the average annual soil loss using all <strong>of</strong> theUS .L.E . factors:1) Take any map symbol designation from an<strong>Elgin</strong> <strong>County</strong> soil map.e.g.BEcBased on the 'TCey to the Map Symbols" <strong>and</strong>"Legend" descriptions provided on each soilmap, this map symbol represents a Berrien(BE) soil which occurs on a complex c slope <strong>of</strong>2 to 5% . For the purpose <strong>of</strong> this example, thecrop/vegetation cover for the soil is fallploughed silage corn, with the previous year'scrop being winter wheat. No conservationpractices have been employed .2) Select the appropriate R, K, LS, C<strong>and</strong> P factorvalues for the soil <strong>and</strong> slope components <strong>of</strong>the symbol from the tables provided .R value for <strong>Elgin</strong> <strong>County</strong> = 90Kvalue for Berrien (BE) soil = 0.18 (Table 9)LS value for c slope = 0.39 (Table 11)C value for fall ploughed silage corn(previous year winter wheat) = 0.48(Table 13)P value for no conservation practices =1 .00(Table 17)3) Using the Universal Soil Loss Equation<strong>and</strong> themetric conversion factor <strong>of</strong> 2.24, calculatethe potential average annual soil loss (A) bywater:A=RxKxLSxCxP=90 x0.18 x039 x0.48 x1.00= 3.0 t/ac/y x 2.24= 6.7 t/ha/y4) <strong>The</strong> potential average annual soil loss for themap symbol is 6.7 t/ha/y.(5) How to Determine Potential SoilLoss at Field SitesSite or field specific interpretations are usefulfor on-farm management purposes because theyprovide farm managers or extension personnelwith a general indication <strong>of</strong> the erosion-reducingeffectiveness <strong>of</strong> various crop <strong>and</strong> managementsystems . <strong>The</strong> example which follows outlines theprocedure which should be used to estimate theaverage annual soil loss for a particular field site.<strong>The</strong> potential soil erosion class for the soil at thatsite can then be determined using Table 18.'Example ,- A particular farm field is thought tohave an ongoing water erosion problem, <strong>and</strong> thefarm manager would like to know if this is so .<strong>The</strong> field is in a corn-corn-soybeans (3yr.) rotation,<strong>and</strong> is fall moldboard ploughed . As aconservation measure, contour farming is carriedout as a st<strong>and</strong>ard management practice .Procedure :1) First determine the soil conditions for the field.For the purpose <strong>of</strong> this example, soilinvestigations indicate that the field iscomposed <strong>of</strong> Tavistock soils (TA) which havea slope gradient <strong>of</strong> 5%, <strong>and</strong> slope lengths <strong>of</strong>200 metres.2) Now determine all <strong>of</strong> theUS.L.E . factor valuesfor the field . For this example they are.R value for <strong>Elgin</strong> <strong>County</strong> = 90K value for Tavistock soil (TA) = 0.27(Table 9)LS value = 1.37 (for combination <strong>of</strong> slopelength = 200 metres, <strong>and</strong> slope gradient= 5%; Table 10)C value = 0.39 * see belowP value = 050 (contour farming; Table 17)* C values for a variety <strong>of</strong> crops <strong>and</strong> rotations aregiven in Tables 13,14,15, <strong>and</strong> 16 . <strong>The</strong> C value forthis particular rotation, however, is not given inthose tables <strong>and</strong> is determined as follows:<strong>The</strong> C value for corn following soybeans, withfall moldboard ploughing (FMP), is 0.47 (Table13)<strong>The</strong> C value for corn following corn, with fallmoldboard ploughing (FMP), is 0.35 (Table 13)<strong>The</strong> C value for soybeans following corn, withfall moldboard ploughing (FMP), is 0.36 (Table13)<strong>The</strong>refore, the rotational C value isC= 0.47+0.35+0.36 =0.393 (years)3) Using the metric conversion factor <strong>of</strong> 2.24,calculate the average annual soil loss (A) bywater :A=RxKXLSxCxP=90 x0.27 x1.37 x0.39 x0.50= 6.5 t/ac/y x 2.24= 14 .6 t/ha/y= Moderate (Soil Erosion Potential Class 3;Table 18)4) Since the tolerable"A" value for deepagricultural soils is 6.0 t/ha/yr (3.0 t/ac/y) orless, the results indicate that the potentialerosion for the farm field is above the tolerablelevel. <strong>The</strong>refore, a change in cropping orconservation practices are needed to reducethe average annual soil loss to a moreacceptable rate.106
To reduce the soil losses on the field in theexample to a more acceptable level, using achange in cropping practices, rearrange theequation to solve for the crop cover factor (C) .Since the U .S .L.E . is calculated in imperial units,the A value which should be used is 3.0 t/ac/yr .<strong>The</strong> C value needed to achieve a 3 t/ac/y soil losswould be:C=A/RxKXLSxP (where A=3.0<strong>and</strong>RxKxLSxP=16 .6)= 3 .0 /16.6= 0.18<strong>The</strong> farm operator would therefore have to choosea crop or crop rotation with an average annual Cvalue <strong>of</strong> 0.18 or less . Some alternatives are : 1) aspring disced or cultivated small grain crop (Cvalue = 0 .18; Table 13); 2) a no-till grain corn crop(C value = 0 .16; Table 13); or 3) one <strong>of</strong> three croprotations which include a forage crop (Table 16) .
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