Date: Sat, 12 May 2001 10:08:14 -0400 Reply-To: Sustainable Agriculture Network Discussion Group Sender: Sustainable Agriculture Network Discussion Group From: Mary-Howell & Klaas Martens Subject: Humus, Organic matter, Chitin, etc In-Reply-To: <200105111042.tfo948.9sq.37tiu8v@emu> Content-type: text/plain; charset="US-ASCII" The recent discussion on soil organic matter has been very good. I would like to add a few more ideas that I feel are important to consider. The work done at Sanborn field in Missouri showed clearly that the use of nitrogen fertilizers was an important cause of organic matter loss. I don't know of any recent studies that look into the role of nitrogen use in soil organic matter depletion but it seems that this factor also needs to be considered. Dr. Albrecht further observed that nitrogen fertilizer also lowered the quality of organic matter in the soil substantially. I also think there needs to be much more discussion about the fact that all soil carbon is not the same - it is held in different ways, and some forms are far more benefical than others. As we consider carbon sequestration and building soil organic matter, perhaps aiming for the highest levels of carbon should not be the only focus. Instead, probably the questions we need to ask is "WHY are we trying to keep carbon in the soil? What should the carbon be doing in the soil? What forms of carbon best achieve these goals?" Healthy plants commonly give off up to half of the carbon they fix as root exudates that feed soil organisms in the rhizosphere. This large and important carbon source has, to my knowledge, not even been considered in recent studies of humus formation and carbon sequestration. It is undoubtedly rapidly incorporated into the bodies of living organisms, but only if there is a healthy diverse population of organisms present. We may not see all of the carbon 'sequestered' through photosynthesis into the living soil if we do not consider this form. We should be actively investigating ways to enhance the dynamic stability of this living form of soil carbon. The carbon holding potential of many soils may actually be depressed, especially under no-till regimes, because the microbial and larger soil animal populations are not optimized. The much maligned mixing of organic matter into the soil and stimulation of microbial feeding serves to mineralise nutrients and make them available to the crop. The common alternative, use of chemical fertilizers, also damages soil and depletes organic matter. I have read the often repeated statements that moldboard plowing reduces earthworm and microbial populations. I urge you all to look harder at recently plowed organic fields before accepting this theory. Earthworm populations explode imediately after plowing on our farm. Instead, plowing probably shifts the populations, as the change in soil structure and additional oxygen will favor some organisms and not ohers. That isn't necessarily detrimental, it all depends on the relative value of different types of organisms. If plowing favors a more beneficial mix of organisms, even if some type of organisms are adversely affected, it seems that in balance, occasional and judicious plowing has significant value, even for carbon sequestration. When direct observations contradict the prevailing assumptions, It seems wiser to reconsider the assumption than to ignore the observations. No-till systems that use cover crops, earthworms, and other soil organisms to loosen, mix, feed and aerate the soil, such as Steve Groff has developed, are achieving many of the benefits of plowing without some of the drawbacks and are important models to learn from. These systems are building active organic matter and sequestering carbon efficiently. But we should not draw conclusions about ALL no-till operations from what is demonstrated on Steve Groff's fields! I recently read of a different no-till system that was being touted for building soil and sequestering carbon using high lignin crop residues, no green manures, and relying on heavy use of chemical fertilizers to maintain yields. This system had impressive carbon levels in the top 2 inches after 40 years of this type of management, but when I took a calculator and figured the total amount of OM in his top 12 inches of soil it was far less total OM than what our conventionaly tilled soils have. I would be willing to bet that the diversity and quantity of living organisms in this man's soil are very low. My point here is that just building large amounts of "recalcitrant carbon" is not the best goal we can work toward. We could do that by dumping lignite coal on our fields. Organic matter is not just a way to store carbon in soil. It should support a large active and diverse community of organisms in the soil. Crops growing in soil with a healthy diverse mix of organisms in it are more disease resistant, more insect resistant, more tolerent of adverse weather, and more productive than those growing in a less active soil.