69 exchangeable of this each element already present in the soils Table 4.1 and this
small increase might not have a substantial agronomic effect. The present research may, however, underestimate dissolution under field conditions as rhizosphere
dissolution of silicates is greater than in soil alone due to chemical modification of the rhizosphere by plant roots Hinsinger and Gilkes 1995, 1997; Hinsinger et al.
2001 and the activities of microorganisms in rhizosphere soil Liermann et al. 2000; Dong et al. 2000; Un Lee and Fein 2000. Furthermore, highly acid soils in
the tropics and elsewhere commonly have extremely low values of base saturation and for these soils the amounts of basic cations released by dissolution of SRFs will
contribute substantially to the stock of plant available basic cations.
4.3.2.2. Soil pH and EC
Adding silicate rock powders increased pH and EC of soil-SRF mixtures, and these increases were increased by prolonged milling. The increases of soil pH
and EC respectively were calculated from the value for treated soil minus the value for the control soil. The mean increases of pH and EC after incubation for 2 and 10
months as functions of milling time are presented in Figures 4.3 and 4.4.
Figure 4.3. Mean increase of pH pH
treated soil
– pH
control soil
of 10 soils receiving SRF milled for 0 initial, 10, 60, and 120 min and incubated for 2 and 10 months 2M
and 10M. Sd is the standard deviation of mean. The mean increases of pH the positive liming effect after 2 and 10 month
incubations for soil-basalt SRF and soil-dolerite SRF mixtures are about 0.2 units T0 to 0.6 units T120, 0.2 to 0.5 units for soil-gneiss SRF, and 0.3 to 0.8 units for
soil-feldspar SRF. Thus all SRFs had a positive liming effect that would be
0.0 0.3
0.6 0.9
2M 10M
2M 10M
2M 10M
2M 10M
Basalt Dolerite
Gneiss K-feldspar
In c
rea s
e of pH
uni ts
Initial 10 min
60 min 120 min
Sd
70
Figure 4.4. Mean increase of EC EC
treated soil
– EC
control soil
of 10 soils receiving SRFs milled for 0 initial, 10, 60, and 120 min and incubated for 2 and 10 months
2M and 10M. Sd is the standard deviation of mean. beneficial to some plants on these acid soils. Basalt and dolerite SRFs increased the
EC of soil-SRF mixtures incubated for 2 months by about 5 µScm for T0 to about 20 µScm for T120 and these values are about doubled for soil-SRF mixtures
incubated for 10 months. Gneiss SRF had a small effect on EC, i.e., increased by about 5 µScm for T0 to about 15 µScm for T120 for both 2- and 10-month
incubations. The effect on EC of adding K-feldspar SRF was small for soil incubated for 2 months, i.e., increased by 4 - 10 µScm, but these values increased
about 3 fold for 10 month incubations. Adding SRFs increases soil salinity but in every case these increases in EC salinity were negligible from an agricultural
perspective Hunt and Gilkes 1992. Thus use of SRFs at rate of about 10 tha equivalent to about 1 in top soil may provide a significant liming benefit and
will not adversely affect soil salinity. Positive liming effects increasing pH due to amendment of acid soils with SRFs have been reported by several workers
Coroneos et al. 1996; Hinsinger et al. 1996; Gillman 1980; Gillman et al. 2002; Coventry et al. 2001; von Wilpert and Lukes 2003 with no effect on salinity being
observed for soils receiving 20 tha of granite dust Hinsinger et al. 1996. It is thought that the increases of pH and EC for soils with added SRFs are
essentially due to consumption of hydrogen ions during dissolution of the SRFs and the concomitant release of basic cations from the powders. Consequently plots of
means of the increases of pH and EC versus the increase of EAC CH
3
COONH
4
- extractable basic cations Figure 4.5 show that close relationships exist. The
10 20
30 40
50
2M 10M
2M 10M
2M 10M
2M 10M
Basalt Dolerite
Gneiss K-feldspar
In c
rea s
e of
E C
µ S
c m
Initial 10 min
60 min 120 min
Sd
71 relationships are quite different for each SRF as the SRFs have different capability
in increasing soil pH of soil-SRF mixtures and released different amounts of each element during dissolution.
Figure 4.5. The relationships between mean values of the increases of pH and EC and the mean increase of exchangeable base cations EAC for 10 soils mixed with
basalt
■
, dolerite
□
, gneiss
▲
, and K-feldspar
∆
SRFs milled for 0 initial, 10, 60, and 120 min under dry condition.
4.3.3. Dissolution in Dilute Acetic-Citric Acid versus Dissolution in the Soil
