Technical note
Alternative soil particle-size analysis by gamma-ray attenuation
E.A. Elias
a,*
, O.O.S. Bacchi
a
, K. Reichardt
a,b
a
Laborato
Â
rio de Fõ
Â
sica do Solo, CENA, USP, CP 96, CEP 13400-970, Piracicaba, SP, Brazil
b
Departamento de Ciencias Exatas, USP, CP 9, CEP 13418-970, Piracicaba, SP, Brazil
Received 12 April 1999; received in revised form 3 August 1999; accepted 3 August 1999
Abstract
In 1992 a new method of soil particle-size analysis was introduced, which coupled sedimentation to gamma-ray attenuation.
The theoretical possibility of an alternative for this method is here presented, which includes the necessity of sieving soil
samples, but simpli®es the gamma-ray measurements. Advantages and disadvantages of this alternative are discussed. # 1999
Elsevier Science B.V. All rights reserved.
Keywords: Granulometry; Soil mechanical analysis; Spectrophotometry
1. Introduction
Vaz et al. (1992) developed a method of soil par-
ticle-size analysis by gamma-ray attenuation which,
like the pipette method, is based on particle sedimen-
tation. It differs, however, from the pipette method for
two reasons: (r1) the pipette is replaced by the gamma-
ray beam; and (r2) sieving does not precede sedimen-
tation. This can be clearly seen in their statement:
``The pipette method estimates sand content indir-
ectly, using a sieve (no. 270) of 0.053 mm diameter
openings. In the new method all fractions are mea-
sured during sedimentation (FFF)'' (Vaz et al., 1992).
The advantages of Vaz' method in relation to the
pipette method are threefold: (a1) no perturbation
of the sedimentation process; (a2) continuous analysis
of particle-sizes and (a3) direct observation of all
particle fractions. The theoretical possibility of a
``Vaz' method alternative'' is here presented, in which
sieving of coarse particles does precede sedimenta-
tion, so that the Vaz' and pipette method would differ
due to reason (r1) only. This alternative would miss
advantage (a3). However, there are, at least, two
additional advantages, shown below.
2. Theoretical
The equation used by Vaz et al. (1992) to estimate
particle concentration C at a time t is
C lnI
0
aIX"
p
À"
w
aD
p
Y (1)
and the relative particle concentration = C
H
= C/C
0
is
C
H
lnI
0
aIXC
0
"
p
À"
w
aD
p
Y (2)
where I
0
and I are the count intensities of the gamma-
ray beams after passing through the absorbing med-
Soil & Tillage Research 52 (1999) 121±123
*
Corresponding author. Tel/fax: 00-55-19-4340021
E-mail address: elielias@cena.usp.br (E.A. Elias)
0167-1987/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0167-1987(99)00070-7