Cooperative Extension Service
The University of Georgia College of Agricultural and Environmental Sciences
PDF
C. Owen Plank, Extension Agronomist
Crop & Soil Science Department
Athens, GA 30602-7272
oplank@arches.uga.edu
Definitions
Laboratory Analysis
Factors Affecting SOM
SOM and Soil Productivity
Nutrient Applications
SOM and Cation Exchange Capacity
Soil Pesticide Applications
Conclusions
References
Soil organic matter is related to the productivity of a soil. Because of this, maintaining SOM is an objective of many sustainable crop production systems. As with several other soil properties, SOM levels can be determined in the laboratory. However, SOM tests are difficult to interpret by the laboratory that performs the analysis and are not very meaningful for most growers.
In the southern United States, 11 of the 13 state-supported public soil testing laboratories offer as SOM test upon request. The North Carolina Department of Agriculture offers a "humic matter" test as an alternative. None of the public laboratories offers SOM as part of a standard or routine soil test, and only one laboratory (Virginia) offers an interpretation for the grower.
There is a reason for this general omission: SOM is influenced by many factors; consequently, scientists have conducted little field research relating SOM to soil nutrient levels, nutrient uptake or plant growth. Unlike the interpretation of soil pH or extractable soil P or K levels, there is no simple interpretation for SOM levels. The test, therefore, does not provide a farmer, a homeowner, a gardener or a crop advisor much quantitative information that is helpful in managing soils or crops. Instead, the information is generally evaluated on a relative or comparative basis. For example, soils with a higher SOM content will have a higher citation exchange capacity (CEC), higher water holding capacity, and better tilth than soils with a lower OM content.
The term SOM encompasses all the organic components of the soil, such as (1) intact plant and animal tissues and microorganisms, (2) dead roots and other recognizable plant residues and (3) a mixture of complex amorphous and colloidal organic substances no longer identifiable as plant tissues. The latter category of these organic materials is referred to as soil humus (or humic material). Humus is a complex array of substances remaining in the soil after extensive chemical and biological breakdown of fresh plant and animal residues; it makes up about 60 to 80 percent of the SOM. This means that the plow layer of an acre of soil with 1 percent organic matter contains 6 to 8 tons of humus. The rest is less stable and partially decomposed organic residues. Humus is the stable fraction of SOM and is relatively resistant to microbial attack. This is the fraction of SOM that is most responsible for cation exchange and is generally classified into three groups (Brady and Weil, 1999):
Fulvic acid: low in molecular weight, light in color, soluble in both acid and alkali, and most susceptible to microbial attack. Depending upon conditions, the half-life (the time required to destroy half the amount of a substance) of fulvic acid is approximately 10 to 15 year.
Humic acid: moderate in molecular weight and color, soluble in alkali but insoluble in acid, and intermediate in degradation potential with a half-life in excess of 100 years.
Humin: high in molecular weight, dark in color, insoluble in both acid and alkali, and most resistance to microbial attack.
The warm, humid climate of Georgia is conducive to microbial activity most of the year and, consequently, SOM does not accumulate extensively. Organic matter content typically is less than 3.0 percent.
Most SOM values are derived from organic carbon (C), because the quantitative determination of SOM has high variability and questionable accuracy (Nelson and Sommers, 1982). Although organic C analysis is reasonably accurate using the traditional wet digestion with acid-dichromate and heat (modified Walkley-Black), modern instrumentation now allows many labs to determine C rapidly and accurately by analyzing the CO2 evolved following dry combustion in an inductive furnace. When Walkley-Black digestion is used for C analysis, organic C is multiplied by 1.724 to give percentage organic matter. It is generally recognized, how-ever, that the multiplier can range from 1.6 to 3.3.
Loss-on-ignition techniques have also been used to estimate SOM. SOM determinations by this method are usually greater than SOM determined by the Walkley-Black procedure.
The amount of SOM in the surface layer of mineral soils can vary from less than 1 percent in coarse-textured, sandy soils to more than 5 percent in fertile, prairie grasslands. The amount is influenced by all soil-forming factors. Jenny (1941) arranged the order of importance of these factors as:
climate > vegetation > topography = parent material > age
Some general statements about SOM levels in virgin soils can be made based on Jenny's work:
Most SOM is found in the zone of maximum biological activity - the topsoil or plow layer. Anything done to this layer will influence long-term buildup or depletion of SOM (e.g., amount of tillage, crop rota-tion, erosion, cover crops, amount of crop residues returned to the soil, fertilization and organic amendments.
