Methods: The trial is a randomized complete block design (approximately 4.5 acres) with three replicates of each treatment. The soil type across the trial is a Valdez silt loam. Baseline soil samples were collected in July 2018 following wheat harvest but prior to tillage. Soil was sampled from 0-6, 6-12, 12-24, and 24-36 inch depths. On July 30, 2018, a cowpea cover crop (Vigna unguiculata cv. ‘Red Ripper', Figure 1) was inoculated with Rhizobium and planted after a pre-irrigation. Pre-irrigation was only applied to the cover crop plots. The cover crop was drill-seeded at 7-in row spacing with a planting density of approximately 50 pounds of seed per acre. A second irrigation was applied approximately one month after planting. End-of-season soil sampling (0-6 and 6-12 inch depths) occurred on October 23, 2018, prior to cover crop termination. Soil properties of interest include bulk density, soil moisture, salinity, pH, total nitrogen (N), and total carbon (C). Soil properties were analyzed by the following methods: pH from the soil saturated paste, salinity by the saturated paste extract, and total N and C by combustion method.
Preliminary Results: Soil properties are presented for the baseline condition (Table 1) and for the end of the first cover cropping season (Table 2). Bulk density averaged 1.0 g/cm3 across sample timings, depths, and treatments. Soil moisture (% by volume) was observed to increase from the baseline condition in the cover crop (“CC”) treatment. At baseline sampling, salinity increased with depth from 0.47 to 2.44 dS/m. After one cover cropping season, salinity increased in both treatments, but increased more in the no cover crop (“No CC”) treatment, averaging 1.22 dS/m from 0 to 12 inches. Soil was acidic, which is typical for the region. The pH averaged 5.5 across sample timings, depths, and treatments, but there may be a trend for cover cropping to increase the pH. Total N and C decreased with depth at the baseline sampling. After one cover cropping season, there was little change from the baseline condition in both treatments.
Summary: The Delta is a unique agricultural region with unique environmental challenges. Some soils in the region are subsided due to oxidation of organic matter, and some soils suffer from salinity, having limited ability to leach salts due to low permeability soils and shallow groundwater. Cover cropping is not a typical practice in the annual crop rotations of the region, and summer cover cropping is particularly rare. After the first year of a three-year study, cover cropping had no observed effect on bulk density, Total N, and Total C. Cover cropping may have slightly raised the pH in the top 12 inches, compared to dry fallow. The cover crop treatment, having received two irrigations, had lower salinity in the upper layers of soil compared to dry fallow. We also observed that the 2018-2019 triticale crop that was planted in the field germinated roughly five days earlier in the cover crop plots compared to the fallowed plots. Thus, it appears that summer cover cropping with a legume has the potential to improve soil tilth at a time of year when the field would otherwise be fallowed and dry with no soil cover, and there could be agronomic benefits to subsequent crops. We will continue to monitor these soil properties in 2019 and 2020, and additionally, we will monitor small grain yields and greenhouse gas (CH4, N2O) emissions.
We would like to thank Dawit Zeleke and Morgan Johnson (Staten Island), Tom Johnson (Kamprath Seed), and Margaret Smither-Kopperl and Valerie Bullard (USDA-NRCS) for their cooperation on this trial. We would like to acknowledge the California Climate Investments program for funding, and our UC colleagues who are cooperating on this grant in other parts of the state (Jeff Mitchell, Will Horwath, Veronica Romero, Sarah Light, Amber Vinchesi-Vahl, and Scott Stoddard).
Survey: We would also like to alert readers of a cover cropping survey that is being conducted by the Contra Costa County Resource Conservation District. The survey is found here. The purpose of the survey is to learn more about cover cropping practices and barriers to adopting cover cropping on-farm. Even if you farm in another county, please consider filling out the survey, which should take about 10 minutes. The survey is open through the end of June. Your responses will help inform CCC RCD and UCCE programming. Thank you for your participation.
The 2012-2016 drought was one of the worst droughts in California history, not solely for the lack of precipitation, but also for its length, high temperatures, low snowpack, and water demand. It's probably safe to say that it won't be our last drought – or even our worst – as we look into the future. That said, what can we do in the California alfalfa industry to better manage for drought and the likely salinity impacts from a lack of water?
