Summary of Final Report:

Adoption of sustainable soil management practices is becoming common in winegrape production in response to an increased awareness of the value of soil health to maintain environmental quality.  Soil health is characterized by the functions it provides to the vineyard (i.e water retention, nutrient supply, carbon sequestration etc.) and is often impacted by vineyard soil management practices (i.e., compost application, no till, cover crops).  However, the magnitude of the effect of these practices on soil organic matter and other soil health indicators is difficult to predict.  A major impediment for the diagnosis and establishment of target values of soil health in vineyards is the large diversity of soils and physiographic conditions across and within growing regions.

During the 2020-2021 funding cycle, we conducted semi-structured interviews to growers paired with ongoing sampling efforts across 32 vineyards in Napa Valley to (i) establish a baseline of soil health indicators within the various Napa Valley soil types; and (ii) examine grower perception and comprehension of these indicators and the desired qualities of a healthy soil relative to production goals.  This was replicated in the Paso Robles AVA during the 2021-2022 funding cycle, and in Lodi during the 2022-2023 funding cycle followed by a no-cost extension during the 2023-2024 cycle to finalize research activities across the three regions.

The activities in this project were carried out through a collaboration between researchers, extension specialists and grower groups including the Napa Valley Grapegrowers Association, Paso Robles Wine Country Alliance, Independent Grape Growers Paso Robles Area, and Upper Salinas-Las Tablas Resource Conservation District.  The data gathered so far for the three regions shows that most grape growers consider soil health to be important for wine grape production due to perceived benefits to reduce soil erosion, improve vine health and grape quality.  Through grower input, researchers were able to extract the soil properties that are perceived as the most important for these beneficial outcomes, namely a balanced soil structure and nutrient supply.  Finally, data collected in the interviews revealed a strong grower knowledge of how the physical and chemical aspects of soil health are related to beneficial agronomic outcomes, as well as strong grower interest in understanding the role of soil organic matter and the soil biome on these outcomes.

Results from the soil health assessment show no significant differences for most chemical, physical, and biological properties such as soil pH or inorganic N between soil categorized as ‘ideal’ and ‘challenging’ by growers.  However, a strong negative correlation was observed between active soil carbon, bulk density, and surface hardness, indicating that soils with more C, had lower compaction issues.  Results of this research were presented 33 times in three years including scientific conferences and outreach events.  Furthermore, we have published one paper and one more is under development to be submitted for publication in October 2024.  Finally, together with the Napa Valley Grape Grower Association, we have developed a handbook of soil health in Napa to be published at the end of 2024.

Proper management of salt accumulation in soils is crucial to prevent long-term reductions in productivity, especially where irrigation water is saline or sodic. Reclamation of saline-sodic soils requires the removal of most of the exchangeable Na+ and the replacement by Ca2+ in the root zone. One of the most effective practices to achieve this result is the use of chemical amendments containing calcium, such as calcium sulfate (CaSO4).

This project started in 2019 intending to compare different forms and doses of calcium sulfate and their effect on the physiology of grapevine growing in a sodic soil located in the southern San Joaquin Valley. The first year was devoted to locate the experimental site, perform a baseline of soil and plant conditions, and applying the treatments over 17 acres in a randomized complete block design with 6 treatments and 4 replications. In the second year, we monitored the soil conditions, plant physiology, and soil composition both on the ground and with the aid of satellite imagery and started to observe the first effects of the treatments.

Since April 2013, this study has closely monitored the effects of organic floor fertility strategies (leguminous cover crops, grape pomace composted with manure) in concert with soil carbon storage techniques (biochar and grape pomace compost) Gathered annually are a number of soil, vine, and grape metrics at the University of California Oakville Station. Preliminary effects of these treatments on soil greenhouse gas emissions (nitrous oxide, carbon dioxide, and methane), plant available nitrogen (ammonium and nitrate), soil moisture, vine vigor, and berry quality were discerned. Perhaps the most important results acquired thus far are the monthly trends and magnitude of soil nitrous oxide (N2O) emissions and mineral N availability. Nitrous oxide is a GHG with 300 times the global warming potential of carbon dioxide (CO2), therefore causing significant potential carbon offsets, and limited N fertility was found to severely restrict harvest yields during the decade preceding this investigation. Periods of large N2O production were largely initiated by substantial rain events. When cover cropped or compost supplemented soils were amended with biochar, significant reductions in N2O were observed compared to organically fertilized controls during individual rain periods. However, when organic N fertilizer was not present, biochar-only plots emitted significantly more N2O than conventional controls. There was little difference throughout the year in plant available nitrogen as ammonium (NH4-N) among all treatments, yet a striking contrast in plant available nitrogen as nitrate (NO3-N). Yields have been restored to ca 4+ tons per acre and although yields for the non-conventional treatments were significantly higher in 2013, they did not differ from conventionally fertilized plots in 2014. Thus, this study demonstrates that organic ground fertilizations used in concert with biochar amendments can increase NO3-N provisions while maintaining NH4-N and decreasing vineyard scale N2O emissions.

No significant changes in carbon leaving the soil as CO2 or methane (CH4), were detected on an annual basis. In yearly assessments of total carbon, we were able to determine that both biochar and grape pomace compost did appreciably increase soil carbon content compared to conventional controls upon application. However, we found that the effect of these treatments on annual increases in carbon were negligible but the study will need to continue over years to discern long-term effects.

