Message-ID: <Pine.GSO.3.94.961010140657.17885A-100000@rccsun> Date: Thu, 10 Oct 1996 14:08:55 +0700 From: Richard Tinsley <mailto:tinsley@AIT.AC.TH> Subject: Mesg from dick tinsley To: Multiple recipients of list DEVEL-L <mailto:DEVEL-L@AMERICAN.EDU>
I have been partly following the discussion on sustainablity and would like to contribute a brief bit I just prepared for a news letter.Dick Tinsley, AIT Bangkok
Article follows: ----------------
Physical Realities in Agricultural Systems II: Nutrient Management
Basics Physics
When considering nutrient management and the contributions of manure and compost to soil fertility and sustainable crop production, the basic law for Physics is the law of conservation of manner. That law states that manner can be neither created nor destroyed, it can only be moved from one place to another and changed in chemical form. When applied to nutrient management in integrated farming enterprises, the need is one of caution to avoid over expectations as to how much nutrient is available and what the total interactions in the system are. There is no magical creation of nutrients taking place with either compost or manure generation. Only nutrients already in the system are being moved around concentrating nutrients in the receiving area and depleting from the accumulation area. The problems are even greater with paddy rice, as the chemistry of the paddy environment is very poor for nitrogen retention, and N is the most critical nutrient.
Nutrient Inflows
Without chemical fertilizer additions, the only sources increased nutrient are:
1. Biological Nitrogen fixation by legumes or blue green algae in paddies,
2. N oxidization by lightning and coming with the rain as nitric acid.
3. Weathering of parent rocks, etc.
These sources do not really amount to enough nutrient to replace what is removed and provide the commercial yield required to feed a rapidly developing economy with an ever growing urban population, and declining rural/urban population ratio.
Of the three sources of natural nutrient addition, nitrogen fixation by legumes and algae is the most effective. In both cases the actual N conversion is by done by bacteria. However, the nitrogen so fixed in to a form available to crops comes at a price in terms of reduced yields. The physics is the amount of energy required to reduce N2 to NO2- and reoxidized to NO3-. If you accurately review the chemistry this represents a 7 step valance change, and that requires lots of energy. The energy has to come from available photosynthate, and represent energy not available for bio-mass production including grain yield. In the case of legumes this invariable results in lower yield when compared to use of chemical fertilizer. When nodules are not fixing nitrogen as happens when N-fertilizer is applied to nodulated legume and the nodules remain green, instead of active pink, the rhizobium bacteria is actually a parasite reducing crop yield while providing no N. Under good conditions legumes and blue green algae can fix approximately 30 kg N/ha/yr or 1/3 the requirement of a good grain crop. The N associated with the stable non fertilized rice yield of approximately 1 -1.5 t/ha, would be largely attributed to blue-green algae fixation limits.
Lightning is a rather strong oxidizing agent that can oxidize atmospheric N2 to NO3- forming weak nitric acid rain that will contribute to the N supply. However, the magnitude of the climatological inflow from lightning is a barely measurable 10 kg N/ha/yr or less. It will also be subject to immediate surface runoff depending on the storm intensity, etc. Nutrient release by weathering rooks in a mature soil is normally less than 1 kg/ha/yr and is not measurable against the background.
Against this rather limited inflow nutrient removal is primarily with the products. When the harvested portion is the reproductive part containing the seeds, the nutrients particularly the more mobile nutrients including N are disproportionately lost for the farm system into the market. This is the result of when most crops mature and start to senesce, the final photosynthate is concentrated in the reproductive activities and away from the residue that is normally used for compost or manure generation. These marketed nutrients must be replaced in order to sustain the system productivity. If not the yields will slowly decline.
Paddy Nitrogen
When paddy rice is a component of the system, the nitrogen conversions associated with the reduced soil condition of the paddy become a major problem with N efficiency. Within the plant most N is held in the ammonia form (NH4+ ) normally associated with amino acids and protein. However, in soils the microbes nitrify NH4+, regardless if it came from minerilization of residues or chemical fertilizer, to NO3- in about 2 weeks. Under aerobic upland conditions this is not a problem as plants can equally uptake either NH4+ or NO3- forms of N. However, under the reduced anaerobic conditions of the paddy NO3- is rapidly denitrified to NO2, in less than a day. NO2 is a gas that immediately escapes into the atmosphere. Thus in paddies there is essentially no carry over N from season to season, and there can be some rapid chemical losses of N from the system that can only be readily replaced with chemical applications of NH4+ N. The NO3- form of N, as you would find in fully mineralized compost or manure, is never recommended for rice applications. Even with good fertilizer management N recovery from paddy is only approximately 50%.
