By Craig Madsen
Understanding and managing the nutrient levels in your soils is an important role whether you are managing 1,000 acres or a backyard garden. In this article, we’ll focus on the role of nitrogen in the nutrient cycle.
Nitrogen plays a critical role in plant production. Nitrogen is a key component of proteins, enzymes, and chlorophyll which is essential for photosynthesis. Too little nitrogen will impact plant production and too much nitrogen can cause problems as well. Excess nitrogen 1) can be lost to the environment, released into the air, or leached out of the soil into surface and groundwater; 2) reduces organic matter in the soil – there is always a balance between the carbon (organic matter) and nitrogen, and nitrogen not used by the plants will be used by the microbes to break down organic matter; 3) excess nitrogen in the plant and soil will result in weed, insect and disease pressure; and 4) excess nitrogen in the plant and water can negatively impact human and animal health.
The atmosphere is comprised of 75% nitrogen which translates to 2000 pounds of nitrogen per foot of atmosphere over every acre of land. How can we convert this free nitrogen to a form that can be utilized by plants? Another question is how can we make the nitrogen we apply more effective?
One of the best ways to make atmospheric nitrogen plant-available is to grow a legume. The rhizobium, nitrogen-fixing bacteria living in the nodules on the roots of the legumes, are responsible for fixing nitrogen. Two important items must in be place for the legumes to fix nitrogen 1) the seed must be inoculated with the specific rhizobia and 2) there must be adequate levels of sulfur, boron, and molybdenum. Sulfur is necessary for good nodulation and adequate levels of all three are necessary for the proper functioning of the nodules. How do you tell if your legume is fixing nitrogen? Get a shovel, dig up some plants, and look at the roots. Do you have nodules? If yes, cut the nodule open and look at the color of the inside of the nodule. If they are pinkish red, then they are fixing nitrogen. If not, then you need to check for mineral deficiencies.
Rhizobium nodules on alfalfa roots
Well-aggregated soils provide habitat for free-living nitrogen fixing bacteria. Well-aggregated soils have areas within the soil colloid that have lower oxygen levels which provides the right habitat for the free-living nitrogen fixing bacteria. How good of soil aggregation do you have? Do a slake test. Allow the samples to dry for 24 hours before conducting the test. https://www.asec.purdue.edu/soilhealth/downloads/SlakeTest,NRCS.pdf
A well-functioning food web is one of the ways natural systems provide nitrogen to plants. It is all about the math. Bacteria are little bags of nitrogen fertilizer. Bacteria have a carbon to nitrogen ratio of 5:1. Protozoa that eat bacteria have a carbon to nitrogen ratio of 30:1. Thus, a protozoa has to eat 6 bacteria to get its 30 carbons but only needs one nitrogen. It releases the 5 nitrogen into the soil and that nitrogen is now available to the plant. Protozoa are often low in numbers in cropland soils. You can grow your own protozoa by making a protozoa tea and applying it to your fields to enhance the nitrogen cycle. http://traceandsave.com/protozoa-and-protozoa-tea/
Different types of nematodes feed on a variety of different foods such as bacteria, fungi, plant, and some are predators. When nematodes with a carbon to nitrogen ratio of 100:1 feed on bacteria they release the extra 19 nitrogen into the soil. A diverse soil food web is important for nutrient cycling.
Worms are also a great source of nutrients for the soil. Worm castings have 5 times more plant available nitrogen than undigested soil and 3 to 7 times more phosphorous, and the list goes on. The worms themselves are also a source of nitrogen when they die. 30 earthworms per 8-inch cube could release up to 30 pounds of nitrogen/acre. Earthworms have the potential to increase pasture production from 10% to 30%.
Another option is to plant a green manure cover crop to provide nitrogen for the next crop. It can be either a legume or a young cereal crop that is incorporated before flowering. https://attra.ncat.org/publication/overview-of-cover-crops-and-green-manures/
There are ways to improve the efficiency of applied nitrogen, a topic for another article.