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Toxic Nitrogen Forms in Fresh Organic Growing Media?

If you formulate an organic growing medium by adding starter organic fertilizers, the growing medium should not be used immediately. It should be allowed to “rest” for 2-4 weeks before using it because various microorganism populations that are required to break down complex organic materials into various intermediary forms of nitrogen need to build up.

Until this occurs, there is a possibility that toxic, intermediary forms of nitrogen accumulate in the growing medium until another group of microorganisms build up to continue the nitrogen conversion process.

This is especially true if a customer makes his/her own growing media as prepackaged growing media has had enough time to rest by the time the customer uses it. The following explains this process. 

Nitrogen Mineralization: As mentioned, when organic fertilizers are blended into a growing medium, the population of microbes that convert organic proteins found in fertilizers into useable forms of nitrogen must build up. The first step in the breakdown of nitrogen into a usable form occurs through a process known as mineralization. During mineralization, microbes start by converting organic matter proteins into ammonia (NH3), which is toxic to plants at levels greater than 2-8 ppm (Figure 1). Mineralization further continues through a different group of microorganisms in which ammonia (NH3) is converted into ammonium (NH4+) (Figure 1). Although ammonium is used by plants, it can accumulate to unhealthy levels for proper plant growth.

Mineralization of organic matters PRO-MIX

"Figure 1. Mineralization of organic matter proteins into usable ammonium through microbial breakdown."

 

Nitrogen Nitrification: After mineralization is completed, nitrification begins. During nitrification, ammonium is converted by bacteria into nitrite (NO2-), which is toxic to plants at a few parts per million. Then nitrite is further converted to nitrate (NO3-) by other bacteria (Figure 2). It takes about 2-4 weeks for the microorganism populations to build up and convert organic proteins into usable nitrogen. After this time, the growing medium should be “stabilized” and there only should be minor levels of ammonia and nitrite at any given time.

Nitrification of ammonium into nitrate through soil bacteria PRO-MIX

"Figure 2. Nitrification of ammonium into nitrate through soil bacteria." 


Since microorganisms are responsible for nitrogen mineralization and nitrification, these processes can be sped up or slowed down based on the following:

  1. Microorganisms are most active at warm temperatures. Growing medium temperatures below 55oF (13oC) slow down nitrogen conversion so ammonia and/or nitrite can accumulate in the growing medium, causing crop damage. Therefore, it is best to wait at least 4 weeks before seed sowing or planting if the growing medium is stored in cold conditions (below 40oF / 4oC).
  2. It takes time to build microbe populations, so depending on the “sterile” nature of the components, it may take longer to build up these microorganisms. If it takes longer, then the “resting” period must also be longer.
  3. A growing medium with a pH above 7.5 or a poor oxygen supply greatly slows nitrification, allowing for the buildup of ammonia and ammonium.

For more information about organic growing media, contact your Premier Tech Horticulture Grower Services Representative:

BLOE         PEEJ             LAWL

Ed Bloodnick
Horticulture Director
US-South East

JoAnn Peery
Horticulture Specialist
US-Central, Canada-Central

Lance Lawnson
Horticulture Specialist
US-West, Canada-West

     
BUET PARS      CHEJ

Troy Buechel
Horticulture Specialist
US-North East

Susan Parent
Horticulture Specialist
Canada-East, US-New England

Jose Chen Lopez
Horticulture Specialist
Mexico, Latin & South America

 

References:

  • Fisher, P., J. Huang, M. Paz and R. Dickson. 2016. "Having Success with Organic Growing Mixes." Grower Talks. Feb 2016, 68-72.
  • Pitchay, Dharmalingam and Gunawati Gunawan. 2017. "Detrimental Effects of Blood Meal and Feather Meal on Tomato (Solanum lycopersicon L.) Seed Germination." Hortscience 52(1): 138-141.

PRO-MIX® is a registered trademark of Premier Horticulture Ltd.

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