The Soils of The Regional Municipality of Ottawa=Carleton

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B. SOIL INTERPRETATIONS FOR WATER EROSION byG.J. Wall and I .J. Shelton, Pedologists, Agriculture Canada, Ontario Institute of Pedology Guelph, Ontario (1) Introduction Soil erosion by water is a naturally occurring process that can be greatly accelerated by man's activity. Any practice that accelerates soil runoff, or reduces the natural protection afforded by vegetative cover, will generally lead to increasing erosion levels . It is commonly held that soil erosion reduces production potential, depletes nutrients, and degrades soil tilth . However, recent studies in the Canadian Great Lakes basin have illustrated the need to look beyond the on-site effects of soil erosion and consider the role of sediments, derived from cropland, on water quality. Any comprehensive soil conservationprogram will recognize the dual nature of the problem of soil erosion by water. The purpose of this section is to provide interpretations of the erosion potential of the Ottawa-Carleton Region soils and soil landscapes . Specifically, the objectives may be summarized as follows : (a) to determine the relative erodibility of surficial soil materials ; (b) to determine the effect of soil erodibility and slope on soil erosion potential ; (c) to provide information onthe effects of different crops and associated cropping practices on soil erosion potential ; (d) to illustrate how the soil maps, in combination with information from the report, can be used to assess site-specific soil erosion problems and alternative solutions ; (e) to provide guidelines for estimating soil erosion potential at the regional scale. (2) Factors Affecting Soil Erosion by Water On-site planning for soil and water conservation requires information on the relationship between factors that cause soil erosion, and practices that may reduce soil erosion . The most important factors affecting agricultural erosion are usually considered to be rainfall runoff, soil erodibility, slope gradient and length, and vegetative cover. Both rainfall and runoff parameters must be considered in the assessment of a water erosion problem . Rainfall-induced erosion is maximum whenthe energy ofthe rainfall is greatest . In Ontario, it is the high-intensity, short-duration thunderstorm activity of the summer months that produces the highest-energy rainfall events . On the other hand, runoff from agricultural land is greatest during the spring months when the soils are usually saturated, the snow is melting, and evapotranspiration is minimal . A good soil and water management program must address itself to rainfall and runoff problems, in both the spring and thesummer periods . Soil erodibility is defined as the inherent susceptibility of a soil material to erode . Soil properties that influence erodibility by water are those that affect the infiltration rate, permeability and water-holding capacity of the soil . Soil properties that resist the dispersion, splashing, abrasion and transporting forces of rainfall and runoff, also influence soil erodibility. Silt, silt loam and very fine sand soils often have the greatest soil erodibility potential, whereas coarse sandy and clayey soils usually have the least inherent soil erodibility. Maintenance of soil organic matter and soil structure, through good soil management, can greatly affect soil erodibility potentials . Soil erosion by water has been found to increase, with both increasing slope gradients and slope lengths . Steep slopes facilitate the runoff of water and reduce the infiltration rate. The potential for erosion on long slopes is enhanced by rapid and voluminous runoff which can generate high erosive energy at downslope positions . Hence the effective slope length should be an important soil conservation consideration in farm field consolidation efforts. The erosion reduction effects of vegetative cover or mulches are well known . Table 12 illustrates the relative effectiveness of common field crops in reducing water erosion potential in Ontario. If unvegetated or bare soil is assigned a numerical value of 1 .0, then the vegetative cover afforded by bean, tomato, or cucumber crops has the potential to reduce soil loss by sheet and rill erosion to approximately 47% of that lost under bare soil conditions (Table 12) . Similarly, a haypasture or permanent pasture vegetation cover has the potential of reducing water erosion to less than 10% of that from fallow or bare land . Table 12 . The relative effectiveness ofcommon field crops in reducing water erosion in Ontario Covertype C-factor value' Fallow land (bare soil) 1 .00 Beans .47 Cucumber .45 Tomato .43 Continuous corn - fallplowed .45 - spring plowed .39 - chisel plowed .27 - zero tillage .08 Corn in rotation .30 Mixedgrains .27 Winter wheat .25 Rye .25 Tobacco .30 Hay-pasture (rotation) .06 Permanent pasture .03 'Cropping-managementfactor, Wischmeierand Smith (27) (3) Measurement of Factors Affecting WaterErosion In order to make meaningful recommendations, with respect to soil conservation practices, one must be able to recognize the significance of a soil erosion problem, and provide appropriate cost-effective erosion control alternatives when a problem is encountered . Although qualitative approaches can be useful in many circumstances, the temporal nature of soil erosion, as well as the difficulty in witnessing sheet erosion losses in the field, make a quantitative approach to erosion assessment and control recommendations more practical . The quantification of the factors affecting agricultural erosion, i .e., rainfall, soil erodibility, slope vegetation and conservation measures, is based on widespread erosion research compiled from nearly 10,000 plot-years of field data, and from rainfall records obtained from about 2,000 weather stations in North America. The resulting soil erosion formula is in extensive use by the Soil Conservation Service of the United States Department of Agriculture, for applying and planning conservation measures that reduce soil erosion to acceptable amounts (27) . It is onlyrecently that the erosion factors have been quantified for use in Ontario (28, 29) .
