Resolving compactness index of pores and solid phase elements in sandy and silt loamy soils

Soil structure is expressed by the size, shape, and arrangement of structural elements. Shape indices of pores and solid phase elements along with the physical soil parameters allow for thorough evaluation of soil structure. Therefore, the aim of the research was the analysis of properties of an index of shape – a compactness index CMP=(16area)/perimeter² – of pore (“pore c-s”) and solid phase element cross-sections (“solid c-s”) ≥100pix² (0.045mm²) of 4 soils. CMP was calculated via image analysis of resin-impregnated soil blocks prepared from intact soil specimens. The morphometric parameters of the objects assigned to selected CMP classes (≤0.2, the lowest compactness; 0.201–0.4; 0.401–0.6; 0.601–0.8; 0.801–1; 1.001–1.2; >1.2, the highest compactness) were compared via 2-way ANOVA for two horizons (A, C) and two textures (sand, silt loam). The usability of CMP in the description of soil structure was then tested. Moreover, the relations of the morphometric parameters of the objects in CMP classes and soil physical and chemical properties (total organic carbon TOC; bulk and particle density; texture; field water capacity FWC; field air capacity FAC; available water capacity AWC; air permeability at −15kPa lgFAP; saturated hydraulic conductivity lgKS) were examined by way of single and multiple linear regressions. For pore and solid c-s of CMP>0.2 their number, area, and average areas in CMP classes decreased with increasing CMP value. The distributions of pore and solid c-s among CMP classes depended on soil texture and structure (aggregate, non-aggregate), allowing for the diagnosis of soil structure status and change. Number and area of objects in CMP classes showed numerous strong relations (R²>0.7) to the soil physical and chemical parameters for the studied soil textures and horizons. The relations differed for pore and solid c-s and depended also on the object shape (spread, compact or very compact). The average areas of the compact and very compact pore and solid c-s increased with the increase of clay and silt content and the decrease of sand content. The number of pore c-s of CMP>0.2 was related to the texture or particle density. On the other hand, the number of solid c-s of CMP>0.2, and the average area of the most spread solid c-s were related to TOC and bulk density. FWC and AWC increased with the decrease of the number of mainly compact and very compact pore c-s by the decrease of the average area of the most spread solid c-s. Both water capacities increased with the increasing average areas of pore c-s of CMP>0.2 and the average areas of the compact and very compact solid c-s. FAC increased with the increase of the number of the compact and very compact pore c-s. lgKS increased with the increase of the number and area of mostly compact and very compact pore c-s. lgFAP increased with the area and the average area of the majority of pore classes and some of the relations were also controlled by the number of pore c-s and the average area of the most spread solid c-s. The study showed moreover that CMP increased with the decreasing size of the objects when measured via computer-aided image analysis. Small cross-sections revealed usually larger CMP values, and large cross-sections were more often classified as irregular or spread. Therefore the analysis of shape of soil structural elements should encompass a wide range of element sizes in relation to the image resolution to obtain the unbiased shape distributions.