Online Articles
about nature - by Boyd
Shaffer,
artist /naturalist
Observations
of a Naturalist
This Article: Nitrogen and the Plant Cycle - A brief overview
OBSERVATIONS INDEX
Nitrogen is the most common element in the atmosphere, and if one measured all the nitrogen contained in the atmosphere above a single acre of land, it would weigh 35,000 tons. The amount of nitrogen in soil is quite low. At its highest it might weigh two tons per acre but is usually much less, and in many northern latitudes it is extremely low. Nitrogen in the atmosphere is chemically inert and will not combine readily with other molecules. It is therefore mostly unavailable to plants and animals that use it to build proteins.
Most of the nitrogen in plants comes from the soil as inorganic compounds and ions that are absorbed by the roots. These are composed of nitrogen, oxygen and hydrogen. They are caused by bacteria and fungi (mycelium) as they break down more complex molecules of dead plant and animal tissue into simpler ones.
Some nitrogen from the atmosphere is converted to ammonia or other nitrogenous compounds by various nitrogen-fixing bacteria. Many of these bacteria gain access to plants through the root hairs (legumes such as lupine, peas, clover, and Alnus such as alder) other bacteria remain free in the soil. This cycle of nitrogen from soil to plants and animals and back to the atmosphere cannot produce more nitrogen in the soil, it is merely being recycled.
Large amounts of nitrogen are constantly being lost by water as it leaches down through the soil and into gravel (in Alaska). Run off from rain and erosion also contributes to this. There is a great loss of nitrogen whenever crops are harvested. The best way to replace the loss is to cover the fields with decomposed plant material, decomposed manure or a high nitrogen fertilizer. Needless to say, that chemical fertilizer is not the best because it does not add bulk and humus to the soil.
Decomposing bacteria use nitrogen and it is not made available to plants until the bacteria die and the nitrogen is released into the soil. Because of this, crops should not be planted in soils where only partially decomposed material has been added. Moreover, when sawdust, leaves or other dead plant material is spread around plants to retain moisture or control weeds, less nitrogen will be available until those "mulches" have completely decomposed. If you do use such covering make sure there is plenty of decomposed material beneath it fist, and replace it every time more mulch, wood chips etc. is added. Wood chips are acid and lime might be necessary too.
Fields, weeds and forested areas are often burned to revitalize the soil, but burning destroys what nitrogen is present, and it can only be replaced by decomposed organic matter or high nitrogen fertilizers. Nitrogen fertilizers used on large farming projects have leached into streams and rivers causing large amounts of algae to grow and in many areas robbed the water of oxygen, hence a loss of aquatic animal life including fish. -End