Restoring soil fertility of degraded hardsetting soils in semi-arid areas with different pastures

To evaluate the effectiveness of a pasture phase in restoring soil fertility, changes in concentration of organic carbon, physical and chemical properties of degraded hardsetting red soils (Alfisols) were assessed at 3 sites in the semi-arid central western region of New South Wales 3-4 years after conversion to different pastures. The pasture species included an annual grass, ryegrass (Lolium rigidum cv. Wimmera), an annual legume, barrel medic (Medicago truncatulata cv. Sephi) as well as a perennial grass, Consol lovegrass (Eragrostis curvula), and a perennial legume, lucerne (Medicago sativa cv. Trifecta). Significant differences in concentration of organic carbon in soils were found only in the top 0-2.5 cm even after 3-4 years under perennial pastures. The concentration of organic carbon under annual grass pasture did not differ from that in the fallow. Corresponding to the organic carbon concentrations, significant changes in water-stable aggregation (detectable to 20 cm depth) and hydraulic properties (up to a 5-fold difference in sorptivity, namely 0.29 mm/h(0.5) under fallow to 1.42 mm/h(0.5) under lucerne) were observed under perennial pastures when compared with that of the fallow. Of the 2 perennial pastures, Consol lovegrass and lucerne were both effective in promoting soil friability as well as water-stable aggregation. However, lucerne increased mineralisable nitrogen more than Consol lovegrass (by up to 4.6 times more, equivalent to 11.6 mg/kg in the 0-10-cm layer). Our results highlight the potential benefits of perennial pastures in maintaining the quality of the fragile hardsetting soils in the low rainfall areas. However, due to the slow rate and the restricted depth of improvement (0-2.5 cm), apart from incorporation of a pasture phase in the cropping system, soil management that maintains soil organic matter level and soil structure, namely reduced tillage and stubble retention, should also be adopted. These findings should be relevant to the management of the hardsetting soils that occupy 12% of the land area of Australia, particularly those in the lower rainfall areas.