Irrigation and soil physical quality: an investigation at a long-term irrigation site

The hypothesis that irrigation improves soil quality by improving its moisture holding capacity compared to dryland (non-irrigated) soils was tested using data on soil physical properties and irrigation frequency and timing (dryland, and irrigated at 10, 15, and 20% v/v and 3-weekly) from a long-term irrigation trial at Winchmore, South Island, New Zealand (soil type: Lismore silt loam). Despite previous work showing that macroporosity decreases and bulk density increases with irrigation and grazing, the most frequently irrigated plots showed no significant difference in these measurements, or in saturated and unsaturated hydraulic conductivity, compared to dryland plots. This implies that the soil had an inherent resilience to soil physical degradation. With time (years), the number of irrigations had declined and date of the first irrigation needed during the irrigation season had become later, leading to the perception of improved moisture holding capacity. This was supported by an increase in soil carbon concentrations in all treatments with time since development. Irrigations were scheduled, on average, at 10.9, 15.3 and 20.6% v/v for 10, 15, and 20% treatments, respectively, until 1997, but at 9.9, 14.1 and 19.2% for 10, 15, and 20% treatments, respectively, afterwards. However, several reasons explain this and refute the hypothesis of improved moisture holding capacity. The most pertinent was that irrigation timing was probably affected by the use of a gravimetric method prior to 1997, and a TDR-based technique since. In addition, no difference was seen in pasture production relative to irrigation with time. Thus, apart from soil C concentrations there was little conclusive evidence to support the hypothesis that irrigation improves soil moisture holding capacity at this site.