>Hi,
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>Estimating LL, DUL and SAT on the basis of a soil's texture and SOM content
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>is very uncertain and gives great differences depending on which estimation
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>method is used (and there are >150 methods!). If one really wants reliable
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>data there is nothing better than measuring it, though this still leaves the
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>uncertainty of measuring in the lab with a pressure plate apparatus or
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>measuring in the field.
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>I have been comparing eight estimation methods for LL, DUL, SAT, especially
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>focusing on the uncertainties in each method (and these were big!). I
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>presented this last year at a crop-modeling symposium in Florence, Italy,
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>from which the article just has been published (the title expresses the high
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>level of uncertainty in these methods....):
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>A.J. Gijsman, S.S. Jagtap, J.W. Jones. Wading through a swamp of complete
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>confusion: how to choose a method for estimating soil water retention
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>parameters for crop models. European Journal of Agronomy, Volume 18, Numbers
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>1-2 (2002), pp. 75-105.
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>Since the water retention parameters are so crucial for the outcome of a
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>crop model simulation, I would recommend to pay due attention to this.
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>Concerning your specific question: there is no need for having the same (or
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>even similar) difference DUL vs. SAT in all soil layers. Layers differ in
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>texture and in soil organic matter (SOM) content, and will thus differ in
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>water-retention parameters. Some of the methods I compared gave DUL < SAT
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>(as it should be!) for certain texture combinations, but DUL > SAT for
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>others, in which case I used DUL = 0.95 * SAT; this is not to say that this
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>is right, but it is surely better than DUL > SAT. For DSSAT, DUL has to be
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>greater than LL and SAT has to be greater than DUL (thus: SAT>DUL>LL), or
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>the model crashes.
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>If you have measured bulk density (BD), I would not modify this to tune the
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>water retention. Sometimes BD values may indeed be suspicious, as I found
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>e.g. recently in an international soils database BD's greater then 2.50
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>g/cm3; knowing that the particle density of granite (i.e. sand without the
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>pores) is about 2.65 g/cm3, such a value is highly unlikely, as it would be
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>a soil without pores and without SOM. In the above-mentioned article we
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>describe also a method from the literature for calculating the (theoretical)
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>BD; this may not apply equally to all soil types, but it is the best one can
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>do, if measured data are missing or clearly wrong.
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>Arjan Gijsman
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>Univ. Florida / Centro Internacional de Agricultura Tropical (CIAT), Cali,
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>Colombia
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