Mechanism(s) of humic acid induced beneficial effects in salt-affected soils

Abstract

There is enough evidence that humic acid (HA) helps to enhance crops yield by promoting certain physical, chemical and biological activities in soil-plant system. Indigenously produced coal derived, HA was added to saline-sodic silty clay soil to determine its effects on activities of alkaline phosphatase, urease, microbial activities, cation exchange capacity and moisture retention of soil. Results indicated that HA treatments significantly (p<0.05) increased alkaline phosphatase activities. Urease activities increased by 2.84, 5.71 and 20.73% with application of 0.5, 1.0 and 2.0 mg HA kg-1 respectively, in normal soil and by 16.7 and 33.6% with addition of 1.5 and 3.0 mg kg-1 in salt affected soils as compared with control (HA0). Microbial activities, measured in terms of CO2 evolved, increased by 15.7, 36.7 and 78.8% at no NPK, 14.8, 37.1 and 66.8% at half NPK, and 15.4, 40.9 and 51.2 at full NPK with addition of 0.3, 0.6, and 1.0 mg HA kg-1soil, respectively as compared to control soil (HA0, NPK0). Cation exchange capacity (CEC) enhanced by 12.3 and 20.7% over the control with additions of 1.5 and 3.0 mg kg-1 HA to saline-sodic soil. Moisture retention of soil increased significantly (p < 0.05) with 1.0 mg HA kg-1 at 1.5 MPa. It is concluded that physico-chemical and biological properties of soil are promoted by HA, which explains for the reported increases in crop yield. Key words: Humic acid, alkaline phosphatase, urease, cation exchange capacity, microbial activities, water retention of soil.