Progress report for GW21-227
Project Information
In recent meetings, growers have expressed a strong interest in understanding soil health for wine grape production systems. The problem is that we currently lack a comprehensive framework to assess soil health for the semi-arid Mediterranean soils of the western U.S. This problem is more challenging for the California wine grape production industry not only due to its large soil diversity but to this crop’s unique management practices and goals. Therefore, soil health for California vineyards might have a different definition and sustainable management requirements compared to other crops (Lazcano et al. 2020).
Therefore, we aim to 1) develop a definition of soil health for wine grape vineyards that meets the unique growers’ production goals, and 2) assess diverse soil health indicators in vineyards across the Napa Valley region.
With the collaboration of growers, this study will help us improve and do targeted sustainable recommendations for conserving soils while helping growers achieve their production goals. We expect to observe trends of soil health indicators among soil types and sustainable management practices.
The objectives of this project were:
- To develop a definition of soil health for wine grape vineyards that meets the unique production goals.
- To assess the variability of diverse soil health indicators in vineyards across the Napa Valley region.
The outreach objectives of this project were:
- To integrate growers' collaboration and participation in defining and assessing soil health for such unique crops as wine grapes.
- To provide several informational talks for growers and develop short farmer-friendly articles (i.e. factsheets) communicating about the importance of soil health, the role of the soil organic matter and soil microbial ecology in soil health, and the main findings of our project.

Samplings and ratings will be performed during the spring of 2021, but for the purposes of this grant, we will focus on the laboratory soil health testing and analysis that will start during August of 2021. The sampling and rating will happen during the spring because this is the time when growers are available to collaborate with us (during the summer and fall, growers are too busy to collaborate). The soil ratings will take place during the same sampling day on each farm (n=32). Following the soil health testing and analysis of samples, we will proceed to organize, analyze, and synthesize the data and finally writing the findings in a farmer/industry friendly report, as well as a scientific paper. A detailed description of the timeline for this project is provided in Table 1. The major milestones that will be accomplished include developing a soil health assessment of the Napa Valley Region for the first time which will be very helpful for growers to decide on performing sustainable soil management practices for sustainable agriculture.
Cooperators
- - Producer
- - Producer
- - Producer
- (Researcher)
- (Researcher)
- - Producer
Research
Objective #1: To develop a definition of soil health for wine grape vineyards that meets the unique production goals.
During the summer of 2020, we conducted semi-structured interviews to 15 wine grape growers of the Napa Valley region to understand their perception of soil health for wine grape production (Gonzalez-Maldonado et al., in preparation). These interviews were key for understanding the grower's knowledge, thoughts, and actions towards soil health and sustainable soil management practices. However, a quantitative approach was integrated to complement the assessment of grower’s perception of soil health for vineyards, a crop that requires unique and specific practices and goals compared to other crops. Therefore, following Mann et al. (2019) as inspiration, farmers were asked to rate two areas where they believe their soils better reflected their production goals (an ideal soil) and the opposite (a challenging soil). After these soil ratings were made by farmers, we proceeded with the second objective of sampling soils and assessing soil health indicators in those exact sites. The purpose of rating soils is to quantify and understand the most important attributes of a soil for wine grape production by comparing them with quantitative soil health data. To complement the ratings, we asked growers to provide a set of detailed information on soil management practices to be integrated into the analysis.
Objective #2: To assess the variability of diverse soil health indicators in vineyards across the Napa Valley region.
Samplings
This project took place in 32 vineyards from Napa Valley, California with the collaboration of the Napa Valley Grape Growers Association and 16 local wine grape growers. Soil sampling, preparation, and storage happened from late February to May of 2021 upon growers' availability for collaboration. Soil samples were taken from selected representative areas of each vineyard and rating (ideal and challenging soils) guided by the grower. Samples were taken for two depth intervals (0-10 cm and 10-20 cm) and two locations (vine row and tractor row). The sampling was divided into two depths and two locations due to organic material inputs and microbial activity usually being higher in the top depth of soils and differences in inputs from differing plants growing. For instance, growers grow cover crops in the tractor rows (or alleys) which might drive different results compared to the vines row. Three sample replicates were taken per vineyard leading to a total of 384 soil samples. The geographic coordinates of the sampling locations were recorded to categorize each vineyard into a soil type (or soil cluster) classification system. Additionally, soil taxonomy information was collected using the SoilWeb online tool (https://casoilresource.lawr.ucdavis.edu/gmap/) including soil textural class and soil series.
