Molecular Characterization of Size-Fractionated Humic Acids Derived from Lignite and Its Activation of Soil Legacy Phosphorus and Lactuca sativa Growth-Promoting Performances

[Image: see text] Humic acid (HA) has been widely used in agricultural production processes because it improves soil nutrients and has a growth-promoting effect on plants. Understanding the relationship between the structure and function of HA is the key to efficient utilization of HA in the field of activating soil legacy phosphorus (P) and promoting crop growth. In this work, HA was prepared using lignite as raw materials by the ball milling method. Moreover, a series of HAs with different molecular weights (<10, 10–50, and >50 kDa) were prepared through ultrafiltration membranes. The chemical composition and physical structure characteristics of the prepared HA were tested. The effects of HA with different molecular weights on activating accumulated P in calcareous soil and root promotion of Lactuca sativa were investigated. Results showed that HAs with different molecular weights have different functional group structures, molecular compositions, and micromorphologies, and the molecular weight of HA can significantly affect the activation performance on soil accumulated P. Moreover, the HA with low molecular weight more easily enhanced the seed germination and growth of Lactuca sativa than that of raw HA. It is expected that more efficient HA can be prepared in the future for the activation of accumulated P and promoting crop growth.