Heidi Webber (and others),

The SLPF fertility value could fall between about 0.70 and 1.00.  A
value of 1.00 is the upper end, and should be used only for the best
soils, such as silty clay loams and silty loams in Midwestern USA that
have no pH or nematode problems.  Other soils will be lower, according
to whether they have low pH, have salinity problems, have nematode
problems, or are highly sandy or leached soils.  Things other than N
that would reduce leaf photosynthesis.  Just for reference, for soils in
Gainesville Florida for soybean, peanut, and bean, we use a value of
0.92, and for North Carolina soils we have used 0.95.  For Ghana we have
used values of about 0.86 to 0.87.  Tell me more about the soil type and
problems and I could give you a guess.

The real point of SLPF is that it accounts for the non N and
non-water-holding aspects of the soil productivity.  We know that the
leaf photosynthesis parameters, LFMAX, are really quite stable and
should NOT be used to calibrate in the growth and yield at a given site
unless you have two cultivars that differ in dry matter growth rate.
For that reason, you should eventually be able to set your SLPF based on
yield and dry matter of crops measured at the desired location.  But I
suppose you don't even have that information.  Once set for a site, the
SLPF should not be changed (for different seasons for example).  The
"productivity" aspects of the cereal models versus grain legume models
may not have been adequately "coordinated" by DSSAT model developers to
allow using the same soil file over multiple crops.  This issue of a
common SLPF value along with "common" rooting function is something that
"we" as DSSAT model developers still need to work further on, since we
now have possibility of same soil following through on a rotation of
different crops.  Yes, the same soil follows through now on rotations,
but rooting and SLPF may not be right for both crops.

Hope this helps.
Ken Boote

Heidi Webber wrote:

>Dear listserv members,
>
>I am a new DSSAT user and wondered if anyone could help me in estimating
>a "reasonable" value for the soil fertility factor (SLPF) to use for
>soils in the Fergana Valley of Uzbekistan.  Our study looked at deficit
>and alternate furrow irrigation for common bean, green gram and soybean
>as second crops after winter wheat.  Unfortunately, I don't know what
>cultivars were used, as they were bought at a local market.  Until I
>determine what cultivars were used, or other cultivars close to them, is
>anyone familiar enough with the soils in the region to suggest an
>estimate of the fertility factor?
>
>Thank you and any suggestions would be much appreciated,
>
>Heidi Webber
>Student in Agricultural Engineering at McGill University, Montréal,
>Canada
>
>