For this grant, focus was given to the soil health indicators analyses performed in the laboratory. The laboratory soil health analyses were performed from August 2021 to December of 2022. After laboratory processing, data organization and analysis will happen from January to March 2023. The findings will be written starting in April of 2023 and will be presented during the summer of 2023. The project will conclude by July 31, 2023. The soil health analyses costs are covered by a grant awarded by the American Vineyard Foundation to the PI Dr. Cristina Lazcano and collaborators.
Soil management practices information:
A detailed list of soil management practices history was provided by growers of the previous three years prior to sampling for each area sampled. Examples of practices include tillage, no-tillage, disking, cover crops, compost applications, among others.
Soil health analyses
Soil Physical Properties
The soil physical properties quantified in this study were gravimetric soil moisture, water holding capacity, and soil texture. We assessed soil infiltration and soil compaction in the field but these remained out of the scope of this grant due to time limitations. In California, water is an important limiting factor, so we need to assess how much moisture potential and availability there is in the soils. Gravimetric soil moisture was measured by calculating the difference between field moist soil and dried soil (105 C°). The available water holding capacity was quantified from capillary soil saturation at field capacity of dried soil. Soil texture was quantified to determine particle size following the rapid method described in Kettler et al 2001. The soil texture method allowed us to classify our soil types according to textural class.
Soil Chemical properties:
Soil pH and electrical conductivity (EC) were measured in a 2:1 (soil:water) paste using a pH and EC electrode (Fisher Scientific, Waltham, MA). Soil pH and salinity (obtained through EC) are important factors that drive soil biological processes including plant growth. Also, soil organic matter % was calculated from total C assessed through dry combustion. Permanganate oxidizable carbon (POXC) was assessed to estimate the active carbon pool of the soil organic matter (Culman et al., 2012). Organic matter has been shown to be an important factor in soil health, so total soil organic matter is an overall good indicator of soil quality. However, active carbon, in particular, has been shown to be sensitive to short and long-term management practices (Culman et al 2012) and is also related to the amount of carbon that could be mineralized; so, this indicator is important to determine how much soil organic carbon will be eventually lost as CO2 rather than being stabilized in the soil (Hurisso et al 2016). Quantifying both total and active soil organic matter is a good way of considering the spectrum of soil organic matter in the soil. Soil organic matter is one of the main drivers of soil health since it has the capacity to influence other soil properties for good like water infiltration, reduce soil erosion, increase soil microbial diversity, achieve higher yields in some crops, among others.
Soil Biological Properties:
Soil biological properties, especially soil microbial diversity and activity, are one of the most important soil properties and drivers of soil health but this area remains largely understudied due to its complexity. Soil microorganisms are essential drivers of key soil processes for agriculture and environmental quality like nutrient cycling, soil organic matter turnover and stabilization, nutrient availability, aggregate stability and reduced soil erosion, water filtration, pest suppression, among others. We used several methods that have been shown to provide useful general information regarding microbial diversity and activity like soil respiration, potentially mineralizable nitrogen, and Phospholipid fatty acids. Soil microbial respiration was measured using a LICOR-850 infrared gas analyzer that quantifies CO2 respired by microorganisms from re-wetted soil samples to 50% water holding capacity and incubated (2 days) in an air-sealed gas jar. Microbial respiration is an indicator of how active soil microorganisms can be when adequate moisture is added. Also, potentially mineralizable N (PMN) was estimated by measuring ammonium and nitrate produced in incubated soils under anaerobic conditions for 7 days in the laboratory (Moebius-Clune et al. 2016). Measuring PMN helps us understand potential nitrogen, which is one of the most needed and limited nutrients, for cycling and availability for plants. Soil microbial diversity was analyzed by a commercial laboratory through a phospholipid fatty acid (PLFA) assay from the 0-10 cm depth which is where typically most microorganisms are concentrated in the soil. The PLFA method is great for studying the structure of the soil microbial community by differentiating bacterial and fungal microorganisms. Studying soil biodiversity helps us understand what type of organisms are present in the soil and obtain an idea of functions they could carry which can have different impacts on nutrient cycling and availability, soil carbon, and ultimately in soil health.
