Nitrogen is one of the three macro-nutrients needed for healthy plants. It is the eldest brother N within the NPK trio. In the tropics, this vital ingredient is often found lacking in cultivated soil. From my experience using the soil test kit in various gardens in Singapore, nitrogen always shows up as “low”.

How can this be! There are tons of nitrogen in the atmosphere, roughly 78%. However, atmospheric nitrogen doesn’t just flow into the soil readily as it is inert.

Well, how do we get nitrogen into your farm then? The easy way would be to buy a bag of chemical fertilizer with a high nitrogen content like an NPK rating of 20-15-8. Humans are now able to fix atmospheric nitrogen into the soil artificially using the Harber process. However, the process uses a lot of energy coming from fossil fuels because the Harber process can only take place under high pressure and high temperature. Michael Pollen, in his book The Omnivore’s Dilemma, wrote that after World War II the US government found a huge surplus of nitrogen that was going to used for making bombs like TNT. This excess ended in the fields.

Other than requiring lots of energy to produce, artificial nitrogen fertilizers pollute the environment. Only a small percentage can be readily taken up by the plant and the rest will be washed away. This extra nitrogen flows into the rivers and lakes, poisoning fish and humans. They also result in eutrophication which reduces oxygen in the water subsequently killing fish and aquatic animals.

In nature, nitrogen fixation occurs through lightning and certain micro-organisms. We can’t do much about lightning but we are lucky to have one of the highest in the world in Singapore. We can however encourage nitrogen fixing organisms to live in our soil. Most legumes like beans and peas help nitrogen fixation by having the rhizobia bacteria in nodules on their roots (see the round balls in the photo). These friendly bacteria produce nitrogen for the plant and when the plant dies the nitrogen is released into the soil for other plants to use.

You know you’re doing something right when you see these!

I took this picture after pulling out a long bean plant that is reaching the end of its life. If you squeeze the nodules, they will burst and pop open. An orange-pink liquid will flow out and that’s the indication that the nitrogen fixing rhizobia bacteria have been doing their jobs.

If a puny long bean plant already has so many nodules, imagine the amount of nitrogen that one of those towering leguminous trees like the raintree fix!

Now, go plant your beans! Read more about green manuring.