A study was conducted over a three year period to evaluate traditional (measurement of leaf water potential (Ψ_leaf)) and new techniques (sap flow sensors and petiole ‘dendrometer’ sensors) to monitor vine water status in the field.  Seventeen red, wine grape cultivars grafted onto 1103P and grown in a replicated trial at the Kearney Agricultural Research and Extension Center were evaluated to determine the appropriateness of Ψ_leafas a means to determine vine water status.  Four of the cultivars, Cabernet Sauvignon, Grenache noir, Petite sirah and Syrah, were used to evaluate sap flow sensors to measure grapevine transpiration.  Lastly, petiole ‘dendrometers’ were attached to Cabernet Sauvignon and Thompson Seedless grapevines to compare daily petiole diameter variations with measures of Ψ_leaf and grapevine water use measured with a weighing lysimeter.

One of the criteria for classification as a grape cultivar being isohydric vs. anisohydric is that midday Ψ_leafbetween a well-watered and water-stressed plant should be similar or at least not significantly different.  If this were the case, Ψ_leaf could not be used to assess vine water status.  Based upon the clear differences in Ψ_leafmeasured mid-morning, midday and mid-afternoon between irrigated and non-irrigated vines across years all the cultivars measured in this study would be classified as anisohydric.  Others have suggested that the Ψ_leaf of a near-isohydric cultivar will decrease in response to soil water deficits but only to a certain level (~ -1.5 MPa for Grenache) while that of an anisohydric cultivar will continue to decrease (values < -1.5 MPa for Syrah).  Midday Ψ_leaf of water stressed vines before and after veraison of all cultivars were not significantly different from one another and were more negative than -1.8 MPa as would be expected for an anisohydric cultivar.  Therefore the use of midday Ψ_leaf would be appropriate for use in an irrigation management program.

The sap flow technique used in this study was able to discern volumetric sap flow and sap flow velocity differences among cultivars differing in canopy size with the exception of one case (Cabernet Sauvignon in 2010).  The technique was also able to differentiate between the two irrigation treatments on a daily and diurnal basis.  These differences in sap flow between treatments were also reflected in grapevine Ψ_leaf measurements.  Lastly, sap flow volume/velocity varied from one day to the other in response to a reduction in evaporative demand across cultivars and irrigation treatment.  However, the values of daily volumetric sap flow in 2011 for the irrigated vines were less than those estimated from the product of the crop coefficient, derived from the shaded area of each cultivar, and daily ET_o.  In addition, the diurnal sap flow velocity across cultivars and irrigation treatments appeared to increase rapidly early in the morning and thereafter leveled off.  They did not follow the diurnal pattern of solar radiation as shown in other research.  Therefore, the sap flow technique as measured under the conditions of this study would not provide a reliable, quantitative measure of grapevine transpiration.  As such, it would also not be a stand-alone technique to assist in an irrigation management program, but has some utility in tracking relative changes in water use associated with stress.

A bend or deflection sensitive transducer was used as a means to monitor vine water status, measuring the diameter of a leaf’s petiole and/or the cluster’s peduncle. Daily maximum and minimum petiole diameters and the daily shrinkage were correlated with vine water status or vine water use.  Petioles started to shrink as soon as water use increased as the sun came up in the morning and continued to shrink until the midday peak of vine transpiration (petiole diameter increased as the sun went down in the afternoon).  The daily petiole diameter contractions remained fairly constant when grapevines were irrigated daily at full ET_c or numerous times a week when being deficit irrigated.  When irrigations were terminated, daily max and min petiole diameters decreased and the daily contractions increased.  Monitoring peduncles provided the extra benefit of assessing growth rate of that tissue.  Peduncle diameter growth rate was reflective of irrigation treatment when measurements were taken between veraison and fruit maturity.  Further research is needed for this promising technique to assess vine water status to include measurements taken between berry set and veraison.

Berry weight at veraison was a linear function of mean midday Ψ_leaf measured between the initiation of irrigation and veraison across cultivars.  The change in berry weight between veraison and September 9 was also a linear function of mean midday Ψ_leaf when measured between those two times across cutivars.  There was a significant interaction between cultivar and irrigation treatment on yield.  Vines with the highest yields that were not stressed between May and veraison or irrigated at 50%of estimated ET_c during that time period did not always have the greatest yield when stressed between May and veraison.  These results indicate that cultivars may respond differently to the imposition of water stress at various times during the growing season.  Tempranillo had the greatest water use efficiency (water used per ton of fruit produced) of all cultivars while Freisa and Malbec the least across irrigation treatments.  It is interesting to point out that Tempranillo was least affected by the irrigation treatments imposed in this study during 2012.  These results indicate that wine grape growers may be able to choose cultivars that are inherently more water use efficient.

This project evaluated the irrigation crop coefficient at 84 vineyards east of Paso Robles, which lie within a region previously designated as a groundwater ?Area of Concern? due to declining groundwater levels. The mid-summer measurements, taken during the peak irrigation water use period, indicated an average crop coefficient of 0.53, with a range from 0.27 to 1.03. The significant degree of variability indicates the wide disparity in theoretical irrigation water requirements for vineyards in the area, and supports the practical observations that ?not one size fits all? with respect to vineyard water requirements. The survey results, while of value in and of themselves, will be used primarily in conjunction with the results on an ongoing UCCE vineyard water use survey, to add value to those measurements. Comparing the volumes of water used between vineyards is not a fair assessment of how efficiently water is managed, because it does not take into account the different water requirements for different vineyards. By having site-specific crop coefficient values in addition to the measured irrigation use data, it is then possible to compare water use as the percentage of ETc applied, which is a more robust comparison between different locations.