes and they do something really special that most other plants in the garden can’t. The roots of these plants create a mutually helpful partnership with rhizobia bacteria which are generally already in the soil. The bacteria settle on the plant roots and make little growths (you’ll see them as pink or white lumps on the outside of the roots) and inside these growths they transform nitrogen from the air into ammonia, a type of nitrogen the plant can actually take up and use to get bigger. The plant, for its part, gives the bacteria sugars it makes during photosynthesis. It’s a win for both of them, and that’s why scientists call this a mutualism.
How Much Nitrogen Do Legumes Actually Add
How much nitrogen plants pull from the air and ‘fix’ in the ground is very different depending on the kind of plant, the soil they’re in, and how long they grow. Experts who study farming think garden peas usually fix about 50 to 80 pounds of nitrogen for each acre during the time they are growing, and clover grown as a ground cover can fix 75 to 200 pounds per acre each year. For those of us gardening at home, this means that growing peas or beans in a raised bed and then cutting them off at the soil line (instead of yanking the whole plant out) leaves the little nitrogen-holding lumps on their roots to break down and provide that nitrogen to the next plants. Doing this can mean you don’t have to add as much, or even any, extra nitrogen fertilizer to beds where you grow beans or peas every two or three years.
Using Legumes as Cover Crops
If you want to use natural processes to get more nitrogen into your soil and improve it, planting legumes as a cover crop is the best method. You can plant crimson clover, winter peas, hairy vetch, fava beans or other similar plants in your garden bed when it’s empty in autumn or early spring. Let them grow for some weeks or months and then chop them and dig them into the soil before you plant your vegetables. As this plant material breaks down, it releases nitrogen slowly, giving the next crop a feed all the time it is growing. Lots of organic farmers do this, and it works very well in a home garden, no matter how big or small the plot is.
Common Misconceptions About Legume Nitrogen
Lots of people think beans and peas (which are legumes) give off nitrogen to the soil around them as they grow. But that’s not how it works. Most of the nitrogen they ‘fix’ from the air stays inside the plant in the little lumps on the roots, the stems, and the leaves while it’s actually growing. Other plants can’t use that nitrogen until the legume plant is gone, or has been chopped up and has started to rot. So, planting beans alongside tomatoes won’t give the tomatoes a big surge of nitrogen this season. The good stuff for the tomatoes happens later, after the old bean plant has broken down and become part of the soil.
Key Takeaway
Peas, beans, clover, and vetch all have a special relationship with bacteria in the ground. These bacteria transform nitrogen from the air into a kind of plant food the plants can actually use. Because of this, they can add a good amount of nitrogen to your garden’s soil and you might not have to buy fertilizer, or at least won’t need to use as much. To get the benefit, you need to cut the legume plants off at the soil surface and let the roots break down in the ground. This releases the nitrogen they’ve collected, to feed whatever you plant next. The best way for a home gardener to use this natural way of improving the fertility of the soil is to plant legumes as a “cover crop” when you aren’t using the garden for anything else.
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