Nitrogen (N) is an essential, macronutrient for the growth of almost all plant life. It makes up around 80% of the atmosphere of the earth but is not available to plants in the gaseous form. It must be transformed into another molecule before it can be used by plants.

The most common form of nitrogen used by most plants is the nitrate or NO3- molecule. A few types of plants prefer their nitrogen in the ammonium form which has a chemical formula of NH4+. Bacteria located in the soil are able to transform ammonium into the nitrate form.

Nitrogen tends to be very water soluble. Therefore, it often leaches through the soil especially in sandy types and is lost to the roots of plants. Unlike other nutrients, it does not build up in the soil so it needs to be replenished each growing season.


Fertilizers - Every bag of fertilizer must contain 3 numbers that represent the percentages of the following nutrients: Nitrogen - Phosphorous - Potassium. So, the first number on a bag of 18-12-10 fertilizer shows that it contains 18% nitrogen.

To take the nitrogen from the air and turn it into nitrates or ammonium requires a large input of electricity. That makes nitrogen the most expensive of the three macronutrients in a bag of fertilizer. BTW - Ever notice that things seem to "green up" after a thunder storm? Well, part of that is due to nitrates being formed by the electrical exchange that occurs in lightning.

Since nitrogen is a universally necessary nutrient for plant growth, it is available in a large number of types of fertilizers. Some of these are useful to the home landscape owner while others are more pertinent to large scale agricultural or horticultural applications.

 

Nitrate Nitrogen: This is the form that is most commonly and quickly used by plants. It is available to plant roots in almost all types of situations in the soil and does not need to be converted to any other form. This form of nitrogen does not become attached to soil particles and moves freely in the soil water. It works best when applied frequently but in low levels to avoid the leaching of excess amounts.

  • Granular Fertilizers - All the typical bags of fertilizer that you find at the local garden center or hardware store contain nitrate type nitrogen. As soon as the granule dissolves in water, it becomes available to the plants.
     

  • Liquid Fertilizers - The standard liquid fertilizers used for container grown plants also contain the nitrate form of nitrogen.
     

  • Specialty Fertilizers - There are a number of special fertilizers that contain the nitrate form but they would rarely, if ever, be used in the home landscape. These would include calcium nitrate, potassium nitrate and sodium nitrate.

Ammoniacal Nitrogen: This type of fertilizer quickly transforms into ammonium once it is placed in the soil. Ammonium will attach itself to soil particles and is, therefore, not as readily lost through leaching as is nitrate. However, before it can be used by plants, ammonium must be changed into nitrate by microorganisms in the soil. For this to happen, the conditions must be right for these organisms to thrive including adequate heat, moisture and oxygen. Once the material is changed to nitrate, it is becomes water soluble and susceptible to leaching.

Finally, ammonium nitrogen products have an acidifying effect on the soil. This is valuable for certain "acid-loving" plants such as Rhododendrons, azalea, blueberry, boxwood, pin oak, heaths, heathers, etc.

  • Urea (46% N) is the most widely used dry N fertilizer. It has an analysis of 0-0-46. Once applied to the soil, urea is converted to ammonia which reacts with water to form ammonium within two to three days (faster under warm conditions). Some volatilization of ammonia can occur when urea is surface applied.
     

  • Ammonium sulfate (21% N) has an analysis of 21-0-0. It acidifying effect is very useful in soils with a high i.e. >pH 7.0. This fertilizer is especially useful for those "acid-loving" plants listed above.

Organic Nitrogen: Organic nitrogen is in the form of protein or other insoluble compounds found in plant and animal products such as blood meal, manure and sewage sludge. In order for the nitrogen to be turned into nitrates that are then available to plants, these materials must be broken down by microorganisms in the soil. The rate at which this takes place depends on soil temperature, oxygen and moisture levels. Therefore, organic forms of nitrogen tend to be slow release and not water soluble until they have been changed into nitrates.

 

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

 

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