Impacts of Long-Term Micronutrient Fertilizer Application on Soil Properties and Micronutrient Availability

Deficiencies of micronutrients in calcareous soils have been reported in different areas of China's Loess Plateau. The objective of this research was to study the influence of the continuous application of micronutrient fertilizers on soil properties and micronutrient availability in this region. The micronutrient fertilizer field plot experiment began in 1984. It included Zn, Mn and Cu fertilizer treatments and the control treatment. The crop system was continuously cropped winter wheat. The soil properties and available Zn, Mn, Cu and Fe were measured. Their relationships were analyzed through correlation and path analysis. After 31 years, the soil pH, CaCO₃ and available P levels decreased; in contrast, the organic matter, fulvic acid, reducing substances and soil moisture levels in the surface soil increased in the micronutrient fertilized treatments compared to the control treatment. Cu and Zn fertilizers promoted the available Cu and Zn levels in the surface and deep soil, but available Mn was not significantly affected by Mn fertilizer. It can be seen from the interaction between the micronutrient availability and micronutrient fertilizers that Zn, Cu and Mn fertilizers can increase the available Fe level; Mn fertilizer can increase the available Cu level, and Cu and Mn fertilizers can increase the available Zn level. This means that Fe, Cu and Zn availability were easy to implement, whereas the soil-available Mn was difficult to improve in calcareous soils on the Loess Plateau. Fulvic acid and organic matter showed a significant and direct effect on the available Zn; the available Mn and Fe were mainly affected by the soil CaCO₃ and moisture; the available Cu was mainly affected by the soil organic matter, available P and reducing substances. These results indicate the importance of organic matter in calcareous soils; it can not only directly affect the availability of micronutrients but also indirectly affect their availability through the indirect interaction with fulvic acid, reducing substances, available P and CaCO₃. The above conclusions suggest that the long-term micronutrient fertilizers changed some important soil properties and increased the micronutrient availability in the loess-derived soil.