Clay minerals as a soil potassium reservoir: observation and quantification through X-ray diffraction

Abstract

Potassium (K) is a major element for plant growth. The K + ions fixed in soil 2:1 clay mineral interlayers contribute to plant K nutrition. Such clay minerals are most often the majority in temperate soils. Field and laboratory observations based on Xray diffraction techniques suggest that 2:1 clay minerals behave as a K reservoir. The present work investigated this idea through data from a replicated long term fertilization experiment which allowed one to address the following questions: (1) Do fertilization treatments induce some modifications (as seen from X-ray diffraction measurements) on soil 2:1 clay mineralogy? (2) Are soil 2:1 clay mineral modifications related to soil K budget in the different plots?(3) Do fertilizer treatments modify clay Al, Si, Mg, Fe or K elemental content? (4) Are clay mineral modifications related to clay K content modifications? (5) Are clay mineral changes related to clay Al, Si, Mg or Fe content as well as those of K content? Our results showed that K fertilization treatments considered in the context of soil K budget are very significantly related to 2:1 soil clay mineralogy and clay K content. The 2:1 clay mineral modifications observed through X-ray measurements were quantitatively correlated with chemically analyzed clay K content. Clay K content modifications are independent from clay Al, Si, Mg or Fe contents. These results show that the soil chemical environment can modify interlayer site occupations (illite content) which suggests that high level accumulation of potassium can occur without any modification of the clay sheet structure. This study therefore validates the view of 2:1 clay minerals as a K reservoir easily quantifiable through X-ray observations.

Long-term effect of low potassium fertilization on its soil fractions

“…Soils of K fix under this treshold tend to have decreasing K avail even at positive K budget (Vopěnka and Macháček 1985). We found that the content of K fix is connected with MLP content, which is known not only to release K to plants but also to fix K from fertilizer (Barré et al 2008). Lower K fix therefore indicates lower capability of soil to retain K. This can contribute to fertilizer K leaching together with highly water-permeable soil at Svitavy and increased precipitation at Chrastava.…”

“…Knowledge about effects of non-fertilization on soil K fractions or soil K bearing minerals was usually gathered from one-site long-term experiments (Blake et al 1999, Scherer et al 2003, Barré et al 2008, Lukin 2012, Hejcman et al 2013. On the other hand, many studies relating K reserves to soils of different mineralogy (Andrist-Rangel et al 2010, Sarkar et al 2013 do not include information about long-term K budget.…”

Available Potassium Evaluation of Gharb El-Mawhoob Soils,

“…In addition, silt fraction may rapidly release a part of the non-exchangeable K and becomes available for plant nutrition. However,K that is present in the clay fraction is adsorbed with medium force energy in the vermiculite and smectite interlayers and may serve as a medium-term K source to plants as long as the available contents available are not excessively high (Barré et al, 2008a & b).…”

Soil Development under Continuous Agriculture at the Morrow Plots Experimental Fields from X-ray Diffraction Profile Modelling

“…Specifically, potassium (K) incorporation or release from 2:1 clay interlayers may lead to the illitization of vermiculite or smectite layers and to the vermiculitization of mica/illite, respectively. For example, an increase in the proportion of illite layers has been reported in temperate and tropical soils following addition of K fertilizers [4,[8][9][10]. By contrast, 126 years of N and K fertilization and liming have only led to marginal modifications of clay mineralogy in the soil under permanent grassland [11].…”

“…This approach is intrinsically limited to sharp, well-defined reflections and does not allow extracting a comprehensive information on (i) extremely small or crypto-crystalline contributions and (ii) complex interstratified contributions owing to their weakly modulated and faint diffraction signatures [28]. Over the last two decades, decomposition of XRD patterns has been combined with simulations of XRD patterns [29,30] to enhance and rationalize the description of soil clay mineralogy and of its evolution as a result of agricultural practices [9,10,15,[31][32][33][34]. Despite significant improvements, prompted in particular by the identification of interstratified contributions, this combined approach remains limited as it does not allow a complete quantitative characterization of interstratified clays that may prevail in soils [21][22][23][24][25]35].…”