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This paper reviews the effects of wastewater sodium on soil physical properties, particularly with respect to irrigation systems. Fundamental sodicity concepts are examined including (i) sodicity definitions, (ii) the effects of sodium on soil properties, (iii) a discussion of factors that impede the infiltration rate and hydraulic conductivity, (iv) the changes that occur in ionic strength of percolating water in soil, and (v) consideration of the wastewater and soil constituents that modify the effective sodium adsorption ratio. Importantly, the ability for soils to assimilate wastewater over time changes, but these changes are not often considered prior to the planning of such irrigation systems, or after the irrigation systems are operating. The general lack of understanding of sodicity is in part due to the considerable variation in sodicity definitions. Exchangeable sodium percentage (ESP) values that are reported to pose a sodicity problem vary around the world due to the different mineralogy of the soils investigated, but variations in threshold ESP values have also been caused by a lack of consideration of the solution electrolyte concentration when determining ESP. In practice, the effects of sodicity may be evident in soils that are well under reported threshold values. When the effects of sodicity are identified, the landholder at least has the opportunity to implement remediation practices. However, more often than not, the effects of sodium from irrigation water are latent, leading to considerable problems following the cessation of effluent irrigation and changed land use.

Pasture-based grazing systems contribute to the excessive nutrients found in some streams in south-eastern Australia. This study investigated phosphorus (P) exported in runoff from a rain-fed dairy pasture (Darnum) and 4 bays of irrigated dairy pasture (MRF). Runoff was monitored for 7 years at Darnum and 2 years at the MRF to identify factors associated with the variation in total P (TP) concentrations between events. The flow-weighted mean annual P concentrations in runoff varied between 3.3 and 28.2 mg TP/L for Darnum and 6.2 and 31.5 mg TP/L for the MRF. The relationships between TP concentrations in runoff and days between fertiliser application and runoff, days between grazing and runoff, and total storm flow were examined using an additive component model that explained 61% and 70% of the variation in log-transformed TP for Darnum and the MRF, respectively. The interval between application of fertiliser and runoff and the effect of year were highly significant and explained most of the variation in TP. Grazing and fertiliser application were identified as the major factors that may affect TP concentrations that the land manager can control (preventable). The estimates of year effect (i.e. the component of TP not explained by the other variables and over which the land manager had no apparent means of control) ranged from 1.60 mg (s.e. 1.99) to 7.14 mg (s.e. 1.90) TP/L in non-drought years (>45 kL/ha runoff annually). The year effect averaged 5.7 and 6.9 mg TP/L for Darnum and the MRF, respectively. It is shown that an additive component model provides a useful structure for investigating similar, field-scale data.

Unnecessary accumulation of phosphorus (P) in agricultural soils continues to degrade water quality and linked ecosystem services. Managing both soil loss and soil P fertility status is therefore crucial for eutrophication control, but the relative environmental benefits of these two mitigation measures, and the timescales over which they occur, remain unclear. To support policies toward reduced P loadings from agricultural soils, we examined the impact of soil conservation and lowering of soil test P (STP) in different regions with intensive farming (Europe, the United States, and Australia). Relationships between STP and soluble reactive P concentrations in land runoff suggested that eutrophication control targets would be more achievable if STP concentrations were kept at or below the current recommended threshold values for fertilizer response. Simulations using the Annual P Loss Estimator (APLE) model in three contrasting catchments predicted total P losses ranging from 0.52 to 0.88 kg ha−1 depending on soil P buffering and erosion vulnerability. Drawing down STP in all catchment soils to the threshold optimum for productivity reduced catchment P loss by between 18 and 40%, but this would take between 30 and 40+ years. In one catchment, STP drawdown was more effective in reducing P loss than erosion control, but combining both strategies was always the most effective and more rapid than erosion control alone. By accounting for both soil P buffering interactions and erosion vulnerability, the APLE model quickly provided reliable information on the magnitude and time frame of P loss reduction that can be realistically expected from soil and STP management. Greater precision in the sampling, analysis, and interpretation of STP, and more technical innovation to lower agronomic optimum STP concentrations on farms, is needed to foster long‐term sustainable management of soil P fertility in the future. Core Ideas Sensitive management of soils and soil P fertility is critical for limiting water quality degradation. Maintaining soil test P (STP) at or below the agronomic optimum reduces the eutrophication threat. STP drawdown in combination with erosion control reduced catchment P loss by up to 62%. The APLE model quickly quantified the magnitude and timescale of potential P loss reductions.