How Nitrogen Gets Into Plants

Nitrogen is necessary for protein synthesis in the plant, which in turn supplies energy to the plant and keeps the plant’s metabolic system running.  On our Earth, nitrogen is the main element in the atmosphere and it is ultimately from the atmosphere that plants get their nitrogen, although they cannot take it up directly.  Some nitrogen comes into the soil as rain and some is fixed from the atmosphere by soil dwelling bacteria and some is supplied by decaying organic matter.  However, nitrogen availability in the plant is often the factor that limits growth.  The development of synthetic nitrogen fertilizers after WWII allowed large increases in crop yields along with all the downsides we now understand – nitrate poisoning of waterways and algae overbloom, loss of soil organic matter and subsequent erosion, loss of native species where synthetic nitrogen has been applied. In regenerative systems, nitrogen cycling needs to be carefully addressed.

Plants absorb nitrogen in several forms

  1. As amino acids.  This is the easiest form of nitrogen for plants to absorb.
  2. As urea (CH4N2O).
  3. As ammonium (NH4+).
  4. As nitrate (NO3-).

For certified organic growers, the use of synthetic urea, ammonium and most nitrate fertilizers are not allowed.  Organic growers often use compost and cover crops to supply nitrogen. In order to be used by plants, organic nitrogen sources require conversion to inorganic forms through mineralization by soil organisms. The rate of mineralization is dependent on soil temperature, pH, moisture and host of other environmental factors. Because of this, it is not always easy to match nitrogen requirements of the plant to nitrogen mineralization in the soil. Sometimes mineralization will exceed the needs of the plant, for example when a succulent young cover crop is incorporated ahead of a crop and releases most of its nitrogen before the main growth stage of the crop, leaving the crop nitrogen limited at the time it needs it most.

To even out the release of nitrogen, there are several techniques.

  1. Application of slow release nitrogen fertilizers such as seed meals and feather meal at planting.
  2. Side dress organic nitrogen sources during crop growth.
  3. Apply liquid organic nitrogen sources such as soluble amino acids during crop growth.
  4. Apply adequate amounts of compost at planting.

Over application of compost can cause denitrification, where nitrates are stripped of their oxygen and the resulting nitrogen becomes tied up at N2.  This can happen in anaerobic environments (in the absence of oxygen). Over application of compost can also result in a buildup of excess phosphorus and potassium in the soil.

Protein synthesis and sucking insects

In the plant photosynthesis creates sugar which, with the addition of nitrogen, makes amino acids.  Amino acids bond into peptides. Peptides form complete proteins.

Enzymes are responsible for building all of this.  Enzyme co-factors need trace mineral keys.  There are many kinds of enzymes and they can be used again and again to facilitate these reactions.  For example, we manufacture enzymes in our digestive system to break apart complex molecules.  However, we don’t manufacture enzymes to take apart cellulose, therefore we can’t digest grass. This is unlike the ruminants whose enzymes enable their grassy lifestyle.

When the synthesis of protein doesn’t complete, the plant is left with sugars and amino acids in the leaves.  Sucking insects are dependent on amino acids and sugars in the plant sap to feed on and if protein synthesis is not complete, insect problems can result.  In order to counteract this there are several minerals that may be applied as foliar fertilizers to help complete protein synthesis and clear up the insect problems.  These are:

  1. Magnesium (Mg)
  2. Sulfur (S)
  3. Molybdenum (Mo)

These may be applied by foliar spray with boron (B) and molasses.  Alternatively, all but the Mo may be soil applied or applied through irrigation.