Water Management during the Growing Season: Dan Putnam wrote a blog article, “Why Alfalfa is the Best Crop to Have in a Drought” (http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=17721), which describes the water use of alfalfa compared to other crops, its adaptations to water-stressed conditions (like being deep-rooted), and ways we can adapt our management in low-water years. In particular, during the growing season, we can optimize water use and alfalfa growth during the early part of the season when yield and quality are highest, and dry down in the later part of the season. Dan's research has shown that the alfalfa will survive and resume growth when moisture conditions become favorable.
Water Management during the Winter Season: In 2013 through 2015, I cooperated with alfalfa growers in the Delta region to understand soil salinity conditions and leaching fractions in fully irrigated fields. I then modelled soil moisture and salinity conditions to understand these conditions during the winter season to help inform our management during the off-season.
Figure 1 (below) shows the daily water balance (precipitation minus crop evapotranspiration, ETc) and the change in soil moisture from field capacity (i.e. soil moisture after free drainage has ceased) at seven alfalfa fields during Winter 2013-14. This figure helps us to visualize why precipitation, particularly in a drought year, is not contributing more to soil moisture for early spring growth or to leaching salts. Total rainfall was approximately 8.2 inches, and for only a few storms (shown as peaks) was there enough precipitation to exceed crop water use (ETc).
The other lines on the graph (labelled Sites 1-7) illustrate the soil moisture deficit from field capacity. Soil moisture is expressed relative to field capacity because a primary interest in this modelling was to understand how much water is available for leaching salts. Until a soil reaches field capacity, we assume the water is held in the soil and not available for leaching. For all sites, the soil was drier than field capacity in the fall after the last cutting and before the first rain event. The lines decrease (i.e. become more negative) until December 1st because crop water use exceeded precipitation, so the crop drew upon soil moisture. On December 1st, there was a rain event that was enough to exceed ETc, so the soil moisture deficit decreased, but soil moisture was still less than field capacity. This trend continued for the remainder of the winter. If there had been enough precipitation to increase soil moisture above field capacity, then water would have been available for leaching, but this did not happen in Winter 2013-14. Precipitation rarely exceeded ETc, and each alfalfa site remained at a soil moisture deficit over the entire winter. In other words, precipitation was never high enough to fill the soil profiles, exceed the soils' field capacity, and leach salts.
Figure 2 (below) represents conditions for water year 2014-15. Total rainfall was approximately 11.8 inches, and precipitation exceeded ETc more frequently than in water year 2013-14. There was a period starting on December 11th where soil moisture exceeded field capacity (for all but Site 5), providing water for leaching. The highest peak on each site's line represents the total water available for leaching after accounting for ETc and filling the soil profile to field capacity. This peak occurred on December 20th and was 0.8, 3.3, 1.1, 1.8, 0, 1.4, and 1.2 inches, for Sites 1-7, respectively. (Site 5 was 0 inches because the soil moisture deficit remained the entire year; thus, zero water was available for leaching.) As this water was available for leaching, we assume that this water drained from the profile, and the lines drop to zero, or field capacity. Beyond December 20th, the daily water balance was never enough to exceed field capacity for any of the sites. (Note: the lines for all sites, except Site 5, overlap after December 20th.) So, no other water was available for leaching over the remainder of the winter season.
Conclusions: The 2012-2016 drought provided limited ability to manage salts with winter rainfall. For seven Delta alfalfa sites, we modelled 0 inches of rainfall available for leaching in Winter 2013-14. We modelled a range of about 0 to 3 inches of rainfall available for leaching in Winter 2014-15, depending on location. As a result, root zone soil salinity decreased in Spring 2015 (data not shown). When winter rainfall is not adequate for effective leaching, however, we need to be creative in our leaching strategies. Leaching during the season may not be advisable for crop health and nutrient management reasons, but we may be able to leverage winter rainfall with irrigation by wetting the soil profile before a rain event. A soil profile that is brought to field capacity with irrigation would likely result in rain water passing through the profile and leaching salts, rather than just soaking into a dry soil. We should also consider field modifications that improve irrigation efficiency prior to planting alfalfa, like increasing on-flow rate, narrowing border checks, or shortening field length, where possible. While drip irrigation in alfalfa is still not widely employed, in those fields that have it, it might be wise to also maintain a surface irrigation system for leaching. Our options are not many, but they could provide some relief when water is scarce.
