66
equations and days were integers in the model; dry
weight can increase from 0.0 g to 0.03 g or higher in
one day. Linear extrapolation to zero was performed on
the four days previous to measurable dry-weight
appearance (and disappearance) to smooth out the curve.
The adjustment did not affect the performance of the
model; the model-derived curves more closely represented
the biology of the system. The model was set to run 123
days from planting. The time duration was set by the
equation describing the leaf dry-weight accumulation;
the soybean fields under intensive study for two
consecutive seasons were harvested 123 and 124 days
after sowing.
The correlation coefficients and the coefficients
of determination were not 1.00 for any of the regression
equations. The field data may be biased, which would
produce a 1ess-than-perfect fit. The field data may not
be accurate or may not accurately represent the actual
plants, again introducing bias. Random variation, a
common obstacle with field data, likely accounts for
most of the variation. Had the correlation coefficients
been less than 0.80 or the coefficients of determination
less than 0.60, some concern as to whether' the
regression equations accurately described the field data
might be warranted. The data were well fitted by the
presented equations (Figures 1 7).