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New Organic Soil Through Permanent No Till in Old Eroded Chilean Soils
No till has been introduced in most American countries as an agronomic system of soil management with high conservation practices to overcome the serious problems of erosion within the region. The adoption of this new system by the farmers has also caused a smaller cost per hectare in grain and forage production as compared with old traditional systems.
On Chequen farm, my family farm located in the coastal range of central Chile, after 20 years of sowing without plowing, significant changes have been observed in the structure and fertility of the old eroded soils. This change has occurred due to the increased levels of organic matter within the soil profile. The increased level of organic matter in the Alfisols soil on Chequén farm is due to the parallel increase of yields in the different rotations of wheat, triticale, corn and lupine. It is this rotation which affects the amount of stubble over the soil.
The rotation of irrigated corn and dryland wheat will annually leave 12 t/ha of stubble over the soil when the stubble is properly managed. The increase of the organic matter at 0.2% annually, which is initially on soil surface and later in underlying horizons, has generated an appropriate nutrition of this layer and has favored the microbiology and endemic mesofauna within this zone. This larger amount of biological activity has increased the organic carbon and with it, its humic coumpounds.
In the corn-wheat rotation, the largest amount of total carbon was achieved in the first 5 cm while in 5-10 cm- and 10-30 cm-depths it was smaller. This is compared with a lupine-wheat and lupine-prairie rotation. The carbon balance in the corn-wheat rotation showed humic compounds, like fulvic and humic acids plus humins, were higher in the 5 cm depth compared with the other two rotations. However, in 5-10 cm- and 10-30 cm-depths, the contents of these compounds were larger than the horizon at 5 cm.
The corn-wheat rotation indicates a larger carbon content that is superior to the rotations of wheat-lupine and prairie-lupine. This can be due to the larger quantity of residues and improved quality, especially for the wheat stubble with high lignin content.
THE CARBON CYCLE
The carbon cycle is vital in the soil's life. Carbon is the essential part of the cellular structure of much animal biomass and vegetative biomass within the soil. The photosynthetic process is responsible for the synthesis of the plant's carbon by absorbing the carbon dioxide (CO2) from the atmosphere. The plants receive the solar light energy through the chloroplasts that contain chlorophyll, a green factory rich in carbohydrates and protein, that form the foundation of life on the planet. Under those conditions the plants absorb CO2 and release molecular oxygen (O2) to the atmosphere, maintaining the gas balance. The soil organic carbon is of great importance as it constitutes the essential part of the organic matter and is a basic compound for carbohydrates made up of carbon (C), hydrogen (H) and oxygen (O).
Any remaining stubble left on the soil will begin decomposition as soon as the water content and temperature are adequate. If the stubble is buried and the soil is moist and contains adequate levels of oxygen, a rapid oxidation of incorporated organic matter will take place because the microorganisms will be nurtured excessively from the stubble. This phenomenon implies quick loss of carbon that escapes to the atmosphere as carbon dioxide (CO2). Another form of carbon loss can also be generated under anaerobic conditions which creates methane gas (CH4). The release of these greenhouse gases may be linked to the effects of global warming.
The oxidation of the organic matter starts when the crop stubble is initially attacked by cellulolitic fungi and later by all kinds of microorganisms that include soil mesofauna. In these circumstances, the rapid decomposition of the organic matter from plowing of the soil surface layer decomposes the soluble carbohydrates and low molecular weight humic compounds which are not very stable. This rapid decomposition and release of CO2 is what prevents the build up of organic carbon within the soil profile.
The carbon cycle is fully a natural phenomenon. It is difficult to balance man's intervention with the natural balance of gas in the soil to that gas in the atmosphere. From historic times until the present, agricultural production has been based on the oxidation of soil organic matter by means of plow and of fire, leaving less organic matter on soil profile every year. This farming practice has extracted valuable nutrients required for the soil biology in this great natural resource. The last fifty years have shown a rapid exhaustion of soil organic carbon which is now in the earth's atmosphere as CO2.