The water erosion formula, A= RKLSCP, used to predict average annual soil loss through sheet and rill erosion, is called the Universal Soil Loss Equation (27), where A - is the computed soil loss intons per acre peryear. R - the rainfall factor, is thenumber oferosion-index unitsin a normalyear's rain . K - the soil erodibility factor is the erosion rate, per unit oferosion index for a specific soil, in cultivated, continuous fallow. This unit is expressed in tons per acre. L - the slope length factor is the ratio of soil loss from a field slope length, to soil loss from a 72 .6 foot plot . S - the slope gradient factor is the ratio of soil loss from the field slope gradient, to soil loss from a 9% plot slope . C - the cropping-management factor is the ratio of soil loss from a fieldwith specific vegetation or cover and management, to soil loss from the standard, bare or fallow condi tion . This factor measures the combined effect of all the interrelated cover and management variables, plus the growth stage and vegetal cover, during rainfall episodes . P - the erosion control practice factor is the ratio of soil loss using aparticularmanagement practice, to soil loss from a field notusing thatpractice. When the numerical values for each variable are multiplied together, the product is the average annual soil loss, in t/ ac/yr (conversion to metric equivalents, t/ha/yr requires multiplication by 2 .24) . It should be emphasized that the formula estimates sheet and rill erosion but does not consider soil losses caused by gully erosion or stream channel erosion . Since the erosion formula does not contain a transport or delivery factor it does not predict sediment load of streams. A brief description ofeach factor in the soil erosion formula follows . (a) RainfallFactor (R) The R-value reflects the erosivity of rains, and is generally related to climatic factors . R-values for Ontario range from 25 to 100 . The Ottawa-Carleton Region has an approximate R value of 50 . The distribution of R-values, for southern Ontario, is shown inFigure 35 and discussed elsewhere by Wall et . al . (30) . (b) Soil ErodibilityFactor (K) The K-value reflects the inherent erodibility of a soil dueto the erosive activity ofwater. Table 13 illustrates the K-values and K-ranges for the soils of the Ottawa-Carleton Region . The K-values were computed by the method outlined by Wischmeier and Smith (27) . These K-values range for a low of .05 for the Uplands Association soils, due in part to the high proportion of medium and fine sands characteristics ofthese soils, to a meanhighof .43 for the poorly drained Bainsville soils of the Castor Association, which contain a large proportion of easily erodible very fine sand and silt . In terms of relative erodibility, the Bainsville soils are about 8 times moreerodible than the Upland soils (.41/ .05), when allother soil loss factors are held constant .
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i The Soils of The III Regional Mun
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ACKNOWLEDGEMENTS . . . . . . . . .
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1 . Soil map index for the Regional
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0 t W. CARLETO 10 L 20 km KANATA GO
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Otta!~? Figure 3. Main towns, trans
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Arnprior Chats Falls Almonte " Carp
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may also contain shaly layers . Out
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Till plains (undrumlinized) _ Esker
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Surficial Geology and Relationship
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HUN nnnnoioion miammoo nonmmnon nou
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GEOLOGICAL MATERIALS SOIL ASSOCIATI
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GEOLOGICAL MATERIALS SOIL ASSOCIATI
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In this survey, boundaries were per
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BASIC LAND AREAS SOIL ASSOCIATION -
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their agricultural use . The domina
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OTTAWA- CARLETON SOILS t TYPE DEPTH
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B3 : Dominantlyvery Rego Gleysols p
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cant component, approximately 600 h
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In some soils, significant layers o
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Taxonomic Components The well-drain
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Taxonomic Components Four component
Page 43 and 44: Correlation to Ottawa Urban Fringe
Page 45 and 46: The well-drained Orthic Melanic Bru
Page 47 and 48: ial which extends in a northwest-so
Page 51 and 52: all Leitrim soils classified as suc
Page 53 and 54: The imperfectly drained St . Damase
Page 55 and 56: Recognized Recognized Subgroups Ser
Page 57 and 58: Soil Materials North Gower soils co
Page 59 and 60: Figure 17 . A poorly drained Orthic
Page 61 and 62: Figure 25 . Manotick association so
Page 63 and 64: Soil Landscape Units Four units wer
Page 67 and 68: sloping areas, groundwater flow, an
Page 69 and 70: from clay to silty clay loam . In s
Page 73 and 74: scapes occurring inclose proximity
Page 75 and 76: Within Rockland units, significant
Page 77 and 78: A . AGRICULTURAL CAPABILITY CLASSIF
Page 79 and 80: Table 7 . (cont'd) R' : T' : V' : W
Page 81 and 82: adequate moisture or a wetness limi
Page 83 and 84: In order to apply the tables, one m
Page 85 and 86: (e) The Capability Rating Tables Ta
Page 87 and 88: 00 Table 9. (continued) Slope Class
Page 89 and 90: Table 9 . (continued) AGRICULTURAL
Page 91 and 92: Table 9 . (continued) AGRICULTURAL
Page 93: Soil Landscape Unit Table 10 . Agri
Page 97 and 98: soil Association Table 13 . K-value
Page 99 and 100: graphic regions . Therefore, the C
Page 101 and 102: Table 17 . continued Crop Rotation
Page 103 and 104: SOIL SOIL MATERIAL DESCRIPTION MAIN
Page 105 and 106: C . LAND SUITABILITY CLASSIFICATION
Page 107 and 108: Table 19 . Land suitability ratings
Page 109 and 110: Table 19 . continued Soil associati
Page 111 and 112: 1 . Marshall, I.B., J. Dumanski, E.
Page 113 and 114: Class Description Sl Slightly stony
Page 115 and 116: Soil Association GRENVILLE Series a
Page 117 and 118: Soil Association Series and (Subgro
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The Soils of The Regional Municipality of Ottawa=Carleton

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