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Impacts
and Management of Soil Acidity under Direct Seed Systems - Effects of
Deneficial Soil Microbes and Soil Fauna Introduction: Acidification of soil is a natural process, although it is accelerated by the use of ammonium-based fertilizers as discussed in other papers in this session. Microorganism adapt to a variety of conditions under arctic tundra, hot springs, the ocean depths, and in subsoil hundreds of feet from the soil surface. Therefore, we should be less concerned about total microbial activity and more concerned about the types of organisms present and what functions they perform relative to crop production, soil quality, nutrient cycling and environmental impact. Soil quality depends on the microbial, chemical, and physical attributes of the soil, but can be degraded by water and wind erosion, loss of soil organic matter and nutrients, soil compaction, and soil acidity (Doran and Parkin, 1994). Soil acidity and nutrient availability: The importance of soil acidity to nutrient availability in plants will be discussed in other papers at this conference. Although soil acidity has a great influence on microbial activity, the relation of soil acidity to plant nutrient availability is probably controlled more by soil chemical reactions rather than by microbial reactions (discussed in other papers). One possible exception is in the rhizosphere or the region of soil immediately around the plant root where plant type and source of fertilizer N control acidity. For example, if the dominant source on N taken up by the plant is ammonium, then the plant will release hydrogen ions and decrease the pH in the rhizosphere. Acidity is also created by conversion of the ammonium form of N to nitrate during nitrification by soil microorganisms. Conversely, if the dominant source of N is from nitrate, hydroxyl ions will be released, causing an increase in rhizosphere pH. All this has implications in microbial activity, nutrient availability, and the potential for attack by plant pathogens. Plants too can alter the rhizosphere pH. For example, low nutrient availability of iron and other metals in alkaline or calcareous soil can be increased by the production of acids exuded by the roots which increase the availability of the metals. Measuring soil acidity: Soil acidity is measured with a pH meter that estimates the activity of hydrogen ions and is measured on a log scale. Acidity increases ten fold with a decrease in one pH unit. Therefore, if the soil pH decreases from pH 7 to 5, the soil acidity increases 100 fold. The reduction of soil pH from a range of 6.5-7.2 in the 1860's to less that 5.7 has decreased yield potential of cereals and seed legumes (Mahler and McDole, 1987) and has increased the potential for cereal diseases (Murray et al., 1992). These aspects will be discussed in other papers. Adaptability of soil microbes: Soil microorganisms have a wide range in pH tolerance and have adapted remarkably well to changes in the environment. In general, fungi seem to dominate acid soils more than bacteria. This has more to do with the inability of bacteria to compete with fungi at lower soil pH than to the better adaptability of fungi at these lower pH's. However, fungi have hyphae and thicker cell walls that may make them more adaptable at lower soil pH. Fungi with their hyphae can grow into favorable micro sites in the soil, whereas bacterial may not be able to do so. Soil actinomycetes (group between bacteria and fungi) prefer alkaline soil conditions. There are, however, exceptions to the preference by bacterial to neutral soil pH. Soil acidophiles can grow in soil pH's of 3.0 or less. One example is Thiobacillus thiooxidans that oxides elemental sulfur to sulfate in soil. This organism has special fats on their cell wall that protect them under the acid conditions they produce. Alkalophiles (alkaline loving) function at pH's of 10.5 in arid regions (Coyne, 1999). Most soil microorganisms known as neutrophiles function in the pH range of to 6 to 8. The adaptability of bacterial to adapt to a wide range of temperatures is well known. Thermophiles found in hot springs and under ocean volcanic vents survive at temperatures about 125 F and even at the boiling point of water. Conversely, psychrophiles grow best at temperatures below 59 F and can be found in tundra soils. Nitrification and mineralization: One of the most sensitive groups of organisms to soil acidity is the nitrifying bacteria that convert ammonium N to nitrite and nitrate. Nyborg and Hoyt (1978) reported higher rates of nitrification at three sites where soils of pH ranging from 5.5 to 5.7 were limed. The release of organic N to available forms (mineralization) also responded to soil lime. Forest soils tend to be more acid from the organic acid