Extractable Silicon in Soils of the South African Sugar Industry and Relationships with Crop Uptake

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1 This article was downloaded by: [Neil Miles] On: 12 November 2014, At: 22:55 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Communications in Soil Science and Plant Analysis Publication details, including instructions for authors and subscription information: Extractable Silicon in Soils of the South African Sugar Industry and Relationships with Crop Uptake Neil Miles ab, Alan David Manson c, Ruth Rhodes a, Rianto van Antwerpen ad & Annett Weigel a a South African Sugarcane Research Institute, Mount Edgecombe, South Africa b School of Agricultural, Earth and Environmental Sciences, University of KwaZulu-Natal, Durban, South Africa c KwaZulu-Natal Department of Agriculture, Environmental Affairs and Rural Development, Pietermaritzburg, South Africa d Department of Soil, Crop, and Climate Sciences, University of the Free State, Bloemfontein, South Africa Accepted author version posted online: 29 Sep 2014.Published online: 10 Nov To cite this article: Neil Miles, Alan David Manson, Ruth Rhodes, Rianto van Antwerpen & Annett Weigel (2014): Extractable Silicon in Soils of the South African Sugar Industry and Relationships with Crop Uptake, Communications in Soil Science and Plant Analysis, DOI: / To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content.

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3 Communications in Soil Science and Plant Analysis, 00:1 10, 2014 Copyright Taylor & Francis Group, LLC ISSN: print / online DOI: / Extractable Silicon in Soils of the South African Sugar Industry and Relationships with Crop Uptake NEIL MILES, 1,2 ALAN DAVID MANSON, 3 RUTH RHODES, 1 RIANTO VAN ANTWERPEN, 1,4 AND ANNETT WEIGEL 1 1 South African Sugarcane Research Institute, Mount Edgecombe, South Africa 2 School of Agricultural, Earth and Environmental Sciences, University of KwaZulu-Natal, Durban, South Africa 3 KwaZulu-Natal Department of Agriculture, Environmental Affairs and Rural Development, Pietermaritzburg, South Africa 4 Department of Soil, Crop, and Climate Sciences, University of the Free State, Bloemfontein, South Africa Reports of sugarcane yield responses to silicon (Si), coupled with mounting evidence that elevated crop Si levels reduce both biotic and abiotic stresses, account for the interest in the Si nutrition of this crop. In terms of managing Si supplies to sugarcane in South Africa, uncertainties exist regarding, first, the reserves of plant-available Si in soils, and second, the reliability of soil-test methods for predicting Si availability. In this study, extractable Si was measured in 112 soils collected from sugarcane-producing fields in South Africa. Soils were selected on the basis of dominant soil types and included Inceptisols, Alfisols, Mollisols, Vertisols, Oxisols, Entisols, and Ultisols, varying widely in chemical properties, texture, and extent of weathering. Extractants employed were 0.01 M calcium chloride (CaCl 2 ) and 0.02 N sulfuric acid (H 2 SO 4 ). Silicon extracted with 0.02 N H 2 SO 4 ranged from 2 to 293 mg kg 1, whereas with 0.01 M CaCl 2 the range was 5 to 123 mg kg 1. With both extractants, extractable Si decreased significantly with decreasing ph, exchangeable calcium (Ca), and total cations. In soils with potassium chloride (KCl) extractable Al+H levels of greater than 0.5 cmol c L 1, extractable Si levels were consistently low, suggesting that soluble Al is implicated in reducing plant-available Si levels. Extractable Si levels were not related to the Bache and Williams P-sorption indices of soils. In the second part of the investigation, sugarcane leaf Si concentrations from 28 sites were related to soil extractable Si levels. The CaCl 2 soil test proved markedly superior to H 2 SO 4 as a predictive test forleafsilevels. Keywords Silicon, soil test calibration, soil tests, sugarcane Introduction Uptake of silicon (Si) varies greatly among plant species. Sugarcane and rice are Si accumulators, and under conditions of favorable Si supply, may absorb more Si than any other mineral nutrient (Snyder, Matichenkov, and Datnoff 2007). There are numerous reports of substantial yield responses following applications of Si to Si-accumulator crops when Received 26 June 2013; accepted 6 May Address correspondence to Neil Miles, South African Sugarcane Research Institute, Mount Edgecombe, South Africa. neil.miles@sugar.org.za 1

