Looking at Carbon and Nitrogen in Action

Profile Picture of Post's Author Bill Powel-Smith | Posted March 8, 2022

The role of carbon and nitrogen is critical to providing soil microbes with the basic building blocks of life. Both carbon and nitrogen sources are endless. Access to these sources, however is not endless in most farm fields today. Recall that carbon is the product of plants taking sunlight and moisture and making simple 6 carbon glucose molecules. This sugar molecule is offered to soil microbes as food in exchange for minerals. According to research, photosynthesis in most farm fields operates at only 30-40% efficiency due, in part, to a lack of access to basic minerals. Below is a partial list of minerals that drive photosynthetic efficiency.

  • Magnesium- the central atom of the chlorophyll molecule, (photosynthesis takes place in chlorophyll, the green pigment in plants,) and each molecule contains 6.7% magnesium.
  • Potassium- influences the activation of enzymes involved in photosynthesis and CO2 uptake.
  • Boron (B) – aids in cellular growth and helps to regulate the uptake of nutrients.
  • Copper (Cu) – works to help plants metabolize nitrogen.
  • Iron (Fe) – assists in the creation of chlorophyll.
  • Manganese (Mn) – essential for chlorophyll production.

Plants access to minerals is driven by microbes ‘mining’ them and putting them into a form plants can absorb. Microbial growth is dependent upon carbon and nitrogen levels in the soil. Significant levels of carbon in the soil are the result of plants transforming energy from the sun into sugar molecules that are pushed through the roots system (as exudates) out into the soil. Microbes in soil that are tilled spend most of their “energy” repopulating. Plants spend their exudate energy rebuilding roots systems. No-till fields on the other hand, have expansive living roots systems alive with robust microbial populations. These no-till populations spend less energy rebuilding and more energy producing and working and caring for the plants in exchange for the carbon sugars plants produce.

The organic matter in soil has a C:N ratio of about 12:1, twelve part carbon to one part nitrogen. Microbes in the soil work to break surface residue down to a ratio of 12:1. Plant residues can be very high in carbon such as 80:1 in straw and 57:1 in corn stalks. On the other hand, young alfalfa hay is relatively high in nitrogen with a C:N ratio of 13:1. The challenge with forage production is we remove significant amounts of crop residue, which is a tremendous source of carbon. And still the soil microbes work to get the organic matter into a ratio of 12:1. In most cultivated, mono cropped fields, soils never reach this ideal ratio.

To understand this better, let’s look specifically at what is happening below the surface of an alfalfa field. Alfalfa, a legume, has rhizobia bacteria that fix nitrogen, making the soil rich in nitrogen. Because we harvest the plant reside as forage, the soil microbes will mine carbon out of the soil to try and reach a balanced C:N ratio of 12:1. By mining carbon from the soil, we actually can lose soil organic matter in alfalfa fields. These soils typically get tighter and less porous and alfalfa stands diminish to an un profitable level in a relatively short period of 4-5 years. Alfalfa, as a standalone crop, does not improve soil health. Tap roots and the fact the soil is left undisturbed for several years does not outweigh the impact microbes have on removing carbon and tightening the soil.

In my next posting, we will look more closely at the balance of minerals on soil health and nutrient availability.

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