Statistical Analyses
The interviews were analyzed by following inductive coding and a grounded theory approach. In summary, the codes are keywords identified and grouped as main ideas from the growers answers. Inductive coding means that codes were not predetermined prior to data analysis but emerged from the interviewee responses.
We evaluated growers’ soil rating factors and quantitative soil health data performing a general linear model using the soil health ranking as response variables, and grower rating, and depth as fixed effects using the "glmmPQL" function from the MASS package (Venables and Ripley, 2002) in RStudio (RStudio Team, 2020). Additionally, descriptive statistics like ranges, medians, averages, and standard deviations were calculated for each soil health indicator. Data from the interviews and growers' ratings were compared to meet objective 1. The relationship between biological, physical and chemical soil health indicators were studied through multivariate statistics such as Principal Component Analysis (PCA). Data visualization using the "ggplot2" (Wickham H, 2016) function from RStudio of descriptive statistics was performed to understand soil health indicators trends by soil type and grower rating in the Napa Valley Area.
Objective 1 Results
Most growers identified themselves as vineyard managers (12), others as both owners and managers (2), and few as only owners (2) leading to a total of 16 participants for the interviews.
Overall, soil health was described as important for the wine grape growers interviewed; however, it had descriptions unique to wine grape production goals. A healthy soil for wine grape production was defined as one that:
1) Allows adequate vine vigor control
2) Produces high-quality grapes
3) Maintains vine health
4) Allows to reach yield targets
5) Has adequate water holding capacity and good infiltration
6) Has “balanced” fertility
7) Resists erosion
8) Needs minimal inputs and interventions
Most growers prioritized vigor control and vine health when describing a healthy and ideal soil for red wine grape production. Many growers stated that inducing water stress to control the vine vigor through deficit irrigation is a technique they use for producing the desired grape quality. They described that the soil plays an essential role in this action through the water holding capacity (WHC) and infiltration. For red wine grape varieties (which is the grape we focused on in this project), growers want a soil that does not retain excessive or too little water. Similar to the WHC, growers described a healthy soil as one that is “balanced” in terms of soil fertility. Often, increasing soil health was related to high fertility which was cataloged as undesirable since it would challenge vine vigor control. A few growers mentioned that low fertility was preferred for vines to thrive and produce high-quality grapes.
- “I would say that I think there’s probably a spectrum of soil health. It’s very rare that anything is black and white, y’know so I think as long as your soil health is somewhere near the center of maybe the two ends... I don’t know what exactly the ends would be.”
- “I think it’s very important. But not in the sense that y’know that it has to be super fertile soil, I just like that it needs to be healthy soil.”
In addition to maintaining healthy vines and allowing adequate vigor control, some growers mentioned that a healthy soil would allow them to reach yield targets and minimize inputs and interventions. Most growers stated that a healthy soil should be self-sustaining and resilient to erosion.
The most important soil properties highlighted as ideal by growers were:
- Coarse soil texture
- Adequate water holding capacity
- Good infiltration
- Adequate (not high) levels of nutrients, especially Nitrogen
- Sufficient (not excess) organic matter
Soil texture was the most highlighted soil property among growers. Fine-textured soils were perceived as undesirable and coarse/gravelly soils were preferred for achieving quality goals. The reason for this is that fine-textured soils have higher water holding capacity which could disbalance vine vigor by increasing it, which is undesired. So, growers tended to prefer coarser and gravelly soils over clayey soils that have lower infiltration.
- “I guess a balanced soil with a little bit more umm, larger particles. Umm like, y’know, I- I’ve always seen that slightly gravelly loam soil … it’ll retain a decent amount of water but will also drain decently well. And you can get umm, you can umm reduce vigor a little bit in this kind of soil. So you have the ability to control vigor.”