4 2 N. Miles et al. grown on Si-depleted soils (Fox et al. 1967b; Savant et al. 1999; Berthelson et al. 2003). Anderson (1991) notes that under field conditions, sugarcane requires a leaf concentration of at least 1% Si; at 0.25% Si the yield drops to about one half. Evidence that elevated plant Si levels reduce biotic and abiotic stresses has served to intensify interest in the Si nutrition of crops such as sugarcane. Beneficial effects of Si include improved leaf erectness at high levels of nitrogen (N) supply, reduced susceptibility to lodging, improved drought resistance, decreased susceptibility to attack by insects and diseases, and the prevention of manganese (Mn) toxicity (Marschner 1995; Savant et al. 1999). Of particular interest, in terms of sugarcane production in South Africa, is the evidence of favorable Si levels in sugarcane providing resistance to the devastating Eldana saccharina stalk borer (Keeping and Meyer 2006; Keeping, Meyer, and Sewpersad 2013). Leaf analyses of sugarcane in South Africa reflect wide variations in Si uptake (van der Laan and Miles 2010). In the eastern parts of the country, where most of the sugarcane is grown, considerable soil diversity is the major contributory factor in this regard. In the relatively high rainfall areas (>900 mm per annum), many soils have suffered severe desilication as a result of weathering, leaching, and crop removals, and leaf concentrations of Si in sugarcane growing on these soils are often less than 0.3%. In the drier areas, however, and in particular in the irrigated northern reaches of the country s sugar industry, soils are well supplied with Si and leaf concentrations are frequently in the range 1.0 to 2.5% (van der Laan and Miles 2010). The severe impact of Eldana saccharina on sugarcane grown in the greater rainfall areas of South Africa creates an urgent need to improve plant-available Si supplies on the leached and highly weathered soils in these areas. This, in turn, implies a requirement for a Si soil test that reliably reflects plant-available Si reserves in soils and is calibrated with crop uptake of Si. In essence, as noted by Savant et al. (1999), the challenge in terms of routine soil testing for Si is to develop a simple-to-use, yet dependable Si soil test. The investigations reported in this article had the objectives (a) of determining extractable Si levels in representative soils of the sugarcane-growing areas of South Africa, (b) of identifying soil properties that relate to extractable Si levels, and (c) of developing a Si soil test that reliably reflects the plant availability of this nutrient. Materials and Methods Soil and Leaf Sampling For the determination of extractable Si levels in soils, topsoil (0 to 200 mm) samples (n = 112) were collected from sugarcane fields in all production areas on the eastern seaboard of South Africa. This region is endowed with great diversity of geology, climate, and soils. Soils were selected on the basis of dominant soil types and included Inceptisols, Alfisols, Mollisols, Vertisols, Oxisols, Entisols, and Ultisols. Fields from which samples were taken had been under sugarcane production for periods ranging from a few decades to more than 150 years. Each sample consisted of four to six subsamples, which were thoroughly mixed, air dried, and sieved through a 1-mm sieve prior to analysis. Selected properties of these soils are listed in Table 1. For the purpose of soil-test calibrations, leaf and corresponding topsoil (0 to 200 mm) samples were collected from 28 sites located throughout the sugarcane-growing areas of South Africa. These soils had clay content ranging from 63 to 570 mg kg 1 and ph (calcium chloride; CaCl 2 ) from 3.9 to 6.5. Soil samples from each site consisted of 30 subsamples taken in a zig-zag pattern across the field; samples were thoroughly mixed, air dried, and sieved (< 1 mm) prior to analysis. Leaf samples comprised

