Pea plants grown at an organic vegetable farm in Denver, Colorado

Pea plants grown at DGS, 2024

Nitrogen Fixers

Nitrogen (along with phosphorus (P) and potassium (K)) is one of the big 3 macronutrients that plants need to survive. They gotta have it. It's so necessary for plant success, in fact, that we dump huge amounts of nitrogen fertilizer in the soil (to make food) despite the fact that it's poisoning our environment and people and is completely unsustainable.

So I realized I better start learning about nitrogen.

Farmers water their crops with hoses outside at an organic farm in Denver

Watering crops by hand at DGS, 2024

Nitrogen Basics

WHAT is nitrogen?

Nitrogen is a colorless, odorless, non-toxic gas that all living things need to survive. It's atomic symbol symbol is N.

WHY do plants need nitrogen?

Plants need nitrogen because:

1. It's an essential element of all amino acids, which are the building blocks of proteins,

2. It's also a component of nucleic acids, which form the DNS of all living things,

3. It's a component of cholorphyll, which is the site of carbohydrate formation (photosynthesis). Chlorophyll also gives plants its green color.

WHEN plants have enough/too much/not enough

With the right amount of N, photosynthesis will occur at a high rate, and the plant will exhibit vigorous growth.
Given too much N, a plant will exhibit excessive leafy growth, delayed fruit, overly succulent leaves and shoots, and burning
If a plant doesn't have enough N, you'll find yellowing on older, lower leaves, stunted growth, and reduced fruit and flowering

WHERE do plants get their N from?

About 78% of our admosphere is made up of nitrogen (dinitrogen (N2)), but, unfortunately, plants can't use the form of nitrogen found in the air: N2. That's because N2 atoms are triple bonded, and that takes energy to break - so it doesn't make sense energetically for plants to mess with the N2 in the air at all.

"So what the hell?", I said to myself. I thought nitrogen was nitrogen but it turns out that nitrogen can take several different forms. Some of these forms, plants are able to immediately absorb. Some of them, they cannot. So I started by taking a look at the different forms of nitrogen and trying to get that straight. It wasn't easy, check out the chart below to see why:

Peas we grew, harvested, and packaged at DGS, 2024

FORMS OF NITROGEN

FORM NAME SOURCE CAN PLANTS USE? PROS CONS

The Nitrogen Cycle

It took me SO long to get all of this straight so don't worry if you're confused, I was too. Allow me to explain:

Of all the nitrogen types listed in the chart above, only TWO of them can be absorbed and used by plants as is: Ammonium (NH4) and Nitrate (NO3).

All of the rest of the forms of nitrogen have to be converted by bacteria in the soil *into* either ammonium (NH4) or nitrate (NO3) before plants can use it.

So when the nitrogen is organic, that is, when it comes from decaying plant and animal matter - which makes up more than 90% of soil nitrogen - it first has to be turned into ammonium (which can be used*!) by soil organisms through a process called ammonification or into ammonia through mineralization. Both of these forms of nitrogen (ammonium* and ammonia, that is) have to then go through nitrification, which is a two step process that first creates nitrite (which cannot be used by plants and can be toxic to plants) and eventually nitrates (which can be used by plants but that are prone to leaching and when over applied through the use of fertilizers can cause environmental damage, more on that below).

**You might be confused by the ammonium (I was) because as I mentioned, it can be used by plants but is often transformed into nitrates through nitrification anyway. This seems to be because nitrates are more readily absorbed by plants and also because too much ammonium can be toxic to plants. One source said that plants have internal ammonium regulators so I'm not sure how toxicity works quite yet. Another source says that if there is any plant preference, it is for ammonium early in the season and nitrate late in the season.

The dinitrogen (N2), which you'll recall is the nitrogen found in abundance in the air, can also be transformed by soil organisms into a plant-useable form through a process called nitrogen fixation - a process that creates ammonia (NH3) which, again, must then go through nitrification to create nitrate (NO3) to be used by plants.

