A comparison of soil and foliar-applied silicon on nutrient availability and plant growth and soil-applied silicon on phosphorus availability.
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2006
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Abstract
A greenhouse study was carried out to investigate the effectiveness of soil-applied
silicon (Si) with that of foliar applications for sorghum growth. Silicon sources were
soil-applied as calmasil (calcium silicate) at two rates (4 and 8t/ha) and foliar applied
Si including pure K-silicate, K-humate and K-fulvate (all three foliar treatments at
rates of 300 and 600 ppm). Another treatment included soil applied calmasil plus low
rate of foliar applied K-humate. The soils used for the greenhouse trial were Cartref,
Glenrosa, Nomanci and Fernwood. Results indicated that application of calcium
silicate to the soil before planting increased sorghum yield and Si uptake in three of
the four soils. Silicon uptake from different experimental treatments followed the
order: Calmasil 8t/ha > calmasil 4t/ha ~ calmasil + 300 ppm K-humate> K-humate =
K-fulvate = pure-K silicate = control. Foliar sprays were ineffective at increasing
yield, Si content of the plant tissues or Si uptake. The concentrations of
exchangeable Ca, Mg as well as soil pH were significantly increased by calmasil
treatments. Extractable AI concentrations were also reduced due to the Iiming effect
of calcium silicate and also possibly formation of insoluble aluminosilicates. The yield
response to applied calmasil seemed to be primarily related to its Iiming effect and
reductions in extractable AI in the Cartref, Glenrosa and Nomanci soils. The dry
matter yield was highest in Fernwood and lowest in Cartref soil. However, there was
no significant yield response to calmasil in Fernwood soil which had an initial pH of
5.8 and insignificant extractable AI concentrations. Therefore application of calcium
silicate had no significant effect on extractable AI concentration in this soil. Yield
response to calmasil may also have been partly due to direct positive effects of
applied Si on crop growth through mechanisms such as increased photosynthetic
rate and reduced transpiration rate, Addition of calmasil increased the concentrations
of Si in the plant tissues and reduced those of N, P and Kin Nomanci and Fernwood
soils respectively. This indicates that nutrient interactions were occurring in the plant. It was concluded that foliar-application is not an effective way of applying Si to a Siresponsive
crop such as sorghum when growing in soils low in extractable soil Si.
This is because Si is accumulated in plant tissues in similar amounts to
macronutrients. It was also concluded that in future, studies of crop response to
applied Si should include the use of non-Iiming source of Si (e.g. silicic acid) so as to
separate a liming effect of calcium silicate from effect of applied Si.
In a laboratory study, the effects of applied silicic acid, calcium silicate and calcium
hydroxide on levels of extractable P in two Si-deficient soils were investigated. Two
soils (Fernwood and Nomanci soils) were treated with two rates of P and three soil
amendments (calcium silicate, calcium hydroxide and silicic acid) and incubated for
six weeks at room temperature. Phosphorus was extracted using Truog, AMBIC and
resin methods, and levels of exchangeable and solution AI and extractable and
solution Si were also measured. Application of calcium silicate and calcium
hydroxide increased soil pH in both soils while silicic acid additions had no significant
effect compared with the control. The pH increase was much greater in the
Fernwood than Nomanci soil because of the low buffering capacity of the sandy
Fernwood soil. Exchangeable AI and concentrations of monomeric and total AI in soil
solution generally followed the order: control ~ silicic acid> calcium silicate> calcium
hydroxide. The lowering of soluble AI concentrations in the silicic acid treatments
was attributed to formation of insoluble aluminosilicate compounds while that in the
calcium silicate and calcium hydroxide treatments was attributed to their Iiming
effects causing a rise in pH.
Concentrations of Si in soil solution were lower in the calcium hydroxide than the
control treatment suggesting the solubility of Si decreased with increased pH.
Additions of both Si sources increased Si concentrations in solution and the effect
was more marked for the calcium silicate treatment. This was attributed to formation
of insoluble aluminosilicates in the silicic acid treatment. Concentrations of H2S04extractable
Si with treatment did not closely follow the same trends as those for Si
concentrations in soil solution. That is, levels of extractable Si were very much higher
in the calcium silicate than silicic acid treatment in both soils. In addition, concentrations of extractable Si in the calcium hydroxide treatment were similar to
control in the Nomanci soil, while for the Fernwood soil, concentrations in the
calcium hydroxide treatment were exceptionally high. It was suggested that liming
with calcium silicate or calcium hydroxide had rendered some Si-containing
compounds in the soil acid-extractable and that the nature of acid-extractable Si
fraction need further study in future.
The quantities of P extracted from the two soils by the various extractants followed
the order: Truog> AMBle> resin. The greatest increase in extractable P induced by
additions of P was recorded for Truog P and the least for resin P. The effects of
Iiming (addition of calcium silicate or calcium hydroxide) on extractable P levels
differed depending on the soil and extractant used with increase, decrease or no
effect being recorded. Such results confirm the complexity of lime and P interactions
which occur in acid soils. Additions of silicic acid had no effect on levels of
extractable P, compared to control. It was suggested that the reason for this was that
phosphate is adsorbed to AI and Fe oxide surfaces much more strongly than silicate.
As a result, additions of Si are ineffective at increasing extractable P levels.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2006.
Keywords
Soils--Silicon content., Soils--Phosphorus content., Soils--Analysis., Phosphorus in agriculture., Silicon in agriculture., Phosphatic fertilizers., Sorghum--Effect of minerals on., Theses--Soil science.