5 Crop Silicon Nutrition 3 Table 1 Selected properties of the soils included in the investigation (n = 112) Soil property Mean Median Range ph (0.01 M CaCl 2 ) Organic C (g kg 1 ) Exchangeable Ca (mg L 1 ) Exchangeable Al+H (cmol c L 1 ) Total Exchangeable cations (cmol c L 1 ) Clay (g kg 1 ) Silt (g kg 1 ) Sand (g kg 1 ) Oxalate Fe (mg kg 1 ) Oxalate Al (mg kg 1 ) Oxalate Si (mg kg 1 ) Si (0.01 M CaCl 2 )(mgkg 1 ) Si (0.02 N H 2 SO 4 )(mgkg 1 ) to 40 of top visible dewlap leaves, taken in a zig-zag pattern across fields. The samples were from actively growing crops, between 4 and 7 months of age and well supplied with N. Samples were not restricted to a particular variety (varietal disposition is largely area specific). Immediately after collection, leaf blades were stripped from their midribs. The blades were dried at 75 C, milled, and pressed into disks for analysis. Analytical Procedures Extractable Si was measured in the soils using two extractants: 0.02 N sulfuric acid (H 2 SO 4 ;1:10soil/solution, 20 min shaking) and 0.01 M CaCl 2 (1:10 soil/solution, 16 h shaking). The extracts were centrifuged and the Si analyzed by colorimetry (660 nm) using the modified molybdenum blue method with oxalic acid to eliminate phosphorus (P) interference. Soil ph was measured in 0.01 M CaCl 2. Exchangeable aluminum (Al) + hydrogen (H) were measured by extraction with N potassium chloride (KCl; 10 min) and titration with 0.05 N sodium hydroxide (NaOH) to a phenolphthalein endpoint. Studies on South African soils have shown that a close linear relationship exists between exchangeable Al+H and exchangeable Al per se, and that Al constitutes the major component of the Al+H determination (Farina and Channon 1991). Acid ammonium oxalate extractable Al, iron (Fe), and Si were determined using the method of McKeague, Brydon, and Miles (1971). The Bache and Williams (1971) P-sorption index (PSI) (18 h equilibration with 75 mg P L 1 in 0.01 M CaCl 2 ) was used to obtain an index of P immobilization in soils. For the development of soil-test calibrations, leaves were analyzed for their Si content using x-ray fluorescence, which is the standard method for leaf Si analysis in the South African sugar industry. The validity of the Si calibration on the x-ray instrument is regularly checked by comparisons with results obtained by dry ashing and the determination of Si by colorimetry (Fox et al. 1967a). Corresponding soil samples were analyzed for Si by the procedures detailed previously. In addition, a modification of the 0.01 M CaCl 2 test was evaluated as a predictor of plant Si uptake; this involved the use of 0.01 M CaCl 2