The organic matter content in soils decreases rapidly the first few years they are cultivated. Research by Giddens (1957) showed that the organic matter content was reduced from 2.30 percent under a typical virgin forest of a Cecil sandy loam soil to 1.59 percent after three years of cultivation, or a 30.9 percent loss. An adjacent area that had been in cultivation for 75 years contained 0.64 percent organic matter, or a loss of 72.2 percent when compared to the virgin forest soil.
After the initial period of heavy loss, a more or less stable organic matter content is reached in soils, and this content does not change appreciably over prolonged periods, even with periodic additions of organic matter. This is because the warm, humid climate in Georgia is favorable for microbial activity and breakdown of organic residues most of the year. Giddens (1957) found that when oat forage war incorporated into a cultivated Cecil sandy loam at a rate of 5,000 pounds per acre, 55 percent and 100 percent of the added organic carbon had disappeared in 12 and 24 months, respectively. More recently, Sumner and Bouton (1981) found that two years of cropping with three summer crops (corn, forage sorghum and pearl millet) significantly increased the organic matter content of a Faceville fine sandy loam. The greatest increase occurred with forage sorghum.
Following the first summer cropping season, the soil organic matter content increased from 1.12 percent to 1.64 percent. Following the second cropping season, however, the organic matter content had decreased to 1.46 percent. The organic carbon and the C/N ratio were also increased, indicating that the material incorporated had a wider C/N ratio than the organic matter originally present. This suggests that, with time, the organic matter will be decomposed until an equilibrium C/N ratio of about 10 (or steady state) is reached. Based on these findings, Sumner and Bouton (1981) concluded that it is unlikely that the higher organic matter content could be expected to persist without further additions. They also found that including winter cover crops into the rotation had little effect upon the soil organic matter content.
The data suggest there is little benefit to be gained in terms of crop residue production and, hence, increased organic matter content of soil from winter cropping with crimson clover, ryegrass or crimson clover-ryegrass. It should be pointed out, however, that winter cover crops can protect soils from erosion and reduce loss of SOM associated with sediments.
A highly productive soil will produce higher yields and more crop residues than a less productive soil. If much of the crop residues remains in the field, the soil is likely to have a higher SOM content than a less productive soil. In this situation, an SOM test is redundant. It tells the grower what he or she already knows: The soil is more productive. Unlike extractable nutrients, soil pH or soluble salts, there is no critically defined critical level below which crop yields are severely limited. Any factor that increases crop residues will ultimately increase SOM to some point where climate, geography and management practices limit further increases.
The benefits of higher SOM are often mentioned in association with good soil and crop management: increased soil aggregation, improved infiltration and drainage and reduced crusting of fine-textured soils, better water-holding capacity in sandy soils, higher cation exchange capacity, and increased nutrient reserves. Although many soils can be made more productive with large additions of organic matter, maintenance of SOM for the sake of maintenance alone is not a practical approach in most farming operations.
The terms soil quality and soil health have recently been used in association with sustainable agriculture. Some attempts have been made to use traditional SOM tests or a test of some fraction of the SOM as an index of the total productivity or sustainability of a particular soil. This indexing has proven difficult because a simple interpretation of organic matter analyses or some fraction of the SOM is not universally applicable.
SOM is a huge reserve of potentially mineralizable nitrogen, sulfur and other nutrients. Organic C is generally highly correlated with organic N. Soil organic matter contains approximately 5 percent N. Early soil tests in Illinois were based on the assumption that approximately 2 percent of the total N (mostly organic N) would be mineralized during a growing season. This would be approximately 20 pounds N/acre/year for each 1 percent organic matter (1% OM x 2,000,000 lb. soil/A x 5% N x 2% N mineralized = 20 lb N/acre). The University of Missouri Soil Testing Laboratory adjusts N recommendations based upon SOM. The N requirements to produce a desired yield minus the N-supplying power of the soil based on SOM will indicate the N rate necessary to produce the yield goal for the selected crop (Table 1).
| Table 1. Nitrogen credits for SOM used by the University of Missouri Soil Testing Laboratory (Buchholz, 1983). | |||
| Soil Texture | Soil cation exchange capacity | Nitrogen rate adjustment | |
| Cool season crops | Warm season crops | ||
| - | ----- cmol(+)/kg ----- | --------------- pounds N/acre -------------- | |
| Sand - sandy loam | <10 | 20 x %SOM | 40 x %SOM |
| Silt loam - loam | 10-18 | 10 x %SOM | 20 x %SOM |
| Clay loam - clay | >18 | 5 x %SOM | 10 x %SOM |
| Note: 1 cmol(+)/kg = 1 meq/100g | |||
Because most crops require large amounts of N (100-300 lb/acre) and because most cultivated soils of the southern United States contain less than 2 percent SOM (which will provide less than 40 lb. N/acre), most laboratories do not test for SOM for the purpose of making (or adjusting) an N fertilizer recommendation. Additionally, the estimated N release rate of SOM can be variable. Instead, several laboratories test for nitrate-nitrogen in samples from the topsoil and subsoil.