Objective 2 Results
Out of the 16 growers interviewed, one withdrew their participation from the sampling; however, one of the growers volunteered for more participation. The soil health indicators performed until the summer of 2022 include POXC, MBC, Respiration (Mineralizable Carbon), pH, EC, PMN, NO3-, NH4+, soil moisture and bulk density. The soil health analyses that remain in progress include aggregate stability, PLFA, particle size analysis (for soil texture), and water holding capacity. As expected, soil depth had a significant effect on many of the soil health indicators including POXC, MBC, respiration, and PMN (Table 1). Mean values were higher in the top depth for most of these indicators (Table 2). In general, the only soil health indicator that showed significant differences between ideal and challenging soils rated by the growers was POXC (p<0.01) (Table 1, Figure 1, and Figure 2). For POXC, mean values were overall higher in the challenging soils. Also, significant interactions between depth and grower rating were recorded for soil MBC and respiration in the tractor row only. Here, ideal soils had overall higher MBC mean values in both depths (Figure 4). Respiration mean values varied by depth with ideal soils having lower values in the top depth but higher values in the bottom depth compared to the challenging soils, trends can be observed in the Figure 3. Most indicators showed high variability (Table 2).
No significant differences were found between challenging and ideal soils physical properties like clay content, bulk density, and soil moisture content although they were strongly highlighted by growers (Table 2). Also, no significant differences were found between ideal and challenging soils for soil infiltration (p=0.615).
Table 1. Generalized linear mixed model analysis p values for the tractor tow sampled. The sites were set as the random effect and Depth and Grower Ratings were set as the fixed effects.
|
POXC |
MBC |
Resp. |
pH |
PMN |
NO3 |
NH4 |
soil moisture |
BD |
|
|
Tractor row |
|||||||||
Depth |
0.000*** |
0.041* |
0.000*** |
0.083 |
0.000*** |
0.217 |
0.961 |
0.003** |
0.383 |
|
Grower Rating |
0.006*** |
0.840 |
0.953 |
0.919 |
0.750 |
0.959 |
0.994 |
0.826 |
0.526 |
|
Depth X Grower Rating |
0.128 |
0.015* |
0.027* |
0.149 |
0.062 |
0.259 |
0.186 |
0.254 |
0.357 |
|
|
Vine Row |
|||||||||
Depth |
0.000*** |
0.004** |
0.000*** |
0.210 |
0.000*** |
0.024* |
0.786 |
0.630 |
0.740 |
|
Grower Rating |
0.000*** |
0.661 |
0.306 |
0.588 |
0.885 |
0.805 |
0.943 |
0.579 |
0.769 |
|
Depth X Grower Rating |
0.229 |
0.233 |
0.207 |
0.368 |
0.240 |
0.881 |
0.083 |
0.424 |
0.293 |
(*** = p <0.001, ** = p<0.01, * = p<0.05)
Table 2. Mean and standard deviations of each soil health indicator for the tractor row sampled.
|
Top Depth (0-10cm) |
Bottom Depth (10-20cm) |
||||||
Soil health indicators |
Ideal |
Challenging |
Ideal |
Challenging |
||||
|
mean |
sd |
mean |
sd |
mean |
sd |
mean |
sd |
POXC |
760.32 |
208.66 |
805.36 |
148.67 |
448.37 |
151.62 |
537.02 |
130.55 |
MBC |
183.42 |
136.43 |
159.30 |
101.73 |
98.85 |
63.08 |
113.07 |
97.65 |
Respiration |
29.54 |
22.74 |
36.74 |
19.01 |
19.73 |
10.88 |
16.29 |
10.56 |
pH |
6.98 |
0.40 |
6.94 |
0.45 |
6.83 |
0.50 |
6.87 |
0.56 |
EC |
109.93 |
62.01 |
218.34 |
399.08 |
80.56 |
59.28 |
186.13 |
441.74 |
PMN kg ha^-1 |
42.27 |
28.00 |
51.74 |
57.58 |
23.85 |
12.73 |
21.60 |
11.24 |
NO3 kg ha^-1 |
7.39 |
5.50 |
7.54 |
5.40 |
7.02 |
10.34 |
6.17 |
4.74 |
NH4 kg ha^-1 |
7.85 |
7.93 |
7.94 |
8.37 |
7.69 |
13.97 |
7.86 |
11.45 |
moisture |
0.21 |
0.09 |
0.21 |
0.08 |
0.20 |
0.12 |
0.19 |
0.06 |
Bulk density |
1.18 |
0.22 |
1.19 |
0.10 |
1.17 |
0.19 |
1.22 |
0.15 |
Table 3. Mean and standard deviations of each soil health indicator for the vine row sampled.
