Fertilizer applications are specified as field records either by “year + day of year” format or as days after planting. A new feature of DSSATv47 is the ability to specify custom fertilizer blends, controlled and slow release fertilizers, nitrification inhibitors, and urease inhibitors by putting all fertilizer properties in an external, editable file. This file, FERCHvvv.SDA can be found in the StandardData directory of DSSAT (where vvv refers to the major release version of DSSAT, e.g., FERCH047.SDA for DSSAT47).
Two parameters describe the release of a slow or controlled release fertilizer:
- NREL50 is the release rate, expressed as the time in days required for 50% release of the applied N
- k is the Sigmoid logistics curve K value. For a step-like function, use k=0.8-2.0, for a linear-like function, use k=0.1-0.2)
The figure above shows the N release rates for various selections of sigmoid-k with an initial N application of 100 kg/ha and a 50% release time of 30 days. The linear and step functions bound the rates of release which are approximated by the sigmoid curve. Note that a sigmoid-k of 0.1 requires a longer release duration (i.e., NREL50 = 30 days is not long enough), resulting in an artifact in the function which we remove with a linear approximation in the first few days.
The fertilizer properties table in FERCH***.SDA also allows specification of nitrification inhibitors and urease inhibitors. These are described by two parameters each: an efficiency of inhibition and a duration. In this case, the process (nitrification or urea hydrolysis) is reduced by the efficiency amount for the duration specified.
With this external file, a user may compose a custom blend of fertilizer by creating a new, unique fertilizer code in the table.