Tuesday, July 24, 2012

Nitrogen Fixing Plants to Fertilize Tea Organically

Recently, Brett of Black Dragon Tea Bar shared a post, titled Organic Tea in Mucha, sharing details of his visit in January of 2010 to a region, Mucha(木柵), in northern Taiwan. If you scroll down through the photos, you will see something that I found very interesting, a photo of a nitrogen fixing cover crop planted in between rows of tea plants. In this post, I want to explain why this practice is used, and how it can be beneficial in terms of reducing the environmental impact of tea production.

Most of us are familiar with a number of nitrogen-fixing plants. Most Americans are familiar with clover, which grows in lawns throughout much of North America:

The above photo shows red clover (center, with the larger, more oblong leaves), and white clover (the smaller leaves around the edge, with a rounder shape). Both of these two species are native to Europe, and not to North America, but have become widely naturalized in North America, especially white clover in grassy lawns.

Here is another nitrogen fixing plant, beans:

These are fava beans. Clover, beans, and the nitrogen fixing plants planted in between the rows of tea plants in Brett's original post, are all legumes. Legumes are a plant family that have the remarkable ability to "fix" atmospheric nitrogen into a form able to be utilized by plants. A few other plants, such as Alders and Bayberries, can also do this, but legumes are the most commercially important and also the most widespread in most ecosystems.

Why is nitrogen fixation such a big deal?

Nitrogen is one of the major chemical elements used in life; proteins, the basic building blocks of life forms, including both plants an animals, are rich in nitrogen. Nitrogen is a very common element; nitrogen gas makes up about 80% of the Earth's atmosphere. Yet in many ecosystems, and in much commercial agriculture (including much tea cultivation), plant growth is limited by available nitrogen. Why?

The answer lies in the electron structure of nitrogen atoms, and the corresponding molecular structure of nitrogen gas, depicted here in this molecular diagram:

Nitrogen has an electron structure which is three electrons away from a stable state, unlike Oxygen, which only is two electrons away. Nitrogen atoms thus achieve a more stable state by pairing up, and "sharing" electrons. This setup forms what can be viewed as a "triple bond", pictured by the triple line between the two atoms in the diagram above. This bond is very hard to break, and the state of nitrogen gas is very stable. The oxygen molecule only has a double bond, which is easier to break. Oxygen gas is thus much less stable, which is why things burn--combustion is just a chemical reaction with the oxygen in air.

Getting nitrogen into a form that can be used by living things like plants and animals, to build complex organic molecules, is somewhat of a big deal. Most life forms cannot do this. Legumes do not even do it on their own; they do so by having special nodules on their roots, which contain anaerobic bacteria (bacteria that function in an oxygen-free environment), which do the work for them.

If you want to learn more about this, I recommend looking at Wikipedia's pages on the Nitrogen cycle, and Nitrogen fixation.

Nitrogen and tea:

The tea plant, like most plants, cannot fix its own nitrogen. Thus, when tea leaves are continuously harvested, nitrogen is continually removed from the system consisting of the tea plants and soil in which the tea is growing. This nitrogen must be replaced, or else the tea plant will eventually start exhibiting stunted growth and stop being productive.

There are several different ways of achieving this:

  • Application of organic fertilizer, such as soil or mulch derived from compost, manure, mulched wood, or other organic sources.
  • Synthetic or mineral-based fertilizers, such as ammonia fertilizers (in which atmospheric nitrogen is converted in an industrial process into a form that can be absorbed by plants), or nitrate-based fertilizers (which can be synthetic or naturally-occurring minerals).
  • Growing the tea plant together with nitrogen-fixing plants, such as the example given in the Black Dragon Tea Bar blog post Organic Tea in Mucha.
  • Rotating crops from year to year, such as growing beans and corn in alternate years, or growing other nitrogen-fixing annual crops alternately with crops with a higher nitrogen requirement. This approach does not work with tea cultivation because the tea plant is a perennial plant and requires multiple years to reach commercially viable production.

Nitrogen-fixing plants can be highly effective at adding nitrogen to agricultural ecosystems. Because these plants have access to an effectively infinite amount of nitrogen (far more than they themselves need), these plants tend to "bleed nitrogen" out into the surrounding ecosystem, through their roots. The plants also have leaves and stems that are extremely high in nitrogen. Nitrogen-rich leaves tend to be more delicate in texture and break down more rapidly than carbon-rich leaves (the leaves of the tea plant, by contrast, tend to contain much less nitrogen than most legumes), so the nitrogen used by these plants tends not to stay tied up in the plant itself very long.

Photo by Sebastianjude, licensed under CC BY-SA 3.0.

Nitrogen-fixing plants do not solve all a plant's nutrient needs. Plants, including tea, also require phosphorus and numerous trace minerals for healthy growth, so nitrogen-fixing plants cannot completely eliminate the need for the application of fertilizers. But they can greatly reduce it.

Another benefit of nitrogen fixing plants over synthetic fertilizers is that the way that the nitrogen enters the system, when provided by nitrogen-fixing plants, tends to be more beneficial to the overall ecosystem. When applying synthetic fertilizers, like ammonium nitrate based fertilizers, a lot of the nitrogen simply runs off into the water, which can create nutrient pollution downstream. Nitrogen fixing plants put the nitrogen directly into the soil and the leaf litter, gradually over time, so they can provide the same benefits to the crops being grown while greatly reducing nutrient pollution downstream.

What is your experience with nitrogen-fixing plants?

Have you ever gardened or been involved in agriculture where you benefited from nitrogen fixing plants, such as beans or other legumes (including both legume crops, and trees planted strictly for their fertilizing properties)? Did you know about nitrogen fixation before reading this post, or is this a new concept for you?


  1. Cool post AZ. My wife is by far the better gardener at our little "urban homestead." (I'm the guy who keeps things watered, hauls rocks, and carts around the compost.) She's read a ton of books and taken some classes on permaculture and organics too. Now I know why she's always planting fava beans and clover all over the place!

  2. Reading the wikipedia article on nitrogen fixation, I was surprised to discover that rooibos is a nitrogen fixator! Imagine, a cover crop that organic tea producers would be uniquely equipped to manufacture...probably a much better ROI than clover....

    1. Yes! Rooibos *IS* a nitrogen fixer (it is also a legume). Interestingly, Rooibos has proved tricky to cultivate, and to my knowledge there are no successful operations cultivating it outside South Africa. So, while it is probably doing a lot of good for its local ecosystem, so far it doesn't seem equipped to pass on these effects to other forms of agriculture...

      There are definitely examples of profitable nitrogen-fixing crops, however...beans being the big one. In PA, farmers have been rotating corn and beans for a long time...often planting them in strips big enough to do mechanical harvesting but small enough to get some benefit from alternating them in the same year...then the next year they switch strips.