At the recent World Congress of the Environment, which took place in June of 1998 in Bonn, Germany, the level of CO2 in the atmosphere in 1850 was reported as 280 ppm and that in 1997 it was 365 ppm. At the present time the increase of CO2 in the atmosphere is 1.5 ppm annually. This increase of CO2 may be partially responsible for the devastating effects of the La-Nino and La-Nina weather patterns.
The improper handling of forest and permanent pasture coupled with traditional farming practices throughout the World, which have continually oxidized the organic matter and release CO2 into the atmosphere, have made large contributions to global warming of the Earth. The excess CO2 in the atmosphere is responsible for 50% of the greenhouse effect compared with other gases. (D. Reicosky, personal communication)
These practices, which have occurred throughout the years, have degraded the soil. Forests, permanent pastures and agricultural soils have been depleted of organic reserves. These depleted reserves have left the soil vulnerable to soil erosion and decreased fertility for growing crops. This increased level of soil erosion has also contributed to higher levels of sedimentation in our rivers and lakes.
The soil biology is composed of a great variety of microorganisms, fungi and mesofauna which have daily food and energy requirements that must be met. These small organisms are directly responsible for formation of soil rich in humic substances and consequently result in higher levels of fertility for the plants.
Nature teaches us daily that the plants, when approaching maturity, remain on the soil surface where they begin a slow and natural decomposition process. This process will generate and formulate the humic substances along with many other different beneficial compounds.
A soil, which has very high biological activity, has the advantage of building up humic compounds. This occurs because of abundant biological excretion generated in metabolic cycles of soil organisms. Keeping stubble on the soil surface enables continuous renovation and proliferation of a favorable environment with no disturbance to cause a change in soil structure. Anytime you change this condition by tillage, you could harm the biology of the soil and along with its native fertility.
Organic carbon is essential for the survivability of the soil's biology. To guarantee that the soil maintains its biological activity farmers must leave the crop stubble on the soil surface.
The farmer that sows without plowing can harvest twice, both the grain and straw. It is of vital importance that the farmer remember "the grain is for man, the stubble is for the soil". Returning the stubble to the soil helps recharge the carbon and is the payment to the soil for the grain that it produced. Fertilizers by themselves only feed the plants and do not nurture the soil (Crovetto, 1996).
Today the fertility of the plants is based on different exogenous contributions that farmers give to the soil. In fact, in a large part of the world, farmers must provide nearly all the nutrition needed by the plants for grain production. This is necessary, as the soils have been depleted of their natural nutrients because the farmers have been feeding only the plants and not the soil. One exception to this is the humid pampas of Argentina. In this region, the soils are still quite young due to limited crop production. The plants are still getting a large amount of nutrition from the mineralization of the organic matter. The farmers that will survive market globalization will surely be those that have worried about soil nutrition, respecting the stubble of their crops.
Appropriate soil nutrition will increase the content of organic carbon and consequently of humus. The humus, while very small (less than 0.010 mm), has a strong negative electric charge which increases the cation exchange capacity (CEC) of the soil. This natural phenomenon, improves the fertility of the soil since the CEC increases retention of useful cations for plant nutrition, like calcium (Ca++), magnesium (Mg++), potassium (K+), sodium (Na+) and ammonium (NH4+). This diminishes the risk of loosing of these basic cations through leaching.
The stubble and soil nutrition, are the basis of a permanent agriculture (Crovetto, 1997).
The humus and calcium can be combined in the soil making this chemical of great importance to increase soil nutrition. Calcium (Ca++), in acid soils, can displace the hydrogen ion (H+) on the exchange complex. This improves soil pH and available phosphate for better plant nutrition. Figure l. The humus increases the negative charge of the soil retaining the useful exchange cations. Without humus or calcium in the soil, farming cannot be productive (García and García, 1982).
The quantity of stubble left on the soil is important, as well as the quality of that stubble that affects the soil biology. Not all stubble has the same value for soil nutrition. It is important to consider the lignin content of the stubble along with the quantity of that stubble. The lignin is directly responsible for the formation of humins. The stubble types which contribute most to the formation of humins are those of small grains (cereals) like triticale, rye, wheat, rice and forest waste.