Some opportunities may exist to adjust the N rate applied to certain crops based upon SOM, but these correlations have not been developed to the point that specific recommendations can be made. Within a state or region, general statements accompanying a soil test report may take this factor into consideration.
For example, the recommended N for cotton in Alabama should be reduced "... where cotton follows a good crop of soybeans or on land where excessive growth has caused problems." South Carolina adjusts the N recommendation for corn depending on soil texture, location and legumes used in the rotation. Likewise, Georgia adjusts N recommendations for most agronomic row and grain crops based on legumes used in the rotation and whether excessive vegetative growth has occurred in the past with crops such as cotton (Plank, 1989). These states recognize that SOM has an impact on optimum N rates, but no specific correlation has been established.
Because most of the S in surface horizons of well-drained agricultural soils is usually present in organic forms, S deficiencies in crops are often associated with sandy soils low in organic matter. There are often high correlations between organic C and extractable and/or total S in soils; however, at this time no public soil testing laboratory in the southern United States is using a calibration of a SOM analysis to make S fertilization recommendations.
In the form of humus, SOM enhances the solubility of minerals and, in turn, nutrient availability. Organomineral complexes can also be formed with ions, particularly metallic ions such as Fe3+, Cu2+, Zn2+ and Mn2+. In many cases, the complexed ions are more available for plant uptake than the non-complexed (mineral) forms of these ions.
Cation exchange capacity is a very important fea-ture of soils. It determines to a great extent the potential fertility level of a soil. It may be defined as the amount of centimole charges (cmol(+) of exchangeable cations that a kilogram (kg) of soil can adsorb. Soil humus has as much higher CEC than clay minerals common in soils of the southern United States (Table 2). A little organic matter, therefore, can greatly influence the CEC of a sandy Coastal Plain soil whose clay fraction is dominated by kaolinite or hydrous oxides. For example, if organic matter has a CEC of 200 cmol(+)/kg, 2 cmol(+) are contributed to the soil's CEC for each percent organic matter present.
For some soils of the southeast, the CEC contributed by organic matter may constitute as much as 50 percent of the soil's CEC. This higher CEC may have consequences in how soil-test results for K, Mg, Ca and other cations are interpreted. Many state soil testing programs recognize this effect by separating soils of the state into groups for interpretation. Groups may be separated by CEC or separated as a result of the cumulative management effects of soils with different CEC. Alabama separates soils by CEC whereas Georgia and South Carolina distinguish between the low CEC soils of the Coastal Plain and the higher CEC soils of the Piedmont by using different category ratings for each region. North Carolina separates soils into three classes based on sodium hydroxide (NaOH)-extractable humic matter: (1) mineral soils, (2) mineral-organic soils and (3) organic soils. Therefore, the broad effects of SOM on the CEC of soils of a physiographic region are generally accomplished by other soil-test calibration techniques, precluding the need to analyze all soil samples for SOM.
| Table 2. Cation exchange capacity of organic and inorganic soil colloids at pH 7.0 (from Brady and Weil, 1999). | |
| Colloid | CEC (cmol(+)/kg) or meq/100g |
| Humus | 200 |
| Vermiculite | 100 |
| Smectite | 150 |
| Fine-grained micas | 30 |
| Kaolinite | 8 |
| Hydrous oxides | 4 |
The persistence, degradation, bioavailability, leachability and volatility of pesticides are directly related to the nature and concentration of SOM. Certain soil-applied herbicides are bound by SOM, and this reduces their availability and requires high application rates to achieve effective weed control. High SOM and its associated microbial activity may also lead to faster degradation of soil-applied herbicides. Therefore, some products specify rates based on SOM. These SOM ranges are generally broad and encompass large soil physiographic regions.
For example, higher rates of many acetamides are recommended for soils with >3 percent organic matter. A grower with >3 percent organic matter would pro-bably know this without a soil test. BladexTM (Du Pont), on the other hand, has a labeled rate for pre-emergence broadcast application on corn that increases incrementally as SOM increases from 1 percent to >5 percent. In general, however, only unique fields (those with heavy application of organic soil amendments) may need an organic matter test for herbicide applications. In Georgia, check the Georgia Pest Control Handbook to ascertain if a SOM test is needed to adjust herbicide rates.
