This 2012 project completed the fourth repeat sampling of soils at vineyards in the Paso Robles Groundwater Basin area. These vineyards are irrigated with groundwater of varying quality, leading to the potential for soil salinity levels to increase over time. Elevated soil salinity conditions will in turn lead to degraded soil quality and reduced vine growth and production. The information on soil salinity levels and trends resulting from this study will enable growers to design vineyards and manage the soil and water to ensure long term sustainable production.
The initial sampling was carried out in 2006 at 100 vineyard sites in the area east of Paso Robles; sampling at the same locations was repeated in 2007, 2009 and 2012. The results have shown a clear trend for increasing levels of the soil electrical conductivity and the soil sodium content; these two factors are the primary salinity parameters of concern in the region. Increases in the soil electrical conductivity imposes additional water stress on the vineyards, and higher soil sodium content leads to degraded soil physical quality with important implications for reducing desirable soil drainage and aeration conditions. The average soil electrical conductivity in 2012 was 3.09 dS/m, which is greater than the standard damage threshold value for sensitive vine rootstocks.
The main salinity toxicity component of concern in the area is boron; this was assessed only in 2009 and 2012, and showed a slight increase over this period. The 2012, over 20%of the sites had soil boron levels that exceeded the standard grapevine damage tolerance range of 0.5-0.75 mg/L. There is concern that boron levels in the groundwater may increase as water levels in the aquifer decline and increased mixing of lower-quality water from deeper in the groundwater basin occurs; thus establishing baseline boron levels will help gauge any such changes over time.
This project has provided an important evaluation of fundamental soil salinity conditions over a broad region, and has identified important trends of interest to the local grape industry. These types of conditions are not unique to the Paso Robles area; other regions of the Central Coast will also have similar conditions due to the use of marginal quality groundwater for irrigation and the lack of effective natural leaching by rainfall. Prudent vineyard managers will assess water and soil salinity factors prior to planting, will choose appropriate rootstocks for the expected conditions, and will manage soil and water to reduce the potential for salinity levels to increase over time.