REPRESENTING AGGREGATE SIZE DISTRIBUTIONS AS MODIFIED LOGNORMAL DISTRIBUTIONS

Historically, the two-parameter lognormal distribution has been the me thod of choice when describing soil aggregate size distributions and g enerally provides a good description. However, the assumptions regardi ng the upper and lower extremes of the two-parameter lognormal ogive c an limit its applicability to many tillage-induced aggregate size dist ributions. Two 3-parameter and one 4-parameter lognormal ogives are pr esented that can more accurately describe a wider range of field-sampl ed aggregate size distributions. Two computational techniques for dete rmining values for the coefficients of these modified lognormal functi ons are discussed. One is a direct computation method, useful for appl ications where computation speed is a factor. The second method uses a nonlinear optimization technique, which will find the ''best fit'' pa rameter values more precisely, but requires more computational overhea d.