We typically add nitrogen to the soil to help plants grow and remain healthy. Last month we took a look at the first of three ways that nitrogen can get into the soil: through the fixation of nitrogen nodules by bacteria on the roots of certain plants, mostly legumes. This month we look at the second way of getting nitrogen: by adding organic nitrogen.

Organic nitrogen can be added to the soil in the form of manure, sewage waste, compost and decomposing roots and leaves, as well as dead animals, insects and microorganisms. When fully composted, these organic products turn into an organic soil material called humus, which can be rich in organic nitrogen (think of it as dead protein material). But being in a sea of soil rich in organic nitrogen doesn’t mean too much to plants when the nitrogen is in a form that they can’t use. Once again we rely on microbes (bacteria, actinomycetes and fungi) living in the soil to convert organic nitrogen into ... inorganic nitrogen. By consuming and digesting the organic nitrogen in the humus, the nitrogen is transformed into a form that plants can use. You may have even heard of these forms of nitrogen — ammonium (NH4+), nitrite (NO2-) and nitrate (NO3-).

Changing organic nitrogen into inorganic nitrogen is known as mineralization. Because humus and our clay soils are negatively charged, the positively charged ammonium (that’s the “+” of NH4+) will be attracted to them — just as the negative end of a magnet is attracted to a positive end. Thus the ammonium will not easily leach out to contaminate groundwater during heavy rains or irrigation. On the other hand, the negatively charged nitrite and nitrate are not attracted to the negatively charged clay soil, so they are left to move down through the soil and into the groundwater, where they can contaminate streams and drinking water. This is why it is important to have just the right amount of nitrogen in your soil for microorganisms and plants to use and sequester. As microorganisms consume the inorganic nitrogen, their “bodies” become storehouses of nutrients unaffected by deluges of rain. Then, as the bacteria and fungi are consumed by relatively larger organisms, such as protozoa, nematodes, earthworms and microarthropods, nitrogen is released to the soil in these predators’ waste products. This nitrogen is in a mineral form and is available for plants to absorb.

How much nitrogen is converted by microorganisms depends on the soil temperature. More nitrogen is released in warm weather, when lots of bacteria and fungi are being eaten. Less nitrogen — or none at all — is released in cold weather, when the microorganisms are essentially inactive. The soil food web allows for the release of other minerals, too.

After all this science talk, it may be helpful to analyze what this means to us organic gardeners. First, it points out the importance of having a wide variety of microorganisms in your soil, for these little fellers are what convert your protein-rich scraps of kitchen peelings and yard debris into nitrogen that is in a mineral form (inorganic nitrogen) your plants can use. Second, it means that you should be composting every bit of material that you possibly can, for compost is what keeps your soil microorganisms alive and happy. And third, it means that we organic gardeners really don’t fertilize our plants at all — rather we feed the soil and the soil feeds our plants.

You’ve probably guessed by now that the third way to add nitrogen to garden soil bypasses the whole microorganism step. Sure enough, a German man came up with a method of manufacturing ammonia almost 100 years ago ... and he won a Nobel prize for it, too. It’s a pretty interesting story because it changed the world. I’ll finish this series next month with a look at synthetic fertilizers.

Glen Andresen hosts “The Dirtbag” heard every second Monday at 10:30 a.m. on KBOO 90.7 FM. He tends his bees on a three-quarter acre organic garden at a retreat in Eagle Creek. He also coordinates Metro’s Natural Gardening Program. Comments and questions may be sent to glan ({at)}pacifier.com or c/o The Portland Alliance.