6 4 N. Miles et al. at a 1:2.5 soil/solution ratio and with 30 min shaking. All soil and leaf analyses were performed in duplicate. Results and Discussion Extractable Silicon in Soils Extractable Si levels varied widely in the soils (Table 1). In the case of the 0.02 N H 2 SO 4 extraction, values ranged from 2 to 293 mg kg 1, while with 0.01 M CaCl 2 the range was 5 to 123 mg kg 1. In studies conducted in the Australian sugarcane-growing areas, 0.01 M CaCl 2 extractable Si levels of <10 mg kg 1 were classified as suboptimal in terms of crop requirements, with mg kg 1 being considered marginal (Chapman, Haysom, and Chardon 1981; Berthelson et al. 2001). Based on these criteria, 64 (57%) of the soils included in this study were suboptimal or marginal in Si. The relationship between Si extracted by the two methods is shown in Figure 1. In soils testing low in Si (less than about 25 mg kg 1 ), amounts of Si extracted were largely similar with the two extractants. However, at greater Si levels, H 2 SO 4 extracted considerably more than CaCl 2. In studies on Australian sugar-producing soils, Berthelson et al. (2003), too, reported that dilute sulfuric acid (0.005 M) generally extracted substantially more Si than 0.01 M CaCl 2. They suggested that the former extractant provided an indication of the capacity of soils to supply Si, while the CaCl 2 extraction reflected readily soluble Si. Highly significant correlation coefficients were found between CaCl 2 - and H 2 SO 4 - extractable Si on one hand and ph, exchangeable Ca, total exchangeable cations, clay, and oxalate-extractable Si on the other (Table 2). Relationships between ph (CaCl 2 ) and extractable Si are shown in Figure 2. These relationships, essentially similar for the two extractants, reflect an exponential increase in extractable Si with increasing ph. In soils with ph values of less than approximately Si in 0.01 M CaCl 2 (mg kg 1 ) y = 0.315x R 2 = Si in 0.02 N H 2 SO 4 (mg kg 1 ) Figure 1. Relationship between silicon extracted with 0.01 M CaCl 2 and 0.02 N H 2 SO 4 in 112 sugarcane-producing soils.

7 Table 2 Linear correlation coefficients for soil properties and silicon extracted with H2SO4 and CaCl2 in sugarcane topsoils (n = 112) Soil property Si (CaCl2) Si (H2SO4) ph (CaCl2) Clay OC Oxal-Al Oxal-Fe Oxal-Si Exch Ca Total cations Al+H PSI Si (CaCl2) 1.00 Si (H2SO4) ph (CaCl2) Clay OC Oxal-Al Oxal-Fe Oxal-Si Exch. Ca Total cations Al+H PSI Significantat1%level. Notes. OC, organic carbon; PSI, phosphorus sorption index. 5

8 6 N. Miles et al Si in 0.01 M CaCl 2 (mg kg 1 ) (a) y = e 0.647x R 2 = 0.56 Si in 0.02 N H 2 SO 4 (mg kg 1 ) (b) y = e x R 2 = ph(cacl 2 ) ph(cacl 2 ) Figure 2. Relationships between ph (CaCl 2 ) and Si extracted with (a) CaCl 2 and (b) H 2 SO , Si solubility is consistently at a minimum. This ph-si solubility relationship, together with the positive correlations among extractable Si, exchangeable Ca, and total cations, is a consequence of changes in the minerals controlling ph and silicon solubility as soils weather (van Breemen and Wielemaker 1974; Friesen et al. 1994; Henriet et al. 2008). Less-weathered high base status soils have clay mineral assemblages dominated by Si-rich minerals such as feldspars, vermiculites, and smectites, which support relatively high levels of Si in solution. In lower ph soils, on the other hand, weathering processes have resulted in clay fractions dominated by low-si minerals such as kaolinites and sesquioxides. Sumner, Fey, and Noble (1991), in commenting on the role of weathering in the desilication process, noted that levels of Si in the solution of acid soils may commonly be an order or more of magnitude less than in the solution of less weathered soils. It is perhaps worth noting that deviations from the trends reflected in Figure 2 may occur when Si-depleted, highly weathered soils are treated with Si-containing products that generate alkalinity, such as limes and slags (Haynes, Belyaeva, and Kingston 2013). This is attributable to the fact that silicate adsorption on the mineral assemblages of highly weathered soils, in particular sesquioxides, increases with increasing ph up to about ph 9.8 (McKeague and Kline 1963; Hingston, Posner, and Quirk 1972), whereas coprecipitation of Si with Al is suggested as an additional contributory factor (Sumner, Fey, and Noble 1991). The relationship between Si extracted with 0.01 M CaCl 2 and exchangeable Al+H (Figure 3) indicates that high levels (>40 mg kg 1 ) of soluble Si occur only where exchangeable Al+H levels are below approximately 0.5 cmol c L 1. Again, this is a consequence of changes in the minerals controlling ph (and hence Al solubility) and Si solubility as soils weather (van Breemen and Wielemaker 1974), and is also consistent with reports of a significant role of soluble Al in controlling Si solubility (Sumner, Fey, and Noble 1991; Hodson and Evans 1995; Lietal.1996; Liang et al. 2007). It is noteworthy that in this study there was no relationship between soil-test Si levels and oxalate Al. The strong correlation (r = 0.91) between oxalate Al and organic carbon (C) (Table 2) suggests that the bulk of the oxalate Al is bound to organic matter, and this fraction of Al, therefore, would not seem to be implicated in controlling Si solubility. In support of this contention is the fact that highly weathered sandy soils have low organic matter, yet also have low Si solubility. It is noteworthy that extractable Si was not correlated with the PSI of the soils included here. In studies on soils of the tropics, Fox and Searle (1978) noted that immobilization of P was related to the degree that soils had been Si depleted, and they found a general inverse

