Substitution of Clay Content for P-Remaining as an Index of the Phosphorus Buffering Capacity for Soils of Rio Grande do Sul

Abstract

Mehlich-1 soil extract is currently used in Rio Grande do Sul (RS) and Santa Catarina (SC) States, in Brazil, to evaluate soil phosphorus available to plants and Mehlich-1 is sensitive to soil buffering capacity. This limitation is overcome partly by soil separation into buffering classes based on soil clay content. However, soil clay content has some technical and operational limitations, such as the underestimation of soil clay levels in soils under no-till. The aim of this study was to evaluate the remaining phosphorus (P-rem) compared to soil clay content as an index of the buffering capacity in soils of RS. The relationship between P-rem and soil clay content was assessed using 200 soil samples from a wide range of locations across the state. In 20 representative soil types, P-rem, soil clay content, P maximum adsorption capacity, total and pedogenic Fe contents, amount of P 2 O 5 needed to increase soil P in 1 mg kg -1 extracted by Mehlich-1 and -3 methods and P capacity factor were determined. Furthermore, a pot experiment was carried out with plants to determine soil P content extracted by Mehlich-1 and -3 at the maximum technical efficiency (MET) and the maximum absorption efficiency (MEA). Correlation analyses were performed between P-rem and soil clay content and, with the attributes of soils involving the P buffering capacity. The relationship between P-rem and soil clay content was described as a decreasing exponential function; however, the adjustment was not adequate to propose critical levels for P-rem. The P-rem index was superior to soil clay index in practically all relationships with the attributes related to the soil buffering capacity of the soils. In some cases, the indexes were similar; however, P-rem was not lower than the soil clay content in any case. The P-rem is a more reliable index to represent soil buffering capacity than the soil clay content in soils from RS, and P-rem might be used to implement soil classes for Mehlich-1 and Mehlich-3 determinations of soil P.

Soil phosphorus availability, dynamic and speciation in long-term high fertilized soils

“…In this sense, the M3 has advantages because it is a multi-element soil test, and therefore reduces the analysis costs, besides facilitating the analysis in routine laboratories (Milagres et al, 2007;. However, extensive work on field calibration of M3 with the crops of interest is required, as already done on a small scale for southern Brazil Rogeri et al, 2016). Linear regressions were established to assess the degree of association between the P content extracted by the soil tests in all treatments and soils ( Figure 3).…”

“…Positive correlation coefficients with P-rem are due to the fact that in more buffered, the P-rem is lower . The higher correlation coefficients obtained between Prr-M3 and P-rem and MPAC than with soil clay content is because P-rem and MPAC are two measures which, besides reflecting soil clay content, also indicate, indirectly, the quality of the clay, if it has more or less phosphate adsorption sites (Alvarez and Fonseca, 1990;Rogeri et al, 2016). Since P-rem and clay content are routinely performed in the laboratories of soil analysis, equations that relate Prr-M3 with these two attributes were established (Figure 4).…”

“…Mehlich-1 is sensitive to soil P buffer capacity (PBC), and therefore the interpretation of its results is dependent on a measure of this factor, usually the soil clay content or the remaining P (P-rem). Mehlich-1 extracts, preferably the forms of P-Ca Rogeri et al, 2016), overestimating, in some cases, the availability of P in soils recently fertilized with phosphate rocks . Thus, there may be an overestimation of P content determined by this soil test in soils cultivated with high doses of limestone, and with different sources of phosphate fertilizers.…”

“…However, due to decades of cultivation, with applications of phosphate fertilizers, liming and various management practices, the soil chemical characteristics are no longer the same as when these soil tests were implemented . Recent correlation studies show that the Mehlich-3 (M3) extractant has good results for P extraction in Brazilian soils, with the main advantage of being a multi-element extractant Rogeri et al, 2016;Simões Neto et al, 2009;.…”

“…However, few studies address the calibration phase, with the definitions of P recovery rates (Prr) and critical P levels (Pcl) determined by these extractants in soils with high P availability. Most of the research in this regard was carried out in the southern region of the country Rogeri et al, 2016), requiring further research involving soils from other regions.…”