Mineralogical,Pore And Mechanical-physical Properties Of Vertic Soils Of Northern China And Their Improvement

Vertic soils have unique qualities regarding their mineralogy, morphology, clay contents and physical and chemical characteristics. Presence of these qualities often makes them problematic or unsuitable for sustainable agriculture. However these soils are cultivated in many parts of the world and considered as substantial resource for sustainable agriculture. Vertic soils of Northern China also possess the same characteristics as rest of the soils in the world. These soils contained high clay contents with smectite mineralogy. Clay minerals have important role in nutrient cycling, physiochemical properties, pedogenesis and pollutant transportation in the soils. The presence of these high clay contents and smectite minerals (mostly montmorillonite) makes the soil hard to cultivate and deteriorates most of the physical properties, which have direct relation to other soil related properties and to crop yield. The soil pore size distribution directly influences many soil physical, chemical, and biological properties. Further knowledge of soil pore size distribution is very helpful for understanding many soil functions and processes. Being a suitable resource for the agriculture in the Northern China, it is imperative to conduct detailed study on the vertic soils from mineralogical, physical and mechanical point of view and find some solutions to overcome the problematic nature of these soils for continued productivity and long term sustainability of these resources for the current and future generations. The current study was conducted in laboratory of College of Environment and Resources Sciences, Zhejiang University, Hangzhou, China. Vertic soils of Northern China were characterized for their mineral, morphological, physical and mechanical point of view as well as improvement in the physical properties by introducing biochar (straw biochar, wood chip biochar), wastewater sludge biochar and fly ash. The salient findings of the study are discussed as under.In first experiment of this study vertic soils of Northern China were extracted, characterize and identified for their PSD, mineralogy, morphology and lattice fringes through Nano Zetasizer, X-Ray-Diffraction (XRD), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM), respectively. PSD value showed that the size of clay minerals in all profiles were normally distributed and were unimodal. The PSD value indicated that clay minerals in soil profiles were presented from nanometer to their high ranges of minerals. The profiles (AH-05, AH-09, SD-01, HN-03, JS-07) had clay mineral ranges from few hundred nm to thousand nm except HN-10 profile which also had small clay particles in tens of nanometers. XRD analysis of clay minerals in vertic soil showed presence of crystalline mineral phases like smectite (montmorillonite, nentronite), vermiculite and kaolinite. Smectite and vermiculite are considered to be responsible for typical behavior of swelling and shrinkage in vertic soils. TEM micrograph also indicated the size of clay minerals from ten to hundreds of nm. Clay minerals in profiles AH-05, AH-09 and HN-10 showed pseudo-hexagonal, hedral and oval morphology whereas profiles SD-01, HN-03 and JS-07 had euhedral-pseudo-hexagonal shape with well-defined edges. The well defined lattice fringes of clay minerals indicated that they were well crystallized and have definite shape. Combined applications of above analytical techniques provide us an advantage to study the clay minerals in vertic soils of Northern China much deeper and further to increase our understanding about the complex nature of these particles.In second experiment of this study, vertic soils of Northern China were investigated for their mechanical and physical properties and cracking parameters. The objectives were to determine the particle size distribution (PSD), mechanical and physical properties (COLE, tensile strength, and shear strength) and cracking and to establish a relationship of these parameters with clay contents and clay mineralogy. Also a detailed study on the crack development in vertic soils and improvement in the cracking parameters under straw biochar (SB), wood chip biochar (WCB), water sludge biochar (WSB) and fly ash (FA) was assessed. PSD values showed that all the investigated soils had high contents of fine clay (>30%) except for profile JS-07. The results revealed that high COLE values were found along depths for all the soil profiles except depths>80 cm and>150 cm for profiles SD-07 and HN-10, respectively. Tensile strength of profiles AH-05, AH-09, SD-01, HN-03 and HN-10 increased with increasing depth. Shear strength showed high values of cohesion for profiles AH-05, AH-09, HN-03 and HN-10 along the depth; whereas, angle of friction was decreased. Cohesion values for profile SD-01 gradually decreased from top layer to bottom layer while no marked differences were observed for shear strength in profile JS-07 with depth. Cracking area increased in profiles AH-05, AH-09, SD-01 and JS-07; whereas, crack length increased in profiles HN-03 and HN-10. Principle component analysis showed that tensile strength, cohesion, cracking characteristics, clay%, CEC and COLE were positively correlated