9 Crop Silicon Nutrition Si in CaCl 2 (mg kg 1 ) Exchangeable Al+H (cmol c L 1 ) Figure 3. Relationship between exchangeable Al+H and silicon extracted with 0.01 M CaCl 2 in 112 sugarcane-producing soils. relationship between Si solubility and P adsorption. This may be true for soils of similar texture, but sandy, highly weathered soils have both low Si solubility and low PSI. Leaf Silicon and Relationships of Extractable Soil Silicon Leaf Si concentrations from the 28 sites sampled for the purposes of this study ranged from 0.10% to 2.13%, with a median of 0.42%. There is considerable debate regarding the level of Si desirable for optimal sugarcane production. Anderson (1991) reported that maximum cane and sugar yields were observed at leaf Si concentrations exceeding 1%, and that at 0.25% Si the yield was reduced by 50%. Reuter and Robinson (1997) report a critical leaf value of 0.70%. The leaf threshold currently used for advisory purposes in South Africa is 0.75%. Relationships between soil extractable Si and leaf Si are presented in Figure 4. Leaf Si levels were more closely related to 0.01 M CaCl 2 extractable Si (R 2 = 0.770) than to 0.02 N H 2 SO 4 extractable Si (R 2 = 0.478). This finding is consistent with Australian work which showed that dilute H 2 SO 4 (0.005 N) was markedly inferior to 0.01 M CaCl 2 as a soil test for plant-available Si (Haysom and Chapman 1975; Berthelson et al. 2001). The 0.01 M CaCl 2 Si test was also reported to reliably predict variation in plant Si availability on volcanic soils differing widely in weathering stage in Guadeloupe (Henriet et al. 2008). A disadvantage of the dilute CaCl 2 extraction method in terms of its use in a routine laboratory is the need for overnight shaking. For this reason, 0.01 M CaCl 2 (1:2.5 soil/solution, 30 min shaking) was included for evaluation here, because this is the methodology that is widely used for soil ph determination; its suitability as a Si test could therefore present opportunities for labor savings in a high-throughput laboratory. With an R 2 of for the plot of leaf Si against extractable soil Si (not shown), this test was inferior to the 16-h extraction with 0.01 M CaCl 2, yet markedly superior to dilute H 2 SO 4. This suggests that there is potential for the adaption of the ph extraction procedure to accommodate the requirements of a Si soil test.

10 8 N. Miles et al. (a) Leaf Si (%) y = x R 2 = (b) Leaf Si (%) Si in 0.01 M CaCI 2 (mg kg 1 ) y = x R 2 = Si in 0.02 N H 2 SO 4 (mg kg 1 ) Figure 4. Relationship between sugarcane leaf silicon concentrations and soil silicon extracted with (a) 0.01 M CaCl 2 and (b) 0.02 N H 2 SO 4. Conclusions Data presented here highlight massive differences in plant-available Si reserves in soils of the South African sugar industry. Close relationships between extractable Si levels and soil chemical factors, in particular ph and total exchangeable cations, point to the importance of weathering in regulating Si supply for crop uptake. In low-ph, highly weathered soils where Si reserves are mostly suboptimal, there is an indication that soluble Al is implicated in depressing Si availability. With a large proportion of the sugarcane production in South Africa being on soils containing suboptimal levels of plant-available Si, it is likely that deficiencies of this nutrient pose a yield limitation and also render the crop more susceptible to biotic and

