[Amelioration effect of humic acid on saline-alkali soil]

A field experiment was conducted to investigate the effects of three types of humic acid, i.e. oxidized humic acid (OHA), aminated humic acid (AHA) and microbial-humic acid (MHA), on physico-chemical properties of saline-alkali soil and maize growth. Results showed that the application of all humic acid materials had no significant effect on soil pH in the current season. However, the soil electrical conductivity (EC), the contents of water-soluble Na⁺ and K⁺, and sodium adsorption ratio (SAR) decreased under all three treatments of humic acid application measured in up to 40 cm soil depth. OHA had the strongest effect in reducing soil EC, while no significant differences were detected among the three tested materials in reducing contents of water-solu-ble Na⁺ and K⁺ and SAR. In addition, humic acid reduced soil NO₃^--N content and increased soil NH₄⁺-N content, soluble organic nitrogen (SON) content and total soluble nitrogen (TSN) content, with higher effects of AHA and MHA than OHA. Moreover, the application of humic acid materials increased the content of soil available phosphorus, which was most significant in MHA. Humic acid addition could significantly enhance the yield and function leaf SPAD value of maize, which did not vary among the tested humic acid materials. The rank of effect size on apparent utilization efficiency of N and P fertilizer was in the order of AHA>MHA>OHA. While OHA treatment had the highest agronomic efficiency of N and P, AHA treatment achieved the highest partial factor productivity of applied N and P.