Agriculture Reference
In-Depth Information
and soil solution from being reached, which stimulates further dissolution (Barker
et al.
1997,
Hinsinger 1998, Harley and Gilkes 2000, Wang
et al.
2000).
Research on plant uptake of nutrients in silicate minerals has focused on release of K and
several researchers have concluded that silicate minerals have potential as slow release fertilis-
ers (Gillman 1980, Coroneos
et al.
1996, Hinsinger
et al.
1996, Hildebrand and Schack-Kirchner
2000), but rocks and minerals high in silica (e.g. granite and feldspar) may be poor sources of
potassium (Blum
et al.
1989, Bakken
et al.
1997, Bakken
et al.
2000). When the dissolution of
ground granite in 20 acid soils from Western Australia was measured, few soils showed an
increase in exchangeable calcium (Ca) or magnesium (Mg) and nine soils showed an increase
in exchangeable potassium (K) (Hinsinger
et al.
1996). Incubation experiments and field and
pot trials have shown that between 1 and 10% of the K in feldspar is released up to 14 months
after application (Sans Scovino and Rowell 1988, Coroneos
et al.
1996, Hinsinger
et al
. 1996,)
and the relative effectiveness of granite as a potassium fertiliser was <0.14 compared to KCl
(Bolland and Baker 2000).
Silicate minerals may also provide plants with Ca, Mg and some micronutrients. Studies
have measured the release of Ca and Mg from silicate minerals and rocks such as amphibolite,
basalt, diabase, dunite, gneiss, granite, phenolite, serpentine, syenite and a volcanic ash (Chit-
tendon
et al
. 1964, Chittendon
et al
. 1967, Gillman 1980, von Fragstein
et al
. 1988, Blum
et al
.
1989, Gillman
et al
. 2001). Chittendon
et al.
(1967) showed that on an equal weight basis the
Mg content of tobacco, white clover and ryegrass plants increased in the order dolomite < ser-
pentine < dunite. Application rates between 1 and 50 t ha
-1
of basalt increased the exchangea-
ble K, Ca and Mg in seven highly weathered tropical soils in Queensland, Australia (Gillman
et
al.
2001). For five soils, 5 t ha
-1
of basalt increased exchangeable cations but on one soil, 1 t ha
-1
of basalt was sufficient to increase exchangeable Mg. Silicate minerals also contain plant
micronutrients but very little work has been done to assess their plant availability. Certain
silicate rocks in both water and 0.1 M hydrochloric acid (HCl) released iron and manganese to
a greater extent than copper and zinc (von Fragstein
et al.
1988).
In contrast to their use as nutrients, silicate minerals have been advocated as soil amelio-
rants (Harley and Gilkes 2000, Hildebrand and Schack-Kirchner 2000) especially for lateritic
soils in tropical climates (Leonardos
et al.
1987, Leonardos
et al.
2000, Gillman
et al.
2001).
Silicate minerals increase soil pH, although not as effectively as lime and to varying degrees on
different soils (Gillman 1980, von Mersi
et al.
1992, Hinsinger
et al.
1996, Hildebrand and
Schack-Kirchner 2000). Gillman
et al.
(2001) found that after nine months, granite applied to
a highly weathered soil at 300 t ha
-1
increased the soil cation exchange capacity from 9 to 14
meq/100 g of soil. Ground silicate minerals may also increase water-holding capacity (Kahnt
et
al.
1986). The highly weathered soils in tropical climates have most to gain from the use of
silicate minerals as soil ameliorants.
For organic farming systems that do not have access to adequate quantities of manure to
balance P lost from soil in harvested products, rock phosphate (and guano) may be used. The
use of phosphate rocks and factors affecting their relative effectiveness were reviewed by Kha-
sawneh and Doll (1978). Their relative effectiveness is affected by mineral properties (reactiv-
ity, particle size, surface area), soil factors (pH, titratable acidity, P and Ca availability and
retention, sand content, organic matter content, moisture and temperature) and plant factors
(P and Ca demand, root structure, rhizosphere pH) (Kanobo and Gilkes 1987, Kanobo and
Gilkes 1988, Hughes and Gilkes 1994, Hinsinger and Gilkes 1997). Although many of the
factors affecting rock phosphate dissolution are known, it is not easy to predict their relative
effectiveness. Sources of rock phosphate have unique reactivity, for which citrate solubility has
become a standard test. In an evaluation of the relative effectiveness of 14 rock phosphates, dif-
ferences in rock phosphate reactivity caused a ten-fold difference in dry matter yield between