11 Crop Silicon Nutrition 9 abiotic stresses. In this regard there is an urgent need for the identification of cost-effective ameliorative practices to address deficiencies of this nutrient. Indications are that dilute CaCl 2 is a useful extractant for the measurement of plantavailable Si reserves in the soil. Information reported in this article suggests that overnight extraction with CaCl 2 may not be necessary, and that with shorter extraction times this test may be adapted for use in a high-sample-throughput routine laboratory. Acknowledgments The authors express their appreciation to Gayshree Naidoo and Kerisha Raghunandan for their analytical work and to Sharon McFarlane and Dr Riekert van Heerden for providing leaf samples from their field trials. References Anderson, D. L Soil and leaf nutrient interactions following application of calcium silicate slag to sugarcane. Fertilizer Research 30:9 18. Bache, B. W., and E. G. Williams A phosphate sorption index for soils. Journal of Soil Science 22: Berthelson, S., A. Hurney, A. D. Noble, A. Rudd, A. L. Garside, and A. Henderson An assessment of current silicon status of sugar cane production soils from Tully to Mossman. Proceedings of the Australian Society of Sugar Cane Technologists 23: Berthelson, S., A. D. Noble, G. Kingston, A. Hurney, A. Rudd, and A. Garside Improving yield and ccs in sugarcane through the application of silicon-based amendments. In Final report SRDC project CLW009. Canberra, Australia: Commonwealth Scientific and Industrial Research Organisation (CSIRO) Land and Water. Chapman, L. S., B. C. Haysom, and C. W. Chardon Checking the fertility of Queensland s sugarland. Proceedings of the Australian Society of Sugar Cane Technologists 3: Farina, M. P. W., and P. Channon A field comparison of lime requirement indices for maize. Plant and Soil 134: Fox, R. L., J. A. Silva, D. L. Plucknett, and D. Y. Teranishi. 1967a. Soluble and total silicon in sugarcane. Plant and Soil 30: Fox, R. L., J. A. Silva, O. R. Younge, D. L. Plucknett, and G. D. Sherman. 1967b. Soil and plant silicon and silicate response by sugar cane. Soil Science Society of America Proceedings 31: Fox, R. L., and G. E. Searle Phosphate adsorption by soils of the tropics. In Diversity of soils of the tropics, ed. M. Drosdoff, Madison, Wisc.: American Society of Agronomy. Friesen, D. K., J. I. Sanz, F. J. Correa, M. D. Winslow, K. Okada, L. E. Datnoff, and G. H. Snyder Silicon deficiency of upland rice on highly weathered Savanna soils in Colombia, I: Evidence of a major yield constraint. In IX Conferência Internacional de arroz para a América Latina e para o Caribe. V. Reunião Nacional de Pesquisa de Arroz. Goiânia, Goías, Brazil: Haynes, R. J., O. N. Belyaeva, and G. Kingston Evaluation of industrial wastes as sources of fertilizer silicon using chemical extractions and plant uptake. Journal of Plant Nutrition and Soil Science 176: Haysom, M. B. C., and L. S. Chapman Some aspects of calcium silicate trials at Mackay. Proceedings of the Australian Society of Sugar Cane Technologists 42: Henriet, C., L. Bodarwe, M. Dorel, X. Draye, and B. Delvaux Leaf silicon content in banana (Musa spp.) reveals the weathering stage of volcanic ash soils in Guadeloupe. Plant and Soil 313: Hingston, F. J., A. M. Posner, and J. P. Quirk Anion adsorption by goethite and gibbsite, I: The role of the proton in determining adsorption envelopes. Journal of Soil Science 23:

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