Dear Wei,
If you talk about the mineral bulk density, do you mean the bulk density of minerals plus pores, but without SOM? As such it was reported in Rawls and Brakensiek (1985) and they had a figure with all its values depending on the clay and sand content. The figure has a lot of smooth curve and when I wanted to work with these values, I just grabbed a ruler and measured them in steps of 5% sand and 5% clay. Of course one should realize that it does not give the true value as it is in your soil, but just an estimate; I have no idea what it is based on.
I wanted to send this file as attachment, but the DSSAT list server does not accept attachments, so I will send it directly to your email address in China. I put sand as the horizontal value and clay as the vertical. Both sand and clay go from 0 to 100%, which is the first row for sand and the first column for clay.
The equation that you mention for the BD doesn¡¯t seem correct to me. You have:
BD = 100 / (SOM% / 0.224 / (100 / SOM%) / mineral BD)
But is should be:
BD = 100 / (SOM% / 0.224 + (100 - SOM%) / mineral BD)
Thus after the 0.224 there should be a ¡®+¡¯ and not a ¡®/¡¯, and you have 100 / SOM% but this should be 100 ¨C SOM%. Yours is not correct because what is needed in the overall denominator (with 100 as numerator) is the SOM with its own bulk density and the non-SOM (or minerals) with its own bulk density (or mineral BD).
I used this method extensively for calculating the BD of all texture combinations that make up the texture triangle (5151 in total) when doing a method comparison on methods for estimating the water retention parameters wilting point (or lower limit, LL) and field capacity (or drained upper limit, DUL). The article that reslted from this is Gijsman A.J., Jagtap S.S., Jones J.W. 2002. Wading through a swamp of complete confusion: how to choose a method for estimating soil water retention parameters for crop models. European Journal of Agronomy, 18: 75-105.
If you are interested I can send you a PDF of the article, but it is close to 1 MB, so your email box should accept messages of this size.
Cheers,
Arjan Gijsman
-----Original Message-----
From: DSSAT - Crop Models and Applications [ mailto:[log in to unmask]]On Behalf Of xiongw
Sent: Tuesday, November 01, 2005 8:12 PM
To: [log in to unmask]
Subject: soil bulk density calculation
Dear Colleagues
We need to calculate the bulk density from the soil survey with sampling points across China, we used the function of Rawls and Brakensiek (1985, 1989) which used in DSSAT4, BD = 100 / (SOM% / 0.224 + (100 - SOM%) / mineral BD). While the mineral BD is not the available input value. the reference of Rawls and Brakensiek is quick old literature which is not available in China. Does anyone knows how to estimate the mineral BD from soil texture information, Or there are lastest references.
Thank you very much in advance.
Wei
Institute of Environment and Sustainable development for Agriculture
Chinese Academy of Agricultural Sciences
100081, P.R. China
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