Characteristics of salt contents in soils under greenhouse conditions in China.

Greenhouse cultivation is expanding in China due to high production efficiency and greater economic benefits. Although the accumulation of soil salinity and nutrients has been observed in greenhouse cultivation areas, the linkage between soil salinity, soil major ions, and farm practices is not clear in China. Few studies have examined soil salinity accumulated in soil layers; thus, a broad investigation is needed in order to understand the potential causes of soil salinity in greenhouse soil. In this study, a short review was given to show the salt contents and the major ion under greenhouse conditions in China. Then, we analyzed a total of 132 soil samples from different parts of China in terms of their soil major ions and nutrient components and investigated the relevant farm practices. Based on survey data from three different types of cultivation areas (open farmland, plastic greenhouses, and multispan greenhouses), we found that cultivation in both greenhouse types resulted in a significant increase in salt content and a decrease in soil pH values, a pattern not shown in open farmland. The linkage between soil salinity and cultivation type was confirmed by soil salinity classification. The proportion of each ion in the soil salt differs significantly between the different management methods, but the variation range of the main ions ranged from - 23.3 to 225.6% for multispan greenhouses and - 22.6 to 430.5% for plastic greenhouses. In addition, the salt source in greenhouses is not unique to those methods, suggesting that different growing practices cause the differences in ion concentration. Removing greenhouse covers during the rainy season can avoid further accumulations of salt, but the subsequent rinsing of soil can lead to the deeper salt accumulations. In addition, increasing salt content may lead to decreasing pH once the natural salt balance is altered. These results show that the soil salinization produced by greenhouse cultivation cannot be ignored.