From danamex@mail.internet.com.mx Sat May 22 18:31:44 1999 Date: Fri, 21 May 1999 13:25:49 -0500 From: Ronald Nigh To: sanet-mg@ces.ncsu.edu Subject: Nitrogen in Organic Agriculutre [ The following text is in the "iso-8859-1" character set. ] [ Your display is set for the "US-ASCII" character set. ] [ Some characters may be displayed incorrectly. ] Bart wrote: >Most non-symbiotes would be doing very well to fix 50 kg/ha nitrogen. >Except in fairly anaerobic environments -- rice, for example -- I >wouldn't go out of my way to encourage them, to say nothing of *add* >them to the soil. They are, however, a delightlfully fascinating >component of creation. > Well, I know this is conventional wisdom, that the non-symbionts are not worth the trouble. In my opinion, however, this is an error. The Cubans have some interesting experiences with Azotobacter achieving higher levels than this. There is also some work in the tropics (Brazil) that suggest Azotobacter can be a significant N-fixer under the right conditions. Also, there is something else to keep in mind: 50kg/ha of N fixed in the context of a complex soil microbiology and good soil structure can have a dramatically different effect on the soil and on crop productivity than does an equivalent or greater amount of soluble N salts or anhydrous ammonia. In an organic soil it just isn't necessary to add such high levels of N as are common on some conventional farms, a large percentage of which ends up in the drainage. Also in the humid tropics we are beginning to suspect that N-fixation by bacteria on the leaf surfaces could be important, in which case Mo given as a foliar fertilizer could be very beneficial. Experience so far seem to bear this out. I don't know how relevant this source could be in more temperate or drier areas. A friend of mine doing reserarch in Oaxaca has found 15 species of bacteria, several of them N fixers, in the exudate from the adventitious roots of Indian corn varieties. This is not precisely a symbiosos of the rhizobium type but is probably part of the rhizosphere microbial populations that feed on carbohydrates the corn plant releases into the soil for that purpose. The point is there is a lot more to N-fixation in agriculture than Rhizobium and there is even more to it that we don't know about. I say don't write off the non-sybionts too fast. They have a great potential to contribute to more adequate N management in future agriculture. I remember reading a few years ago that the nitrogenase enzyme was remarkably uniform in different organism. It has two metaloproteins, one with Moly, Iron and Sulfur and the other with an Fe-S group. Its uniformity, according to what I read, suggests that it evolved only once in the history of life. There may be more recent research I am unfamiliar with. I recall that V can substitute for Mo in some metaloproteins but I thought the enzymatic activity would be considerably lower. I remember some speculation of possible V-based N-fixation systems and would like to have any references on that. That would be very interesting. Finally, we should always remember that N-fixation is only one link in the nitrogen cycle, albeit a critical one. But proper N management means paying attention to other important links, to create conditions that retain organic N in the soil biomass as much as possible. Regards, Ronald Nigh Dana, A.C. Mexico, D.F. & San Cristóbal de Las Casas, Chiapas Tel. y FAX 525-666-73-66 (DF) 529-678-72-15 (Chiapas) danamex@mail.internet.com.mx To Unsubscribe: Email majordomo@ces.ncsu.edu with the command "unsubscribe sanet-mg". If you receive the digest format, use the command "unsubscribe sanet-mg-digest". To Subscribe to Digest: Email majordomo@ces.ncsu.edu with the command "subscribe sanet-mg-digest". All messages to sanet-mg are archived at: http://www.sare.org/san/htdocs/hypermail