The composition of different stubbles harvested on Chequén farm highlight the high content of lignin of soybean stubble (11.9%). However the total contribution of this stubble is low due to the small amount of plant biomass. At 9.3% lignin, wheat stubble also provides an important contribution of lignin. (Table l).
The corn-wheat rotation on Chequén is the most effective in producing humic compounds, what is coincident with higher soil fertility.
Table 1. Composition stubble* on Chequen farm.
|Corn stover with leaves without ears||
|Grain sorghum straw Without seeds||
|Soybean straw with leaves and pods without grains||
|Wheat straw with leaves and spike without grains||
|Triticale straw with leaves and spikes without grains||
(*) Base dry matter
No-till allows the stubble to remain over the soil which increases biological activity producing both micro and macro pores within the soil profile. These pores will provide the soil with the proper aeration required for plant growth. The plant roots also generate a large amount of channels within the profile that will create a means to increase water infiltration while also improving the field capacity or availability of water for the plants. The channels or conduits generated by the biology of the soil in no till farming, are generally rich in humus, which can absorb 15 times their weight in water.
The stubble is a vital source of carbon and consequently of humus. The consumption of humus varies with the soil texture. Soils which contain larger soil particles such as sand require a much larger quantity of stubble to generate humic compounds than do the finer clay soils. According to Fuentes (1994). the mineralization of the humus in a clay-loam soil has an annual rate of 1.3%. This would indicate that the humic replacement should be of 620 kg of humic compounds per hectare or equivalent to 2,550 kg. of stubble per hectare since the isohumic coefficient for the wheat straw is 0.22 (defined as the quantity of humus formed from a unit in weight of crop stubble added). The reference quantity maintains the necessities of the active soil biology. This means that if we want to increase the soil organic matter content, we should apply a quantity larger than 2,550 kg straw /ha.
Not all the stubble and organic compounds have the same isohumic coefficient. The stubbles that contributes more humus to the soil are those that have wide carbon to nitrogen ratios. The lower nitrogen in the stubble, the greater is its ability to contribute to the level of humic compounds. Nevertheless, the addition of nitrogen to the stubble with high relationship C:N favors this contribution. In this sense, the lignin content of the stubble will contribute to a better quality humus, expressed in humins. For this reason, the stubble of cereals have great importance in the formation of humus in the soil as they can contribute up to 20% of their dry weight. However, the manures of domestic animals only contribute 10% of their dry weight in humic materials. The manures are less stable and, consequently, decompose faster (Fuentes. 1994).
The small contribution of humic materials in the manure is due to the digestive system of the ruminant animal. One can degrade a large part of the carbon content, especially stubble, biologically through the ruminal liquids and its complex anaerobic enzymatic-digestive system. For this reason, the contribution of humic materials of the manure is notably inferior. This refers basically to soil nutrition and not plant nutrition.
The manure of domestic animals should be applied on the soil surface as soon as possible after having been excreted. The sooner that manure is applied the greater the availability of the nutrients for the soil and or the plants. The losses of carbon, nitrogen and other useful components by leaching or gaseous conversion, will be directly proportional to the time which has elapsed since the manure has been excreted until its surface application. The manure should be applied on the soil surface, preferably before a rain or irrigation and most favorably with low temperatures. The losses from gasification of the ammonia (NH3) content in the manure when it is applied on soil surface is insignificant compared to the losses of carbon and damage to the soil when it is incorporated by tillage.
MAIN COMPONENTS OF ORGANIC MATTER
The soil organic matter is made up of many different compounds whose nitrogen content, carbohydrates, fiber and diverse minerals give origin to cellulose, hemicellulose and lignin. The order of decomposition of the organic components in the soil is: cellulose, hemicellulose and lignin. This order of decomposition does not necessarily compare with that of the mammals, especially ruminant animals. (Table 2).
