A comparison of soil and foliar-applied silicon on nutrient availability and plant growth and soil-applied silicon on phosphorus availability.
Matlou, Mmakgabo Cordelia.
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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.