Figure 1. The daily water balance (i.e. precipitation minus ETc) and the change in soil moisture from field capacity for Winter 2013-14 at seven Delta alfalfa sites. This model shows that there was no water available for leaching. All rainfall was soaked up and held by the soil.
Figure 2. The daily water balance (i.e. precipitation minus ETc) and the change in soil moisture from field capacity for Winter 2014-15 at seven Delta alfalfa sites. This model shows that there was some water available for leaching in mid-December, ranging from about 0-3 inches, depending on location.
I recently visited a bean field in the southern part of the county with a PCA. From a distance, the beans in certain areas of the field appeared to be drying up and dying. A closer look showed that the leaf margins were drying up first before the whole plants declined. Pulling up plants by the roots, they appeared to show some reddish root lesions. Soil moisture was good – it seemed neither too wet nor too dry, but there was white crusting on the soil surface of the furrows.
As I was thinking about what could be happening with the beans, a couple things were running through my mind. The patchiness of the problem in the field and the reddish roots made me think that Fusarium root rot (Figure 1) may be a problem. The PCA believed that there had been tomatoes in the field the previous year but that there may have been beans in the field just two years ago. I wondered whether the white crusting on the soil was due to salt. The PCA said that he thought the field was irrigated with groundwater.
To put something behind my hunch, I sent plant samples up to the disease diagnostics lab at UC Davis. Tests confirmed that both Fusarium and Rhizoctonia inoculum were present on the plant roots and that the Fusarium inoculum was particularly high. Fusarium spores can survive in the soil for several years, and UC IPM guidelines suggest rotating out of beans for at least three years in Fusarium-affected fields. Unfortunately, Fusarium spores will live in the soil even when bean hosts are not present.
Stress conditions in the field can worsen Fusarium infection, particularly conditions of too much or too little water, compaction, and salinity. We tested the soil salinity at this site and found the electrical conductivity (EC) of the surface soil to be around 5.0 decisiemens per meter (dS/m). Beans are very sensitive to salinity, and yield declines are expected when rootzone soil salinity is as low as 1.0 decisiemens/meter. It would appear that salinity could be stressing the beans and causing them to be more susceptible to the Fusarium inoculum in the soil. Because this grower is irrigating with groundwater, I would recommend that he get his water tested for salinity. If the water salinity is acceptable, then he should consider how he will leach the field this winter, perhaps augmenting rainwater with irrigation water (assuming normal-to-low precipitation this winter). If his groundwater is high in salts, then he should consider using a different water source for irrigating and leaching (if available) and rotate to more salt-tolerant crops, like small grains, for at least three years.
In addition to the production manuals previously mentioned, I also consulted UC production manuals produced in the 1950's, including Dry Edible Bean Production in California (1954), Blackeyes: Costs of Production, Suggestions on Growing (1956), and Production of Dry Edible Lima Beans in California (~1951).
In late August, I visited a kidney bean field that was exhibiting stress symptoms, like necrotic leaves and stunting. Even though the field was near harvest, the consultant asked if I would take a look at it. Upon pulling up some plants, I noticed some brick red lesions on the roots, and when I pulled the roots apart, I saw that the brick red color ran the length of the conducting tissue. This is characteristic of Fusarium root rot (Fusarium solani).
Fusarium root rot can be a problem in mid- to late- season beans but generally only when the plants are experiencing some other stress. The other stresses may include lack of moisture, too much moisture (and low oxygen), poor nutrition, or salinity. I will speak to salinity below. Fusarium root rot chlamydospores can survive in soil for years, so the UC IPM recommendations are to rotate out of beans for at least three years. While there are no resistant varieties, some varieties are more tolerant than others, so check with your seed supplier for variety recommendations.
In the case of this field, I wondered whether salinity was the stress that encouraged the Fusarium root rot. Beans are considered sensitive to salinity, with yield reductions expected when the electrical conductivity of the soil saturated paste (ECe) exceeds 1.0 dS/m or when the electrical conductivity of the seasonal average applied water exceeds 0.7 dS/m. I sampled soil from about the top eight inches and found the ECe to be 1.025 dS/m. This barely exceeds the guideline salinity target, but it indicates that salinity could have been contributing to plant stress. The crop consultant was going to follow-up by testing the irrigation water salinity. Overall, I hope that winter rains will come and leach the salts below the root zone; nevertheless, the presence of Fusarium root rot would guide me away from planting beans in this field for a few years.