with each other, whereas negatively correlated with angle of friction, indicating that these properties were subjected to clay% and smectite content. These correlations might be responsible for low productivity (low yields) in Northern China. The vertic soils were subjected to desiccation cracking and crack length increased at high temperature (50℃). High evaporation rate and tensile stress were responsible for the development of cracks. The vertic soils showed development of cracks with reduction of water contents. At 45% water contents, soil achieved the development of primary and secondary cracks and started to consolidate. Addition of amendments had significantly improved the cracking properties of vertic soil. The straw biochar (SB)and fly ash(FA)treatments significantly decreased the cracking density and cracking length,respectively. The improvement in the cracking parameters is very essential for the sustainable soil quality and production in the vertic soils of Northern China.In third experiment of this study, pore size distribution of six soil profiles at different depths along with three clayey soils, collected from the topsoil (0-20 cm) of vertic soils in Northern China, were analyzed using the nitrogen gas adsorption (NA) and mercury intrusion porosimetry (MIP). Total porosity and total volume decreased in profiles AH-05, AH-09, HN-03 and HN-10. Percentage of macropores, ultra-micropores and cryptopores increased in profiles AH-05 and AH-09 while percentage of ultra-micropores and cryptopores increased in profiles HN-03 and HN-10. Profile JS-07 showed high percentage of macro, meso and micrpores. Differential curves of equivalent pore diameter suggested that macropores and mesopores dominated at upper layers while ultra-micropores and cryptopores were attributed to lower layers. The effect of organic matter on the pore size distributionof three clayey soils was evaluated. The differential pore curve of clayey soils by the NA method exhibited that the pores with diameter <0.01 μm accounted for more than 50% in the pore size range of 0.001 to 0.1μm. The differential pore curve of clayey soils by the MIP method exhibited three distinct peaks in pore diameter range of 60 to 100μm,0.3 to 0.4μm and 0.009 to 0.012μm, respectively. In clayey soils, the ultramicropores (0.1-5μm) were determined to be the main pore class (on average 35.5%), followed by macropores (>75μm,31.4%), cryptopores (0.007-0.1μm,16.0%), micropores (5-30μm,9.7%) and mesopores (30-75μm,7.3%). It was found that OM greatly affected the pore structure and pore size distributionof aggregates in clayey soils. In particular, OM removal reduced the volume and porosity of 5-100μm size pores while increased the volume and porosity of<5μm size pores in the 5-2 and 2-0.25 mm aggregates of clayey soils. The increase in volume and porosity of<5μm size pores may be attributed to the disaggregation and partial emptying of small pores caused by the destruction of organic matter.In fourth experiment of study, ameliorativeeffects of soil amendments on the mechanical and physical properties of vertic soils were studied. The potential soil amendments were three types of biochar (straw biochar (SB), woodchips biochar (WCB), waste water sludge biochar (WSB)) and fly ash (FA). The use of these amendments, which can be obtained from natural sources or from agricultural and industrial by-products, can lead to a greater sustainability in agriculture and conservation of soil resources. In order to evaluate the effect of amendments, the physical parameters like soil consistency limits (PL, LL and PI), tensile strength (TS), shear strength (C and φ), swelling potential (SP) and swelling pressure (SPr) were studied. The results showed that all amendments had significantly improved the above mentioned physical properties. No specific trends were observed for consistency limit; however, PI decreased for all the amendments applied and LL showed correlation with cp and TS. Significant reduction (p< 0.05) in the COLE was found for all the amendments and the effect of WSB and FA was more pronounced. The values of TS decreased significantly (p<0.05) for WCB, WSB and FA amendments while higher reduction rate i.e.,37%,43.99% and 61.32% at 2,4 and 6% was observed for SB amendment. The C values decreased significantly for all the amendments and effect of biochar (SB and WCB) was more prominent, as comparedto other amendments; whereas, no specific trends were observed for φ. The swelling potential (SP) and swelling pressure (SPr) capacity of soil also improved under the application of amendments. The SP and SPr significantly decreased after the application of amendments. Higher reduction in the SP values was observed for FA application (24%,22% and 19% at 2,4 and 6% rates) compared to control (52%). Pearson correlation indicated that all the physical properties i.e., COLE, TS, C, φ, SP and SPr were significantly (P≤0.01) correlated with each other. Cluster analysis showed that LL, PL, C and φ fell intoone group whereas SP, SPr, COLE, and TS formedanother group respectively. This correlation indicated that many of the physical properties of studied soil were under the effect of shrinkage and swelling phenomenon and use of amendments significantly improved the physical parameters of vertic soils.