Dear DSSAT list,

            In my previous email I asked some questions about prediction of
maize yields, as the simulated results did not match observed trial results.
The DSSAT list recommended I change the soil fertility factor level to
better match my observed data.  This was also to take account of the
addition of phosphorous fertilizer in my actual trials.

            At present I do not have values for initial soil N. So I have
used the default value in Xbuild of 0.1 ppm of ammonium, 1.2 ppm of nitrate.
Then I changed fertility factor to range from 1 to 0.60 for a 3-year study
to see the results.   Year 1 had the fertility factor of 0.6 to simulate the
first year of fertilizer application where there had been none previously.
The fertility factor values for year 2 and 3 were increased to account for
the increased base fertilizer levels due to the previous year’s applications
of fertilizer. I found a better match of simulated to observed results. I
also then changed the initial N values together with a change in soil
fertility factor and found the best match between simulated and observed

          My question is which would be the appropriate factor to adjust in
this case between soil fertility factor and initial N value? It seems that
soil fertility factor is very sensitive to the prediction of above ground
biomass yields.

           I have not yet analyzed my own initial soil N. But when I have
this real data I will use them instead of an automatic/estimated value. I am
wondering if I should keep the value for the fertility factor the same for
each year then.

           Can anyone provide more information about the function of soil
fertility factor in the DSSAT model? I have read some papers but there seems
to be little information on this and I would like to understand its use in
DSSAt a little better.

           I appreciate everyones comments on my approach.

Thanks for your kindness

Sutisa