Effects of oxidized/sulphonated humic acid on the availability of Cu, Zn, Feand Mn in fluvo-aquic soil

Objectives The paper studied the effects and acting mechanism of humic acids with different structures on the availability of micronutrient elements, which will help choosing proper humic acids to make functional specific fertilizers containing micronutrient elements.

Methods Indoor thermostatic incubation method was adopted using fluvo-aquic soil as tested material. The tested humic acids included the original humic acid (HA), oxidized humic acid (YHA) and sulfonated humic acid (SHA), each of them was mixed with dry soil in ratio of 30, 100 and 300 mg/kg, respectively. The soil water content was adjusted to 60% of the field water holding capacity before loaded into a culture bottle. The treated soils were cultured at 25℃ inside an artificial growth chamber. Samples were taken at the 3rd, 7th, 15th, 30th and 60th day of culture to determine the available copper, zinc, iron and manganese contents.

Results The effects of three humic acids on the availability of Cu, Zn, Fe and Mn in soil were in order of HA>YHA>SHA. 1) HA of 30–300 mg/kg significantly increased the available Cu content in the soil, especially within 15–30 days it was increased by 51.3%, significantly higher than YHA and SHA did. 2) The available Zn content in the soil increased within 15 days of culture, the increase by 30–300 mg/kg HA were 11.8%–20.3%, significantly higher than those by YHA and SHA. 3) HA increased the soil available Mn content by 5.6% within 15 days' culture, and the increase was higher than that by YHA and SHA. SHA increased available Mn by 13.6% when the application rate was 300 mg/kg. 4) After applying 30–300 mg/kg HA, the soil available Fe content was increased by 4.3%–7.2%, and the application of SHA or YHA increased the iron availability of the soil only at 30 mg/kg and 300 mg/kg, respectively.

Conclusions The application of humic acid can significantly improve the availability of copper in fluvo-aquic soil, but the effect on the availability of zinc, manganese and iron in fluvo-aquic soil shows a phase change. The effect of different structure humic acids on the availability of micronutrient elements in fluvo-aquic soil is quite different. HA has the greatest influence on the availability of those elements. In view of this, when humic acid is used to enhance the availability of micronutrient elements, factors such as the nature of the humic acid and the application period should be considered.