The critical ESP value: does it change with land application of dairy factory effluent?

Decisions about the tolerable loading of sodium during land application of dairy factory effluent (DFE) are often based on the risk of exceeding a critical exchangeable sodium percentage (ESP) in the soil. The concept of a critical ESP value is based on overseas research that investigated the effects of saline irrigation water on soil structure, rather than on experimental evidence based on research using DFE. The problem is that sodium may degrade the structural stability of the soil and eventually cause surface ponding and effluent run-off. The aim of this investigation was to determine the effects of land treatment of DFE on key soil physical properties and to establish if a single critical ESP value is appropriate for these situations. Intact soil cores (200 mm diameter x 200 mm deep) were collected from three sites at the Clandeboye dairy factory in Canterbury covering two periods of application (4, 10 years) of land treatment of DFE, plus an adjacent control site that had not received DFE. The soil at the Clandeboye land treatment sites was a Lismore stony silt loam with a mixture of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). The results show that the soil from DFE land treatment sites had greater wet aggregate stability than soil from the control site. Treatment of the soil cores in the laboratory with a sodium solution decreased the wet aggregate stability and the hydraulic conductivity of the soil. The greatest reduction in hydraulic conductivity occurred in the control soil with a smaller decrease in the hydraulic conductivity in the soil from the DFE treatment sites. A decrease of 50% in hydraulic conductivity occurred at an ESP value of 3 in soil from the control site, whereas in soil from the DFE sites this 50% reduction did not occur until an ESP of 13 was achieved. These results show that the critical ESP value may change following the application of DFE to soil, and that the use of a single ESP value in land-use decision making is therefore too simplistic.