I'm going to include the definitions of ammonification, mineralization, nitrogen fixing, and nitrification below and will go into nitrogen fixing in more detail in another section. It's all confusing but I think a basic grasp is ok for our purpose (which is to figure out how to not pollute the environment and ourselves through the proper regulation of N on our farms).

DEFINITIONS: THE NITROGEN CYCLE

Mineralization/Ammonification:

Conversion of organic nutrients (those bound to carbon and hydrogen) found in dead plants, animals, and other organic waste, releasing inorganic forms of N (and other inorganic nutrients) that plants can readily absorb, namely, ammonia (which must undergo nitrification first) and ammonium respectively (which can either be used directly or often goes through nitrification to become nitrates)(sources vary on the difference between mineralization and ammonification so this is what I'm going with).

Nitrogen fixing

The process wherein atmospheric nitrogen gas (N2) is converted into ammonia.

Nitrification:

Slow process

A biological process wherein ammonia (NH3) or ammonium (NH4) is converted first into nitrite (NO2) and then nitrate (NO3) by bacteria in the soil, making nitrogen available for plant uptake.

This whole process is called The Nitrogen Cycle! (See the chart below).

Nitrate nitrogen is the form taken up by plants in the largest quantity. In warm, moist soils, ammonium rapidly converts to nitrate by soil microbes to be better absorbed by plant roots.

Phew! And that should be good enough for now!

The Nitrogen Cycle

https://science.howstuffworks.com/life/biology-fields/nitrogen-cycle.htm

Farmers on a local organic farm in Colorado check out the nice soil

Farmers from The DeLaney Community Farm visit DGS as part of a farm swap, 2024

Nitrogen Fixation

Firstly, and most importantly, little if any nitrogen will move from your nitrogen fixing plant to other surrounding plants while that nitrogen fixing plant lives. The idea that you can use nitrogen fixers, like legumes, to add nitrogen to the soil as companion plants is unfortunately untrue. Here's how it actually works:

HOW do nitrogen fixing plants work?

Nitrogen fixing describes the process during which symbiotic rhizobia bacteria invade a plants root hairs and begin to multiply in the outer root tissue. The plant, in response, forms a little blanket of tissue over these bacteria to protect them. The plant then supplies energy to these bacteria from their photosynthetic process in the form of carbohydrates. As thanks, the bacteria take nitrogen from the atmosphere and presto change-o, they convert it into ammonia, which, as mentioned above, can be converted in the soil into nitrates that plants can readily absorb! How incredibly kind!

Whatever affects the plant will also affect nitrogen fixation. If plant growth is reduced due to drought, low temperature, or limited nutrients or disease, nitrogen fixation will also be reduced. The optimal temperature range for white clover, for example, indicates an optimum temperature range from 55 to 80 F with sharp declines in N2 fixation above and below this range. No N2 fixation occurred below soil temperatures of 48 F.

Factors that influence the quantity of nitrogen fixed:

1. Level of soil nitrogen

2. The rhizobia strain infecting the legume

3. Amount of legume plant growth

4. How the legume is managed

5. Length of growing season

*If given a choice, legumes will remove nitrogen from the soil before obtaining nitrogen from the air through N2 fixation.

Also, a common misconception is that the nitrogen is released into the soil from the legume roots. Until that plant is terminated, however, it's going to need to use that nitrogen for itself - studies show that little if any nitrogen is passed onto companion plants while the nitrogen fixer lives. Most of that nitrogen is found in the topgrowth of that plant - for perennial legumes about 75-80 percent of that plant's nitrogen content is in that top growth. For crimson clover it's 90%. In order to release that nitrogen from the legume for use by other plants, the plant either needs to decompose or be eaten by animals, such as livestock, and then be excreted in its manure and urine (most of the nitrogen ingested by animals is not retained in its body). It's been found that 80 - 90% of the nitrogen eaten by grazing livestop passes through however 50% of the nitrogen in urine is lost through volatilization and another problem is that animal excreta distribution is sub-optimal (unless interrupted and changed) because animals tend to excrete around water sources or under shade trees.