Ammonium Nitrogen

Nitrate nitrogen (NO3-N) and ammonium nitrogen (NH4-N) are primary indicators of water quality. Ammonia: comes from sewage and landfill leachate. It is very toxic to aquatic life. High concentrations of these nutrients may indicate a well construction problem or an environmental impact in the vicinity of the water supply. Nitrate and ammonium nitrogen are measured for all surface and ground water samples. High concentrations of these nutrients may require follow-up sampling to confirm the status of the water supply. Most plants have a high tolerance for nitrate and ammonium nitrogen and utilize these forms efficiently. The recommended upper limit of nitrate nitrogen for human consumption is 10 ppm. Most surface and ground water ideally contains under 3 ppm nitrate nitrogen.

Ammonium Ions do not move far in the soil with water because they soon become attached to clay or humus and then resist leaching. Ammonium ions can be taken into the plant roots only if in the soil solution. If they are attached to the soil particles, they are converted to nitrate ions in 2 to 3 weeks, depending upon:

  1. number of bacteria,
  2. soil moisture
  3. and temperature

An important step in the soil nitrogen cycle is the conversion of ammonium (NH4+) to nitrate (NO3-). This process results in nitrogen in the form most used by plants--nitrate.

Ammonium has a positive charge. Clay particles in soil have a negative charge. Since opposite charges attract, ammonium ions stick to clay. Therefore, compared to nitrate, they are not as available to most plants, nor do they leach away with heavy rains or irrigation.

Nitrate has a negative charge and is repelled by clay, remaining in the soil water solution. Thus, nitrate is readily available to plants and is leached very easily.

Very specialized bacteria convert ammonium to nitrate. These bacteria require oxygen, so they do not function well in saturated soil or at great depths. Like all living things, they require water to live, so the soil must be moist. These bacteria are not active at temperatures at or below freezing. Activity increases as the soil warms, with a maximum conversion rate at soil temperatures around 80-90°F.

If excessive amounts of ammonium are converted to nitrate, plants cannot use it all right away. If there is adequate water to carry the nitrate below the rooting zone, the nitrogen will be lost from your garden forever. The wasted nitrate may end up in the groundwater that supplies your drinking water.

 

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