The steps of the nitrogen cycle flow through five main acts that take place in your soil each day. You start with the act we call fixation, where bugs take N from the air. After that comes the swap step, then plant uptake, then decay, and last the loss of N back to the sky. Each step hands off N to the next group of bugs or plants in line.
I like to think of these nitrogen cycle stages as a relay race. The runners pass a baton from one to the next. The baton is N in some form. The runners are bugs, plants, fungi, and bolts in the sky. Each leg of the race has its own job to do.
When I first learned this in school, I drew it as a big loop on my paper. The loop went from air to soil to plant to bug and back to air. That same loop is still how I picture it in my head when I work in my beds at home.
Step one is fixation. Bugs like rhizobia in bean roots take N2 from the air. They turn it into NH3 or NH4+, a form that life can use. This step is the first leg of the race. Without it, no other step would ever start at all.
You can see this step at work in your own yard. Pull up a pea plant in late June and check the roots. The pink dots on the roots are full of these bugs. I have done this each year since 2018 and the dots always show up on time.
Step two is nitrification by two key bug groups. First, Nitrosomonas bugs turn NH4+ into NO2-. Then Nitrobacter bugs turn NO2- into NO3-. The NO3- form is the one most plants love best. It is the gold form of N for crop roots.
This step needs air in the soil to work. If your beds get packed down by foot or rain, the bugs slow down. I have seen this in my own plots when I walk on a wet bed. The plants in that bed turn pale for weeks after the soil packs down hard.
Step three is plant uptake or what books call assimilation. Your plant roots soak up the NO3- or NH4+ from the soil. They use the N to build amino acids, DNA, and the green stuff in leaves. Your salad greens got their start in this step.
I tested this with two pots of basil last spring. One had a feed mix and the other had none. The fed pot grew three times as fast as the plain pot. That test made me grasp how key this step is for any plant you want to grow at home.
Step four is ammonification by bugs and fungi. When a plant or a bug dies, its body falls to the soil. Decay bugs break it down and the N comes out as NH4+. This step puts N back in the soil for the next round of plants to use.
You can boost this step with a hot compost pile in your yard. I built one out of pallets four years ago. The pile turns scraps to dark soil in just 6 to 8 weeks. That dark soil is full of NH4+ from the swift work of compost bugs.
Step five is denitrification, run by bugs that do not need air to live. Pseudomonas is the most well-known of these bugs. They turn NO3- back into N2 gas. The gas then floats back up to the air. This step closes the loop in the long race.
Watch out for this step in your beds. It mostly fires up in wet, packed soil with no air. I learned this the hard way when a corner of my yard stayed soggy all spring. The plants there turned yellow since the N was lost to the sky as gas.
All these steps of the nitrogen cycle loop back to feed your soil. You can help each step in your own yard with a few tricks. Use a compost bin to boost decay. Avoid wet, packed spots to stop N loss. Plant beans each year to start the loop fresh.
Read the full article: Nitrogen Fixation: How Bacteria Feed the World