SOM is a valued component of any sustainable production system. Many production practices can influence the long-term buildup or depletions of SOM (e.g., amount of tillage). Long-term increases in SOM in a particular field reflect a consequence of improved production practices and possibly higher productivity.
Most research with SOM has focused on the effects of soil management on SOM, not the effect of SOM on management. Laboratory techniques are available that allow reliable estimates of SOM or humic matter in cultivated soils. These measurements, however, have little short-term value to the grower except for adjusting rates of certain herbicides.
Adams, J.F., C.C. Mitchell, and H.H. Bryant. 1994. Soil test fertilizer recommendations for Alabama crops. Agron. and Soils Dep Ser. No. 178. Auburn University, AL.
Brady, N.C., and R.R. Weil. 1999. The nature and properties of soils. 12th Edition. 881 pp. Upper Saddle River, NJ: Prentice Hall.
Buchholz, D.D. 1992. Soil test interpretations and recommendations handbook. Univ. Missouri, Dep of Agronomy Mimeo. Columbia, MO.
Clemson University. 1982. Lime and fertilizer recommendations based on soil-test results. Coop. Ext. Serv. Cir. 476. Clemson, SC.
Doran, J.W., D.C. Coleman, D.F. Bezdicek, and B.A. Stewart (ed.). 1994. Defining soil quality for a sustainable environment. 244 pp. SSSA Spec. Pub. No. 35. Soil Sci. Soc. Amer. Madison, WI.
Giddens, J. 1957. Rate of loss of carbon from Georgia soils. Soil Sci. Soc. Amer. Proc. 21:513-515.
Jackson, M.L. 1958. Soil chemical analysis. Englewood Cliffs, NJ: Prentice Hall.
Jenny, H. 1941. Factors of soil formation. New York, NY: McGraw-Hill.
Magdoff, F. 1992. Building soils for better crops: Organic matter management. 176 pp. Univ. Nebraska Press, Lincoln, NB.
Mitchell, C.C., and W.G. Blue. 1981. The sulfur fertility status of Florida soils I. Sulfur distribution in spodosols, entisols, and ultisols. Soil Crop Sci. Soc. Fla. Proc. 40:71-76.
Nelson, D.W., and L.E. Sommer. 1982. Total carbon, organic carbon, and organic matter. p. 539-579. In A.L. Page (ed.), Methods of Soil Analysis, 2nd Ed. ASA Monogr. 9(2). Amer. Soc. Agron. Madison, WI.
Plank, C.O. 1989. Soil test handbook for Georgia. Univ. of Ga. Coop. Ext. Ser., Athens, GA.
Rasmussen, P.E., and H.P. Collins. 1991. Long-term impacts of tillage, fertilizer, and crop residue on soil organic matter in temperate semiarid regions. Adv. Agron. 45:93-134.
Schnitzer, M. 1991. Soil organic matter - the next 75 years. Soil Sci. 151:41-58.
Sumner, M.E., and J.H. Bouton. 1981. Organic matter maintenance in forestry nurseries. Georgia Forest Research Paper. No. 24. p 3-6. Georgia Forestry Commission Research Division.
Tisdale, S.L., W.L. Nelson, and J.D. Beaton. 1985. Soil fertility and fertilizers. pp 754. New York, NY: Macmillan Publishing Co.
Tucker, M.R., and R. Rhodes. 1987. Crop fertilization based on N.C. soil tests. North Carolina Dep Agric. Agron. Div. Cir. 1. Raleigh, NC.
Wallace, A., G.A. Wallace, and J.W. Cha. 1990. Soil organic matter and the global carbon cycle. J. Plant Nutr. 13:459-466.
Southern Regional Fact Sheet, SERA-IEG-6*1. Adapted from the original manuscript prepared by C.C. Mitchell, Extension Agronomist and Associate Professor, and J.W. Everest, Extension Weed Scientist and Professor, Department of Agronomy and Soils, Auburn University, AL 36849. The Southern Region Fact Sheet on Soil Testing and Plant Analysis series presents timely information of interest to users of soil testing and plant analysis services in the Southern Region of the United States. It is reviewed by the Southern Extension and Research Activity Information Exchange Group #6 (SERA-IEG-6) on soil testing and plant analysis and published by each cooperating state's Agricultural Experiment Station and Cooperative Extension Service.
Bulletin 1196 /February, 2001
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