Groundwater irrigation and phosphogypsum application: their influence on soil properties and pasture production in northern Victoria

Abstract

Saline–sodic irrigation water [EC_iw = 0.1, 0.8, 2.5, 4.5, 7.5 dS/m, SAR_iw = 3, 5, 11, 17, 29 (mmol_c/L)^0.5] was applied to lucerne plots over 1991–95 followed by further irrigation [EC_iw = 0.1, 0.8, 4.5 dS/m, SAR_iw = 3, 5, 17 (mmol_c/L)^0.5] of perennial pasture in 1997–99 with and without added phosphogypsum (PG = 0, 5 t/ha; May 1998). The later irrigation treatments and PG did not decrease exchangeable sodium percentage (ESP) in the B horizon nor alter bulk soil physical properties. Microaggregate stability to wetting decreased with increasing residual sodicity and decreasing soil electrolyte concentration. The stability of topsoil aggregates was explained by higher organic carbon content while subsoil aggregate instability was related to higher clay content and higher ESP. Pasture yield decreased by 2 t/ha with an associated increase in soil ESP from 9 to 15%. Pasture yield was highest for EC_iw = 0.8 dS/m and this treatment marginally increased N uptake in leaves. Higher yields for this treatment could not be readily explained by soil physico–chemical measurements.