production and can nitrify at pH's below 4.4 (Coyne, 1999). Forest soils also tend to be dominated more by fungi than by bacteria. No till soils at the surface also tend to be dominated by fungi, whereas tilled soils tend to be bacteria dominated (Hendrix et al., 1986). This is probably because tillage destroys the hyphal network that fungi produce. Effect of acidity on microbial activity: Soil acidity is known to reduce microbial activity in agricultural soils. Microbial activity in soil can be measured in four general ways: 1) numbers (plate counts) of organisms that grow on a petri plate supplemented with nutrients; 2) activity of specific enzymes; 3) respiration that measures the production of carbon dioxide; and 4) methods that measure changes in microbial communities. All methods have their advantages and drawbacks. For example, plate counts only estimate from 1 to 10% of soil organisms as many don't grow on artificial medium. Enzyme activity is an indirect method and many different methods are available. Respiration is usually a good method of estimating microbial activity except for no till soils. One drawback in measuring microbial activity for no-till soils is that these soils are not normally tilled and when disturbed during sampling and handling, microbial activity can increase appreciably. Recent methods that measure changes in microbial communities can provide some indication how different organisms adapt to changes in management such as no till. Liming studies conducted
in 1992 and 1993 at Pullman and Genesee are shown in Table 1. Lime was
applied at 2.68 Mg ha-1 (1.2 tons acre-1) in the fall of 1991 at Pullman
and in fall of 1992 at Genesee as a broadcast application to barley residue.
Peas were planted in spring, followed by winter wheat in fall. Liming
increased soil pH through the first year at both locations and into the
following year. Microbial biomass (weight of soil microorganisms) increased
with liming as well at both locations, suggesting that the addition of
calcium increased pH and stimulated growth of microorganisms. Respiration
was not affected by lime at Pullman, but was at Genesse in the fall of
1993, suggesting that residue breakdown could be increased by liming.
Liming increased dehydrogenase enzyme activity only in spring of 1992
at Pullman. Pea yield was not affected by lime, although winter wheat
yield was increased at Pullman. These studies point out that liming generally
increases microbial activity the first year after application. In year
three of both studies, the effects were less apparent.
Nitrogen fixation in legumes: Another sensitive function of soil acidity is N fixation in legumes. Soil acidity can limit growth and persistence of rhizobia, the nitrogen fixing bacteria in soils (Graham, 1998). The fast-growing rhizobia that inoculate peas and lentils are generally more sensitive to acidity than strains that nodulate soybean, although acid-tolerant strains exist. Failure to nodulate in acid soils is due to both the lower numbers of rhizobia and to the failure of attachment of rhizobia to infectible root hairs. Although it is common to lime acid soils, the areas involved especially in the Palouse and the cost and availability of limestone often limit this practice. Alternative practices include acid-tolerant inoculant strains and host plants and lime pelleting (coating) of inoculated seed. There is evidence that available Ca rather than low pH may effect nodulation. Liming resulted in increased nodulation in common bean and alfalfa (Buerkert et al, 1990; Pijnenberg and Lie, 1990). This may be because of the direct effect of H+ concentration, the presence of toxic levels of aluminum and manganese, or deficiencies of calcium, phosphorus, and molybdenum at low pH. Acid soils in the Palouse are deficient in molybdenum (MO) and respond to either seed treatment or to liming. Legume seed is routinely treated with MO in this region. Our observations with MO- treated pea seed show that seed harvested from these fields contain more MO than from fields not treated. Consequently, seed treatment is essential, especially in seed lots obtained from non-treated seed. From our liming trials in the Palouse of surface applied material to a 25-year no till field, spring wheat yield increased significantly in the first year, but lentil yield was not affected in year two. Furthermore, nodulation in lentils was not affected by liming. These results are surprising. One explanation is that liming tends to increase mineralization and nitrification, increasing the level of soil nitrate that is known to reduce nodulation. Another factor is that our native stains of rhizobia are somewhat adapted to the relatively acid conditions and that liming reduced their effectiveness. Effect of acidity on the soil micro and macro fauna: The soil fauna