Master Gardener: How plants use nitrogen

Gardeners know that plants need a supply of nitrogen to survive and thrive. To understand how nitrogen can actually be used by plants, it is helpful to expand our knowledge of nitrogen. Where does nitrogen come from? Unlike carbon dioxide, which green plants can harvest from the atmosphere through photosynthesis; nitrogen gas (N2) is not available for plants’ above-ground parts to use. Plants obtain nitrogen through the ‘Nitrogen cycle’. The air we breathe consists of 78 percent nitrogen, which is in the form of nitrous oxides. Rain water dissolves these oxides and nitrogen enters the soil, which plants then absorb by drawing in water through their roots.

Other sources of nitrogen: Nitrogen is also added to the soil through nitrogen-fixing bacteria present in the soil and in the root nodules of legumes. Animals introduce nitrogen into the soil through the discharge of urine and feces. Gardeners most often apply nitrogen fertilizers to garden beds (natural or synthetic). A natural fertilizer could be decaying organic matter such as compost, leaves and lawn grass clippings.

Importance of nitrogen: Nitrogen is an essential component in chlorophyll, which is the pigment that provides green leaves and stems with their color. Nitrogen promotes vegetative growth in plants. Lawn grass, leafy green vegetables and plants grown primarily for their ornamental foliage require high levels of nitrogen for best performance. Fruiting and flowering plants, however, have less need for nitrogen and may fail to produce viable fruits and attractive flowers if they receive too much of this nutrient.

Soil Testing: Nitrogen levels are constantly changing in the soil, so a soil test provides a reading only of the amount of nitrogen available at the time of the test. You should use tests as a guide to determine initial nitrogen needs in a new garden or to monitor the nitrogen levels in existing gardens.

Nitrogen Problems: Plants suffering from nitrogen deficiencies usually develop pale green or yellow leaves. Older leaves may yellow completely. Insufficient nitrogen can cause some plants to wilt and sometimes growth becomes stunted. On the other hand, too much nitrogen can burn foliage or even kill plants. (This usually occurs if the nitrogen fertilizer comes in direct contact with the leaves or stem.) A high nitrogen concentration in the fertilizer requires dilution in the soil so that it does not cause plant damage. Note: On sandy soils, nitrogen applied early in the season can be easily leached out of the root zone with heavy rainfall or irrigation. Nitrogen deficiency may result, as well as an increased potential for nitrate contamination of groundwater. Nitrogen fertilizer should be applied in a series of smaller applications on sandy soils.

References: Carl J. Rosen, Peter M. Bierman and Roger D. Eliason, University of Minnesota