The Soils of Elgin County - Agriculture and Agri-Food Canada

erosion losses will occur . Potential soil erosionlosses are greater than 33 tonnes/hectare/year(>15 tons/acre/year) .Potential soil erosion classes have beendetermined for the soils identified on the soilmaps for the County . Some soils shown on themaps occur on a number of surface slopes or slopeclasses, and a range of erosion classes wastherefore determined for those soils . The potentialsoil erosion class, or the range of classes, for eachsoil are given in Table 9.(3) Assumptions1 . The amount and monthly distribution ofrainfall is assumed to be relatively constantthroughout the County.2. Representative slope lengths for each slopeclass were used to calculate the LS valuesappropriate for Elgin County landscapes .3. Average levels of . management are assumed .These would include good soil managementpractices that are feasible and practical undera largely mechanized system of agriculture.4 . The average annual soil losses calculated foreach map symbol are based on representativerainfall, soil, slope and possible crop andmanagement conditions . The results give ageneral indication of the erosion potential ofthe soils represented in a map symbol, relativeto those in other symbols. Since variations insoil conditions do occur within areasdelineated on the soil maps, estimations forspecific sites require detailed informationcollected on the site anda separate calculationfor each unique combination of conditions .(4) How to Determine Average AnnualSoil Loss from the Soil MapsThe average annual soil loss, or "A", for eachsoil identified in the symbols on the soil maps canbe determined under various crop or managementconditions . The soil loss under bare fieldconditions can be determined by multiplyingtogether the rainfall (R), soil erodibility (K), andslope factors (LS). The soil loss which occurswhen specific crops are grown, or when specificmanagement practices are employed, can bedetermined by multiplying together all of thefactors which compose the equation. Thedescriptions which follow explain how the twotypes of interpretations are made.(a)Determining average annual soil loss underbare field conditionsIn order to determine the average annual soilloss (A = R x K x LS) for the symbols shown onthe soil maps of the County, the U.S .L .E. must beapplied to each of the soil types identified in thesymbols. Since the U.S.L .E. results in imperialunit values (tons/acre/year) for soil losses, thelosses which are calculated using the equationmust be multiplied by 2.24 to convert to metricunits (tonnes/hectare/year) . The steps outlined inMethod 1 indicate how to determine the averageannual soil loss using the R, K, and LS factors.A simplified means of determining theaverageannual soil loss can also be used . Average annualsoil losses are given in Table 12 for selected Kvalues and slope classes mapped in the County .The procedure for determining the average annualsoil loss from Table 12 is outlined in Method 2.For K values not shown in the table, the soil losscan still be estimated by interpolating betweenappropriate K values which are given . Soil lossvalues arrived at usingTable 12 are in metric units(t/ha/y). When using Table 12, it is important toremember that the table has been compiledspecifically for Elgin County, and the soil lossvalues are based on estimated slope lengths foreach slope class appropriate for Elgin Countylandscapes. The use of the table in areas otherthan Elgin County, therefore, is not appropriate.When the average annual soil loss has beendetermined for a particular soil using either ofthese methods, the potential erosion class for thatsoil can then be determined using Table 18 .Method 1The U.S .L.E . can be applied to soil typesidentified in the map symbols to determine thepotential average annual soil loss. Under barefield conditions, the equation is A = R x K x IS.An example is as follows :1) Take any map symbol designation from anElgin County soil map .e.g .BV>TAb>cBased on the "Key to the Map Symbols" and"Legend" descriptions provided on each soilmap, 40% to 80% of the area represented bythe symbol consists of Beverly (BV) soils whichoccur on complex b slopes of 0.5 to 2%, and20% to less than 40% of the area consists ofTavistock (TA) soils which occur on complexc slopes of 2 to 5%. A potential averageannual soil loss for each of these soils musttherefore be determined.104