Kinetics of soil potassium sorption-desorption and fixation
SCHNEIDER A.1, TESILEANU R.2, CHARLES R. 2, SINAJ S.2
1INRA, UMR 1220, TCEM, B.P. 81, 33883 Villenave d'Ornon, France
2Station de Recherche Agroscope Changins-Wädenswil (ACW), CP 1012, CH-1260 Nyon, Suisse
Introduction
The contribution of non-exchangeable K in plant nutrition is significant, particularly when exchangeable K content is low.
Release–fixation of K is a process that depends on time and concentrations of K and Ca (Mg) in the soil solution.
The soil ability for K release and fixation (
b
) is a parameter characterizing the soil release and fixation capacity deduced from sorption–desorption experiments performed at the Ca concentration of the soil solution, but its estimation is laborious.Fixation capacity (
FC
), which measures the fixed fraction of a K supply, is more easily estimated. Both parameters,FC
andb
, conceptually characterize the same process, i.e. fixation and fixation–release, respectively and should thus be equal.Objectives of this study were to asses : (i) the influence of time on soil K dynamics (
b
andFC)
, and (ii) ifFC
could be used as a surrogate ofb
.Material and methods
▪ Two soil samples from a long-term fertilizer experiment (Swiss Research Station Agroscope-ACW), corresponding to two K-treatments K0 and K+ with exchangeable-K: 4.2 and 10.9 mmol kg-1, respectively.
▪ The sorption–desorption experiments were performed at the Ca (Mg) concentration measured on the soil solution extract of both soils. Three periods of contact were tested: 2 h, 1, and 8 d. For each tested time, exchangeable-K was extracted (neutral M AcONH4) to enable the estimation of the change in mobile non-exchangeable K (D
N
) during the experiments (Schneider, 1997a,b). From the relationship between DN
and the initial constraint (f
) the ability for K release and fixation (β
) was deduced by linear regression: DN
=βf
, withf
=v
/m
(c
i –c
0).▪ K fixation was studied in a factorial design: K supply (
S
K: 0, 2.5, 5 mmol kg-1) and duration of contact (2 h, 1 d, 8 d). At the end of the contact the exchangeable-K content was extracted by NH4OAc. The amount of K not extracted by NH4OAc was assumed to be fixed. Fixed-K (DN
t) was calculated as DN
t =S
K +Kex
t0 -Kex
tS, whereKex
tS was the exchangeable-K content at timet
of the treatment receiving the non-nilS
K dose, andKex
t0 was the average (three replicates) of the exchangeable K content at timet
of the control treatment (S
K = 0). Fixation capacity (FC
) was calculated:FC
= DN
t/S
K.Conclusions
▪ K sorption-desorption increased slightly with time, particularly for the low K status soil. However, the influence of time was almost negligible after 1 day of contact.
▪
FC
was also time-independent in case of the high K status soil (K+), but increased slightly in case of the low K status soil (K0).▪
β
was well correlated withFC
. However,FC
systematically overestimatedβ
by a constant non-nil value. Thus, fixation capacity did not appear as a perfect surrogate of the soil ability for K release and fixation.
References
Schneider A. 1997a. Eur. J. Soil Sci.. 48, 263-271.
Schneider A. 1997b. Eur. J. Soil Sci., 48, 499-512.
-10 -5 0 5 10 15 20
0.0 0.2 0.4 0.6 0.8 1.0
cf (mM) DK (mmol kg-1 )
-3 -2 -1 0 1 2 3 4 5 6
-100 -80 -60 -40 -20 0 20
f (mmol kg-1) DN (mmol kg-1 )
Fig.1. Sorption–desorption curves
Soil K0 (open symbols) and soil K+ (closed symbols), after 2 h (triangles), 1 d (circles), and 8 d contact (squares).
Fig. 2. DN as function of the initial constraint (f) b = the slope of the linear regression DN = f(f)
12th International Symposium on Soil and Plant Analysis, June 6-10, 2011, Mediterranean Agronomic Institute of Chania, Crete, Greece
0.0 0.1 0.2 0.3 0.4
1 10 100 1000
t (h)
FC
0.0 0.1 0.2 0.3 0.4
0.0 0.1 0.2 0.3
b FC, FC-c0
Soil t c 0 b 0
h mM L kg-1
2 0.033 54 0.166 b 0.295 b
K0 24 0.018 68 0.203 ab 0.310 b
192 0.021 79 0.247 a 0.390 a
2 0.265 24 0.044 c 0.174 c
K+ 24 0.243 24 0.069 c 0.242 bc
192 0.250 25 0.058 c 0.170 c
b FC
Fig. 4. FC and FC-c0 as a function of β FC-c0 (closed symbols) is the fixation capacity calculated for the supply that could be really fixed (SK-0.5c0)
Fig. 3. FC as a function of time
Soil K0 (rhombuses) and soil K+ (squares), SK=2.5 and 5 mmol kg-1 (open and closed symbols). The lines correspond to the fitting a reversible first- order model.
Results
▪ The characteristics of the sorption-desorption curves and fixation capacity reflected the K status of both studied soils (Fig. 1, Tab. 1).
▪ The influence of time on K sorption-desorption was limited.
β
and
FC
increased slightly with time only in case of K0-soil (Figs 2 and 3).▪ A reversible first-order model fitted relatively well the fixation data (Fig. 3), but a parabolic model was more adequate (resuts not shown).
▪
FC
systematically overestimatedβ
by a constant non-nil value (Fig. 4). This could not be explained by the fact that all the K supply was not entirely available for fixation.Tab. 1. Characteristics of K dynamics deduced from sorption-desorption and fixation experiments