Vine Row |
Top Depth (0-10cm) |
Bottom Depth (10-20cm) |
||||||
|
Ideal |
Challenging |
Ideal |
Challenging |
||||
|
mean |
sd |
mean |
sd |
mean |
sd |
mean |
sd |
POXC |
613.40 |
158.51 |
732.79 |
138.04 |
361.01 |
118.25 |
477.06 |
150.19 |
MBC |
143.27 |
120.55 |
137.19 |
77.67 |
78.11 |
72.51 |
89.81 |
72.23 |
Respiration |
23.74 |
17.06 |
33.52 |
24.92 |
13.92 |
8.41 |
14.37 |
10.17 |
pH |
7.23 |
0.45 |
7.10 |
0.56 |
7.07 |
0.53 |
7.02 |
0.61 |
EC |
127.57 |
68.32 |
233.77 |
401.88 |
98.21 |
54.28 |
206.24 |
431.89 |
PMN kg ha^-1 |
34.74 |
21.27 |
38.05 |
30.90 |
17.83 |
11.68 |
18.62 |
13.39 |
NO3 kg ha^-1 |
10.29 |
8.39 |
10.86 |
9.91 |
7.97 |
6.66 |
8.95 |
8.00 |
NH4 kg ha^-1 |
9.74 |
23.31 |
5.86 |
5.70 |
5.33 |
4.91 |
5.29 |
5.36 |
moisture |
0.19 |
0.06 |
0.20 |
0.10 |
0.20 |
0.07 |
0.20 |
0.08 |
Bulk density |
1.24 |
0.14 |
1.23 |
0.14 |
1.21 |
0.16 |
1.23 |
0.20 |
Figure 1. Permanganate Oxidizable Carbon (POXC or active carbon) boxplots of the challenging soils (orange) and ideal soils (blue) for the tractor row. The black circles represent the values of the vineyards sampled. The columns represent the two depths sampled (A=0-10cm, B=10-20cm).
Figure 2. Permanganate Oxidizable Carbon (POXC or active carbon) boxplots of the challenging soils (orange) and ideal soils (blue) for the vine row. The black circles represent the values of the vineyards sampled. The columns represent the two depths sampled (A=0-10cm, B=10-20cm).
Figure 3. Mineralizable carbon or microbial respiration boxplots of the challenging soils (orange) and ideal soils (blue) for the tractor row. The black circles represent the values of the vineyards sampled. The columns represent the two depths sampled (A=0-10cm, B=10-20cm).
Figure 4. Microbial biomass carbon boxplots of the challenging soils (orange) and ideal soils (blue) for the tractor row. The black circles represent the values of the vineyards sampled. The columns represent the two depths sampled (A=0-10cm, B=10-20cm).
Work in progress:
This project is still ongoing and scheduled to be completed in August 1st of 2023. The following soil health analyses remain in progress: water holding capacity, aggregate stability, phospholipid fatty acids (PLFA) and particle size analysis (for soil texture). Multivariate analyses and more in depth data analysis will be performed upon completion of soil health indicators analyses.
Research Outcomes
Education and Outreach
Participation Summary:
The following educational/outreach product was conducted: 2 Oral presentations of preliminary results in the Agronomy Society of America 2021 annual meeting (Tri-Societies) in Salt Lake City, UT.
- Growers' Perceptions of Soil Health for Wine Grape Production in Napa Valley
- Evaluating Important Soil Health Indicators for Winegrape Production in Napa Valley
The following educational/outreach products are currently in progress:
- Journal Articles: 2
- Understanding Napa Valley Grape Grower Perceptions and Attitudes about Soil Health (in preparation).
- Assessing the variability of soil health indicators in collaboration with grape growers in Napa Valley, California (in preparation).
- Industry-oriented article: California Agriculture peer-reviewed open-access journal of the University of California Division of Agriculture and Natural Resources (UC ANR)
- Factsheets: 2
- Summary of Findings: What is a Healthy Soil for wine grape production? Assessing Soil Health Across California Vineyards
-
Educational: Role of soil organic matter and biological properties in vineyard soil health
- Webinars, talks and presentations: (specific events will depend on the acceptance of coordinators)
- California Small Farm Conference: Understanding and defining soil health in wine grape production; Soil organic matter, soil microbes and their role in supporting soil health.
- Grape Field day: Summary of Findings: What is a Healthy Soil for wine grape production? Assessing Soil Health Across California Vineyards