The main compound for generating humus in soil is cellulose which can constitute about 50-70% of the dry matter of stubble. Cellulose is formed by a long chain of glucose molecules (C6H12O6) and is gradually hydrolyzed under natural environmental conditions (Rosell, 1997. personal communication).
The hemicellulose is a polysaccharide that is hydrolyzed slowly in an acid reaction where the cellulose produces pentosans which spread and accumulate in the soil. (Rosell, 1997. personal communication).
The lignin is a precursor of the more resistant humic material to biodegradation. The humin is formed by molecules of substituted fenilpropane. This compound generates a recalcitrant three-dimensional structure to the natural and biological physical agents. The enzymes and other depolymerizatión mechanisms (hydrolysis, oxidation-reduction, etc.) favor the decomposition of the lignin and their humification (Rosell. 1997. personal communication).
Soil humic compounds
Humus is a generic name for selected forms of carbon and is present in the soil in the forms of: fulvic acid, himatomelanic acid, humic acid and humins (Labrador, 1996). Humus has as fundamental characteristic of small size in all its forms. This compound possesses a net negative electric charge that is greater when humus is of finer particles. The humic particles cannot always be classified according to the chemical composition of the original organic material, but can be classified by molecular weight and their reaction to acid, alkaline compounds, and alcohol.
Table 2. Division of the Humic Substances
Modified after Labrador, 1996.
Fulvic acids are characterized by their relatively low molecular weight and can be associated with the polysaccharides. They are soluble in alkaline and acid reactions. For their larger comparative content of carboxylic acid, they possess a great capacity to dissociate native minerals in the soil. This characteristic in influenced strongly in their genesis. The anionic colloids can form stable complexes with polyvalent cations like Fe+++, Al+++, Cu++, etc. (Labrador, 1996). This remarkable edafic function decreases the phosphate fixation on the aluminum sesquioxides or iron complexes.
A complex mixture of humic compounds like fulvic and humic acid soluble in alkalis and alcohol, produce himatomelanic acid (Labrador. 1996).
Humic acids are the major humic compounds in the soil. They are insoluble to the acids and alcohol and they have a medium molecular weight. They are thin and flat particles bonded to each other, those that form a reticular spongy material (Labrador, 1996). This can be one of the most remarkable physical-chemical characteristics of the humic acid, because it enables a great capacity for water retention and a strong anionic charge that improves CEC noticeably. The humic acid regulates the process of oxididation-reduction of the edafic system, providing some oxygen to the plant roots.
Similar to the fulvic acids, humic acids can form complex substances, especially through metallic ions. According to Schnitzer (mentioned by Labrador), the humic acid is the main factor in genesis of soil and the formation of a good soil structure and in the availability and mobility of certain plant nutrients. It is also important in the agrochemicals controlled persistence and degradation in the soil profile (Labrador, 1996).
Our experience in the no till soils of Chequen farm, herbicides show a very little residual effect or active presence of the original molecule. Herbicides are degraded by the active soil physiology and, without a doubt, are affected by the presence of humic compounds in general.
Humin is the more stable component in the soil because of its high molecular weight. Humins are insoluble to the degrading chemical agents and remain strongly bonded to the finest mineral colloids in the soil (clay particles). This characteristic probably allows it to remain in the soil profile for a long time when the soil in not disturbed. The humin structure is basically derived from lignin content of the organic matter that results in a unique structural relationship.
Figure 2 explains the stability and behavior of humic compounds in soil. Humic compounds are prominent in the humins, the largest molecular weight material that increases cation exchange capacity (CEC) that contributes to better soil quality (Collins et al., 1997). This favorable condition has been observed in the Chequen soils after 20 years of sowing without plowing or harrowing.
The soil CEC has increased from 11 meq/100 g to 26 meq/100 g during this period.
