104 gneiss and K-feldspar SRFs greatly increased soil pH, whereas the application of K
as K
2
SO
4
did not affect soil pH. Soil pH increased by up to about 2.5 and 3.5 respectively for soils BSN-1 and SCP-11 receiving gneiss. The maximum increase
of soil pH due to the K-feldspar was about 1.5 for both soils. However, the efficiency of K-feldspar SRF as a liming material ratio of increase of soil
pHapplication rate expressed as unit mass of the SRF was 2 – 5 times higher than that of gneiss SRF. These results are consistent with the findings of the laboratory
experiments described earlier see Figure 4.3 in Chapter 4.
Soil EC
For experiment A with soil WP-6, the application of CaCl
2
increased the EC of soil by about 400 µScm, but the applications of basalt and dolerite decreased the
EC of soil WP-6. Most plants grown on soil WP-6 receiving CaCl
2
did not survive so that CaCl
2
added to this soil and other soluble nutrients from basal fertilizers added after each harvest remained in the soil. Consequently, the EC of soil at the
end of experiment for the CaCl
2
treatments increased greatly and was much higher than the soil EC for the Ca-SRF treatments. For soil MR-5, the application of Ca
fertilizers had no or little effect on the EC of the soil. The applications of Mg fertilizers had no effect on the EC of soils WP-6 and MR-5.
For experiment C using soil BSN-1, the application of K as K-feldspar SRF increased soil EC by about 375 µScm, but there was no effect for gneiss SRF and
K
2
SO
4
. The plants for the nil K treatment for soil SCP-11 did not survive after the 3
rd
harvest, so that the basal fertilizer salts added to the soil after the 3
rd
harvest remained in the soil causing a higher EC value i.e., about 120 µScm relative to
initial EC value i.e., 21 µScm, see Table 5.1 for soil SCP-11. In summary, the trends of EC values versus the application rate of K fertilizers for soil SCP-11 seem
to be similar to those for soil BSN-1 as described above.
5.3.6. Quantity of Nutrients Dissolved from SRF in the Soil
To provide estimates of dissolution of SRFs in the soil which reflects the benefits of milling in improving the effectiveness of SRFs, two main assumptions
were applied: 1 the amounts of plant-available nutrient initially in the soil for the control treatment were equal to the cumulative nutrient uptake H4 +
105 CH
3
COONH
4
-extractable nutrient in soil at the end of experiment and 2 there was no fixation of nutrients by soil from applied fertilizers during the experiment. The
calculation of the dissolved nutrients from SRFs is presented in Appendix B13, and the percentages of nutrient dissolved from SRFs are presented in Figure 5.13.
Figure 5.13. The percentages of dissolved Ca and K from SRFs in the soil for the glasshouse experiment of 12 months. Detailed calculations used to provide the
values in this figure are presented in Appendix B13.
As shown in Figure 5.13, the percentage dissolution of applied nutrients from SRFs decreased but the quantities of dissolved nutrients increased see
Appendix B13 with increasing application rate. This decrease in percentage of nutrients dissolved from SRFs may be due partly to the increase of soil pH liming
effect. An identical result has been observed for rock phosphate RP fertilizer Kanabo and Gilkes 1988 where the increase in pH and decrease in exchangeable
acidity due to initial dissolution of RP decreases the subsequent dissolution of RP. This negative feed back mechanism does not operate for potassium chloride and
other fertilizer salts that simply require water and not soil acidity to dissolve. A consequence of the reduction of percentage SRF dissolution as application rate of
Soil WP-6
5 10
15 20
25 30
35 40
333 666
1332
Rate of Ca mgkg as Basalt or Dolerite SRF
D is
s . C
a o
f to ta
l Basalt
Dolerite
20 40
60 80
100 120
140 160
225 450
900 450
900 1800
Rate of K mgkg as Gneiss SRF Rate of K mgkg as K-feldspar SRF D
is s
. K o
f to ta
l Soil BSN-1
Soil SCP-11
106 SRFs increases is that the relative agronomic effectiveness of SRFs e.g., relative to
KCl is rate dependent i.e., there is no constant substitution or relative effectiveness value.
The percentages of Ca dissolved for basalt SRF was about half of that for dolerite SRF, and the percentages of K dissolved for gneiss SRF was about 3 - 5 fold
higher than for K-feldspar SRF. These trends are consistent with results of laboratory experiments Chapters 3 and 4, and dissolution of SRFs in the soil-plant
system was larger than in soils alone see Chapter 4 or in a dilute organic acid see Chapter 3 presumably due to plant root-induced dissolution Hinsinger and Gilkes
1995, 1997; Hinsinger et al. 2001, and the mass action effect of plants removing dissolved elements Ca, Mg, K from solution thereby promoting additional
dissolution. Dissolution of K from gneiss SRF at an application rate of 225 mg Kkg was about 100 , and the additional soil K equivalent to about 47 of added K in
gneiss SRF was released as a consequence of increasing soil pH liming effect including organic matter mineralization and release of OM-K. In case of K-feldspar
SRF, the dissolution of K was much lower than for gneiss SRF. Dissolved K for gneiss SRF was probably originated mostly from biotite see Table 2.1 in Chapter
2. This more soluble K from biotite than feldspars microcline may be due to K release from biotite is by simple exchange of the interlayer cation and congruent
dissolution is not required Rick 1968; Feigenbaum et al. 1981; Bakken et al. 1997. Dissolution of K from K-feldspar SRF may be increased by applying a longer
milling time e.g., 90 min, see Figure 2.11 in Chapter 2. Clearly, high-energy milling greatly improved dissolution of nutrients from SRFs, thereby increasing the
agronomic effectiveness of most the SRFs used in this experiment.
5.4. Conclusions
