Kenel,
 
I assume you know the factor, SLPF, in the soil file that we call the fertility factor.  The fertility factor has an origin, in that it should be 1.00 for a soil that has no fertility limitations (a good midwestern cornbelt soil with no soil-borne disease or nematodes).  This has nothing to do with N supplying ability, as you should be able to apply enough N to bring yield to its climatic potential.  The SLPF value can be less than 1.00, for soils that are limiting (lower pH, limiting in P, K, S, or micronutrients).  For example, we use 0.92 for our sandy soils in Florida.  Values of 0.86 to 0.87 have been used for soils in West Africa (actually not the poorest of the soils either).
 
It is not set automatically.  UNFORTUNATELY, the DSSAT default soil creation program puts a 1.00 in the SLPF slot every time.  Not a good thing, but?  The way to set it, is to have a field that has no water limitations (irrigated or adequate rainfall), and no N limitations (quite sufficient N fertilization).  Then you set the SLPF to give the correct slope to the dry matter accumulation rate over time (or maybe to the final biomass value, but that is less secure).
 
I hope this helps.
 
Ken Boote

________________________________

From: DSSAT - Crop Models and Applications on behalf of Delusca Kenel
Sent: Thu 9/27/2007 2:38 PM
To: [log in to unmask]
Subject: Fertility factor



Hi,
We are doing some water-limited simulations for a few crops in Québec. We would like to enter among other parameters the fertility factor. However, we have some qualitative information (very good, good, etc..) about the fertility level of the soils. We would like to know if DSSAT can calculate the value (if yes, how he does it?) of this parameter or if there is scale to derive its value from qualitative info.

Many thanks,
Kénel Délusca,
Université de Montréal