Role and importance of nitrogen in your soil

Organic NZ Magazine: September/October 2004
Author: Holger Kahl

Nitrogen is an important building block of proteins, nucleic acids and other cellular constituents which are essential for all forms of life. Nitrogen is such an important key nutrient element for plants that it warrants careful management, and – if mismanaged – can lead to severe environmental problems.

Organic Growing tutor and Soil & Health co-chair, Holger Kahl outlines the role of nitrogen and how to get it right.

We are surrounded by nitrogen on this planet. Vast amounts can be found in the earth’s crust and in the atmosphere. Nearly 79% of the air we are breathing is nitrogen.

However, the vast majority of living organisms have practically no direct access to this nitrogen. Most atmospheric nitrogen must be fixed by micro-organisms before it becomes available to other life forms.

All the nitrogen-fixing organisms are prokaryotes (bacteria). Some of them live independently of other organisms – the socalled free-living nitrogen-fixing bacteria. Others live in intimate symbiotic associations with plants or with other organisms (e.g. Rhizobia). Most of the nitrogen in our bodies has once been fixed by these organisms.

Chemical manufacturing of nitrogen fertilisers is nowadays another pathway for atmospheric nitrogen to get into plant available forms.

For plants nitrogen is the nutrient in most demand – but too much is as bad as too little excess of nitrogen or an imbalance of nitrogen compared with other nutrients can make plants more prone to pest and disease attack.

There is a well-known direct relationship between the amount of chemical nitrogen fertiliser applied to plants and aphid attack: the higher the amount of nitrogen the more aphids on the plants.

For farming and gardening systems the following simplified nitrogen cycle outline movements and forms of nitrogen.

Most of the nitrogen in the soil exists in the form of organic matter of some kind. When organic matter breaks down (mineralisation) the nitrogen ends up as ammonium (NH4+) which can turn into ammonia gas under alkaline conditions (pH above 7).

Ammonium is converted into nitrate by micro-organisms (nitrification) under aerobic conditions (= sufficient air in the soil). Under anaerobic conditions nitrate can be changed to ammonia gas or nitrogen gas, both of which will escape into the atmosphere.

Real net gains of nitrogen (as opposed to recycling nitrogen e.g. composting of manures) are via rain (a very small amount), free living micro-organisms (a considerable amount) and Rhizobia bacteria in symbiosis with legumes (by far the largest amount).

Losses of nitrogen occur via removal of harvested plants and animals and ammonia/nitrogen gas.

For plants, nitrogen is the nutrient in most demand – but too much is as bad as too little. A very large, and potentially the most environmentally damaging loss of nitrogen can happen via the leaching of nitrate. The majority of soil nitrogen is relatively immobile. However, when nitrogen is converted to nitrate it becomes very mobile.

Nitrate, (NO3- ) is a negatively charged ion (anion). Very little nitrate can be stored in the soil and negatively charged soil colloids (such as clay and humus) largely repel it. Therefore when water drains through the soil nitrate leaches out.