Correlative Study Between The Characteristics Of Saline-alkali Soils With Cracks And The Corresponding Spectral Features

Soil salinization is severe land degradation, which severely damages the soil properties, significantly decreases crop yields and has a relatively large detrimental impact on the ecological environment. Thus it is urgent and important to find a new method to determine the salinity levels of the saline soils.The soda-saline soils in Songnen Plain have relatively strong expansion and shrinkage properties, therefore, it is very common that the soil surface starts dry-shrinking and cracking as water evaporates especially after a rainfall. The crack characteristics reflect the salt content and other properties related to salinity and also affect the spectral reflectance of soda-saline soils to a large degree. A research for non-destructive monitoring of soil properties of soda-saline soils, therefore, can be laid throuth the study of crack characteristics of soda-saline soils and the spectral response to the cracked soils.The paper was completed with support from the National Natura l Science Foundation of China. In the study, soil samples with different salinity levels were selected from Songnen Plain, the soil properties of the samples were then determined. After that a desiccation cracking experiment was conducted in a laboratory, the cracking procedure of soda-saline soils were recorded using digital camera and analyzed based on the extracted characteristics such as crack length, crack area and textural features of the cracked specimens. On the other hand, the spectral features were measured and the spectral responses to soil properties were analyzed and compared for the cracked specimens, the unracked region of the specimens and the 2 mm soil powders of the specimens, respectively. The main conclusions are as follows:1. For the saturated soil specimens, their dehydration processes can be divided into three stages such as rapid evaporation stage, uniform evaporation stage and stable stage. The cracks generate and grow in the rapid evaporation stage; in the uniform evaporation stage no more cracks generate while the surface of soil specimen continue to shrink, however the crack patterns are still changing; in the stable stage, the drying process are almost completed and the surfaces of specimens keep unchanged during residual evaporation.2. The cracked specimen, the un-cracked region of the specimens and the powder soils of the specimens have almost the same absorption features. There are two slight spectral absorption features at the wavelengths of 990 nm and 1150 nm due to the resonant effec t of Na Cl, while there are also three s light absorption valleys at the wavelengths of 1470 nm, 2000 nm and 2170 nm around because of the resonant effect of Na HCO3.3. The reflectance of cracked specimen is more able to reflect the change of salt contents, especially that reflectance above 1400 nm, which is regarded as the sensitive region of soil salinity. The reflectance at 1990 nm has the highest relevance with soil salinity thus the linear model based on 1990 nm for soil properties can offer relative high prediction accuracy. Moreover, the multiple linear prediction model based on the reflectance of 990 nm, 1470 nm, 1990 nm and 2170 nm can offer more accurate estimated results compared with the univarite model based o n the reflectance of 1990 nm.4. The water films between soil partic les produced by the exchangeable cations can reduce the mechanical properties and physical properties of the soils and therefore influence the crack characteristics to a certain degree while the clay contents of the soda-saline soils are similar. The crack length can be considerd as an important characteristic to represent the crack degree of soil specimens. The crack lengths have significant positive relationships with Na+, EC, total salt content with correlation coefficient above 0.9; the crack lengths also have certain relationships with p H, HCO3- and CO32-.5. Texture features can describe the surface of cracked specimen from another aspect, their relevance with Na+, EC, total salt content, Cland HCO3- is a little higher than the crack length.6. The validation results showed that the crack length and texture features of dried soils under natural conditions have obvious relationships with the salt content of soils, which is the same conclus ion with the experimental results, indicating that the prediction models based on crack lengths and surface texture features can realize the measurements of salinity of soda-saline soils under natural conditions.The above research results can provide a new method for the rapid and nondestructive detection of salt properties of the soda-saline soils, and also lay a foundation for the dynamic observation of salinity changes in a large area based on remote sensing.