are probably more affected by tillage than by changes in soil acidity. Kladivko (2001) in describing the changes in the soil fauna with tillage, suggests that no till probably affects the macro fauna (large arthropods, earthworms) more than the micro fauna (protozoa and nematodes) because larger organisms are more sensitive to changes in tillage. Meso fauna (mites, acarids and springtails) are intermediate in size and in combination with the micro flora (bacteria and fungi) play a dominant role in the soil food web fauna in recycling nutrients and carbon. Since soil acidity reduces populations of soil bacteria, then those organisms feeding on bacterial (protozoa and beneficial nematodes) would logically decrease in population. However, there is limited information on the effects of soil acidity on the soil fauna in general. Studies conducted at WSU comparing the soil fauna under conventional and no till did not show any apparent differences, as their populations were generally low under both management systems. However, exceptions were noted in the earthworm populations. Earthworms appear to be favored more under legumes and mustard than under cereals. Higher populations are also noted under no till, although many no till fields have few or no earthworms. Of the soil macro fauna, earthworms appear to respond well to liming. In a liming trial established under no till in 1999, earthworms increased from 25 to 140 per square meter in the spring of 2000 from an application of 4 tons/acre. Stratification
of soil acidity under direct seeding: A potential problem of continuous
direct seeding is the potential buildup of soil acidity in the zone of
N fertilizer application. Under tilled systems, the surface soil is normally
inverted and mixed with soil lower in the profile that usually contains
higher amounts of soil bases (higher in pH). Figure 1 compares soil pH
with depth under conventional and no till management at Touchet (14 years
no till) and at Palouse (25 years no till). This trend was observed in
five of six paired conventional-no till sites where soil pH was substantially
lower in the 2-4 inch zone under no till as compared to conventional management
(Bezdicek et al. 1998). This zone is where the majority of starter fertilizer
(often 16-20-0 + S) is applied and close to where the majority of N is
applied. See other articles in this series by Bob Mahler, Greg Schwab,
and Tim Paulitz. Soil pH above this zone at the soil surface is usually
higher in pH from deposition of bases from surface residue. Soil below
this zone is higher in pH as well. A number of questions arise on whether
long-term no till fields need to be tilled periodically to redistribute
soil nutrients and to bring up higher pH soil to the soil surface. Our
limited work on surface liming of no till fields suggests this practice
is impractical and uneconomical as lime tends to stay at the soil surface.
One possible solution is seed applied lime where smaller quantities could
be applied in the seed row. These studies will be presented in a paper
by Greg Schwab.
Bezdicek, David, John Hammel, Mary Fauci, Dennis Roe, and Jon Mathison. 1998. Effects of long-term direct seeding on soil properties on northwest farms. Proceedings, PNW Direct Seed Cropping Systems Conference, January 7-8, Pasco WA, 1998. Buerkert, A., K. G. Cassman, R. de la Piedra, and D. N. Munns. 1990. Soil acidity and liming effects on stand, nodulation, and yield of common bean. Agron. J. 82: 749-754. Coyne, Mark. 1999. Soil Microbiology-An Exploratory Approach. Delmar Publishers, Albany, NY. Doran, J. W., and
T. B. Parkin. 1994. Defining and assessing soil quality. p 3-21. In Graham, Peter. H. 1998. Biological dinitrogen fixation. pp 322-345. In David M. Sylvia, Jeffery F. Fuhrmann, Peter G. Hartel, and David A. Zuberer (ed). Principles and application of soil microbiology. Prentice Hall, New Jersey. Hendrix, P. F., R. W. Parmelee, D. A. Crossley, Jr., D. C. Coleman, E. P. Odum, and P. M. Groffman. 1986. Detritus food webs in conventional and no-tillage agroecosystems. BioScience 36: 374-380. Kladivko, Eileen J. 2001. Tillage systems and soil ecology. Soil Till Res 61: 61-76. Mahler, R. L. and R. E. McDole. 1987. Effect of soil pH on crop yield in Northern Idaho. Agron. J. 79:751-755. Murray, T. D., C. C. Walter, and J. C. Anderegg. 1992. Control of Cephalosporium stripe of winter wheat by liming. Plant Dis. 76:282-286. Nyborg, M., and P. B. Hoyt. 1978. Effects of soil acidity and liming on mineralization of soil nitrogen. Can. J. Soil Sci. 58: 331. Pijnenborg, J. M. W. and T. A. Lie. 1990. Effect of lime pelleting on the nodulation of Lucerne (Medicago sativa L.) in acid soil: a comparative study carried out in the field, in pots, and in rhizotrons. Plant Soil 121: 225-234. |
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