Sutisa,  interesting results.  To me I think the fertility factor works
only for legumes, not for wheat and maize. I could be wrong.  Anyway, if
it does, I think you should keep it constant for the three years because
"overall soil fertility" including phosphorus status should NOT change
much from year to year.  The adjustment to initial fertility levels
would be more appropriate for your case, I think. Please don't also
forget about the initial soil moisture, since these crop models are very
sensitive to water and N stress.  This is my two cents for your
consideration.  If disagree, please disregard it.  John


-----Original Message-----
From: DSSAT - Crop Models and Applications
[mailto:[log in to unmask]] On Behalf Of sutisa pinitpaitoon
Sent: Tuesday, June 27, 2006 5:13 AM
To: [log in to unmask]
Subject: fertility factor

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