Figure 2. Characterization of the Soil Organic Carbon relative to their stability and transformation
|2,000---------------------||increase molecular weight||300,000|
|45%----------------------||increases carbon content||62%|
|2%-----------------------||increases nitrogen content||5%|
|30%----------------------||increases oxygen content||48%|
|500-----------------------||It increase acidity and CEC||1.400|
|--------------------------||It diminishes lignin content|
Collins et al., 1997
Humins are very desirable and their persistence will depend on how the farmer manages his soil. Any soils under conventional tillage will stimulate the oxididation-reduction processes and, consequently, will affect the stability of all the humic compounds.
On the contrary, no till or direct seed farming stimulates the formation of all the humic compounds when permanently adding organic matter to the soil. The carbon balance is significantly higher in the corn-wheat rotation, compared with the wheat-lupine rotation and prairie-lupine, especially the humins content. The carbon balance in Chequen soils explains that the humin content, humic and fulvic acids are superior in the first 5cm depth in the corn-wheat rotation, compared to the other rotations. This is due to the large amount of stubble (carbon) input that this rotation contributes to the soil organic carbon. The corn-wheat rotation contributed the biggest content of total carbon to the soil, although only in the first 5 cm of the profile as the figures 3 and 4 explain.
Figure 3. Total carbon in Chequen soils
|c/w5||c/w10||c/w30||l/w5||l/w10||l/w30||p/l5||p/l 10||p/l 30|
c/w = corn-wheat
l/w = lupine/wheat
p/l = pasture/lupine
C = carbon
5-10-30 = cm depth
Figure 4 Carbon balance in Chequen soils
|c/w = corn/wheat 5cm||1.82||1.28||0.46||0.74|
|c/w = corn/wheat 10cm||0.3||0.18||0.1||0.07|
|c/w = corn/wheat 30cm||0.21||0.15||0.09||0.08|
|l/w = lupine/wheat 5cm||0.77||0.62||0.14||0.2|
|l/w = lupine/wheat 10cm||0.34||0.51||0.11||0.1|
|l/w = lupine/wheat 30cm||0.21||0.33||0.16||0.07|
|p/l = pasture/lupine 5cm||0.66||0.5||0.17||0.23|
|p/l = pasture/lupine 10cm||0.28||0.34||0.08||0.07|
|p/l = pasture/lupine 30cm||0.23||0.23||0.12||0.06|
|Meaning of abbreviations:|
|CH = carbohydrates; FA = fulvic acid; HA = humic acid; Hum = humins|
The direct seeding or no till farming system improves soil nutrition, it stimulates soil biological activity, increases the soil organic matter content and enables the formation of humic substances which improves soil physical, chemical, and biological conditions.
The soil nutrition is based on the stubble that the farmer leaves on the soil surface. This is the price the farmer has to pay to use the soil resource. Feeding the soil should be a daily process, which implies the presence of stubble should be permanent.
The surface stubble decomposes to form different humic compounds, basically depending their on their C:N relationship and lignin content. The lignin is responsible for the humin formation and at the same time provides the soil more stable humic materials that last longer.
The farmer should not only be concerned about the quantity of stubble that leaves on the soil, but also of stubble quality. Those with high carbon/nitrogen relationship are the most beneficial to the soil.
COLLINS, H.P.; PAUL, E.A; PAUSTIAN, K.; AND ELLIOT, E.T. (1997): Characterization of Soil Organic matter relative to its stability and turnover. CRC Press. Inc. New York. pp. 52-54.
CROVETTO, C. (1996): Stubble over the soil; the vital role of plant residue in soil management to improve soil quality. American Society of Agronomy. Inc. Madison. pp. 245.
CROVETTO, C. (1997). La cero labranza y la nutrición del suelo. 5ª National congress of AAPRESID. Publishing Víctor Trucco. Argentina. pp. 73-78.
LABRADOR, J.; (1996): La materia orgánica en los agrosistemas. Mundi Presses. Madrid. pp. 24- XX
FUENTES, J.L.; (1994): El suelo y los fertilizantes. Mundi Presses. Madrid. pp. 55-71.
GARCIA, J. and GARCIA, R.; (1982): Edafología y fertilizacion agrícola. Aedos. Barcelona. pp. 24-44.