Compost and Manure
With regards to nutrient management, these two items are very much the same. Neither one generates nutrients. What both do basically nothing more than de-carbonate the residues and thus concentrates the nutrients in one place, but within the total farm enterprise, it is a zero sum gain. If I had the choice I would prefer feeding to the animal as the poor beast might be able to extract some energy, where as with composting all the energy goes to the microbes and puff into the air as CO2. It is quite possible for a poorly designed or propaganda intended study to discount the bulk reduction during composting and manure production and measure an apparent nutrient gain within the residue. With compost there is also a reasonable potential for volatilization of NH4+ after it is released and before being nitrified. The best that can be said is the nutrients have been moved to a more convenient place and the volume reduced to something easier to manage. When looking at a fresh rice field and noting the nice bright green areas were the buffalo munching on the previous residue left his pies. It is correct to note the rich green and attribute it to nutritional benefits of the manure. It might be equally correct to also note the surround yellow crop as the source of the beautiful green spot. A real zero sum example. One item that may need to be look at is the ratio of accumulating area to application areas. The ration is probably 20/1 or more. Thus 20 ha of accumulated compose or grazing manure to apply adequate nutrient for 1 ha of crop.
Please also note that stubble and crop residues are not real good ruminant feed, and would rarely be voluntarily selected by any self-respecting cow or buffalo. Again as the plant matures and moves all the mobile nutrients for reproduction, the straw and stover also under change to more ligneous forms that are considerable less digestible even for the ruminant than the fresh stalk or tillers. The residues really provide little more than bulk and fiber. In most cases there is not enough mineral nutrition in the stubble to maintain animal health. Even with their poor heads embedded in a straw pile, animals normally loss considerable weight during the dry season. When they are loosing weight the animals may actually be contributing mineral nutrient to the manure. Please consider this an involuntary contribution and could jeopardize relations with the animal rights people.
Bulk
It must also be remembered that residues are very bulky. Most must be handled at wet weights in which case nearly 90% may be water. Even after dried out the material more than 90% will be carbon, oxygen, and hydrogen, with less than 10% actual nutrient of some kind. The bottom line is for every 1 ton of fresh residue you may get 10 kg of nutrient, a rather back braking ratio when you are the person hauling it around. By composting and manuring with a 60% carbon concentration you will remain with a back aching 30 kg nutrient/t. Escaping the drudgery of hauling manure and compost was one major benefit of the chemical generation of farming.
Immobilization
Another consideration is the fate of incorporated organic manner in the soil as it relates to release of nutrients particularly nitrogen to higher plants. As the soil microbes breakdown the organic residues there is open competition between the expanding microbe population trying to take advantage to the new substrate or incorporated organic material and the plants seeking nutrients. In this competition the plants loose out completely, and while decomposition is being undertaken all released nutrients as well as chemical fertilizer nutrients will go to the microbes leaving the plants with none. The process is well know under the name Nitrogen Immobilization. It is reasonable common to incorporate organic manner and have the crop turn yellow for the couple weeks. After most of the organic manner has been decomposed and the microbe population is residing, only than will the nutrients be available to the crop. Meanwhile, the crop will suffer nutrient stress and yield loose.
Increasing Soil Organic Manner
Finally, the potential to increase soil organic manner by incorporating organic residues, is not really worth considering. The nearly complete breakdown of residues by soil microbes within a rather brief period means it would take enormous volumes of residue to make a measurable increase in organic manner. Under ideal warm moist soil conditions, 99.9% of residues will disappear within a month. If not, the world would eons ago have become one giant peat bog. The reality is the soil organic manner is in equilibrium with crops being grown. The healthier the crop the higher the soil organic manner. It must be recognized that at least 50% or the crop bio-mass is associated with the root system, from which most soil organic manner is derived.
While there is a major need to keep track of nutrients as the interact within the total farm enterprise system, it is important to keep track of the physical realities in which they operate. When recycled with in the farm., there are always concentrations and depletion in what remains a zero sum operation. While it is possible to concentrate nutrients for promoting an intensive integrated area around a fish pond or home stead, the nutrients had originate someplace. In the areas observed in North East Thailand, the fishpond integrated areas represented only about 10% of the farmer holdings. The rest was mostly in annual rice with nutrient supplements. With these addition it could be possible to "sustain" the integrated area on recycled materials, but not the entire farm enterprise system.