- Author: Michelle Leinfelder-Miles
- Author: Radomir Schmidt
The term ‘soil health' has become a common term in agricultural research and management. While most of us are familiar with testing soil for chemical properties, like nutrients, salinity, and pH, soil health also considers soil physical characteristics – like compaction, aggregation, and water infiltration – and biological characteristics – like soil respiration, active carbon, and nitrogen mineralization.
These properties influence the soil's ability to function, and enhancing these properties can improve soil functioning to grow crops and produce ecosystem services. We often relate soil health to management practices like crop rotation, cover cropping, reducing tillage, and adding compost because these have been shown to increase soil functioning in agricultural landscapes. They are also some of the practices that are financially incentivized by the CA Department of Food and Agriculture Healthy Soils Program.
There is a regulatory framework for diverting green waste from landfills to make compost. In 2014, AB 1826 was passed in California, which required businesses to recycle organic wastes and jurisdictions to set up organic waste recycling programs to divert green waste from landfills. In 2016, AB 1383 established organic waste reduction targets (75% reduction by 2025, compared to 2014). The bill also required jurisdictions to do education and outreach. Green waste diversion is expected to reduce greenhouse gas emissions by 4 million metric tons per year and increase food recovery by 20 percent. Agricultural land could serve to receive green waste compost recovered by this regulatory framework.
Our project objectives were to learn whether green waste compost improves soil nutrient status or other soil health characteristics, whether it improves alfalfa yield or quality, or if its application affects greenhouse gas emissions from the system. Alfalfa was chosen for this study because it has a large footprint on the state's agricultural landscape and because it has a high phosphorus (P) and potassium (K) nutrient need which compost could help supply. Also, as a ‘high-traffic' crop, alfalfa soils can have poor physical traits (e.g. compaction, water infiltration), which could potentially be ameliorated with compost.
The study was conducted on commercial farms in Yolo and San Joaquin (SJ) counties. The Yolo site had a mineral soil with high clay content (approximately 50 percent clay), and the SJ soil was a mucky clay with high organic matter (approximately 8 percent). We are comparing two green waste compost rates (3 and 6 tons per acre) to the untreated control. Compost applications were annually (2020-2022) surface-applied in the fall/winter ahead of rain.
Our preliminary results indicate no statistically significant differences in total carbon and nitrogen among treatments (Fig. 2). There is a trend, however, for compost to increase carbon at the Yolo site, which is inherently low in organic matter. An interesting observation about the SJ site, where the soil is inherently low in K, is that the compost increased soil K (statistically significant, Fig. 3). The compost analysis showed that the product was roughly 1 percent K. Therefore, the 3-ton compost rate should have added approximately 50 lb of K per acre, and the 6-ton rate approximately 100 lb of K per acre. Based on the amount of change in soil K and the compost analysis, the compost was likely what contributed to the increase in soil K. This appears to be translating into higher tissue K (Fig. 3), and in turn, higher yields (though neither tissue K nor yield are statistically higher than the control, Fig. 4).
Greenhouse gas emissions have not differed among treatments (Fig. 5), indicating that the carbon that is added by the compost is not being respired from the system. There are higher CO2 emissions at the SJ compared to the Yolo site, which we attribute to the inherently higher carbon of the SJ soil. Additionally, we have observed that the soil acts as a methane sink. This is noteworthy because methane is a more potent greenhouse gas than CO2.
Based on our experiences working on this project, we have the following guidance for growers interested in applying green waste compost. While green waste compost is a relatively cheap input, transport cost can be high. In 2021, we estimated that material plus hauling cost was approximately $27/ton and spreading was an additional $10/ton. The highest demand for compost is in the fall. To ensure availability, growers should aim to purchase compost in the spring or summer and store it on-site until fall. Ordering the compost in spring or summer also tends to result in a higher quality product delivered (i.e. less trashy). Timing compost application can be a challenge (i.e. after all harvests but before soil gets too wet), so having the compost already on-site may help in getting it applied more readily. We still have more data to analyzed for this project, so more information will be forthcoming. We want to thank the growers in Yolo and San Joaquin counties for collaborating with us on this project.
- Author: Michelle Leinfelder-Miles
- Author: Rachael Long
- Author: Radomir Schmidt
Since Fall 2020, I have been evaluating the effects of applying green waste compost on established alfalfa. The three-year project includes two trials – one in the San Joaquin County Delta and the other in Yolo County – and is a collaboration with Rachael Long (UCCE) and Radomir Schmidt (UC Davis). The project is supported by a CA Department of Food and Agriculture Healthy Soils Program (CDFA HSP) demonstration grant. Our interests are in evaluating whether compost enhances soil carbon and nitrogen storage, improves soil physical characteristics (i.e. improved water infiltration, reduced compaction), reduces greenhouse gas emissions, and/or boosts alfalfa yield.
Compost is decomposed organic matter from plants or animals and may be classified by the carbon-to-nitrogen ratio (C:N). The C:N is the relative amount of carbon and nitrogen in the material. Plant-derived composts (like green waste compost) have a high C:N, and animal-derived composts (like composted manures) have a low C:N. A material with a ratio greater than 30:1 is considered a high C:N material. The ratio is important because it affects microbial metabolic functioning and plant-available nitrogen. Both high and low C:N composts promote soil functioning by increasing soil carbon that is in a form easily accessible to microbes. That, in turn, can improve soil biological activity and physical conditions. With a high C:N material, however, nitrogen may be immobilized (“tied up”), so soil nutrient monitoring is important in order to stave off impacts to crops.
The San Joaquin County trial is approximately 20 acres, and there is no history of compost application at the site. The soil is a Peltier mucky clay loam that is considered partially to poorly drained. Compost applications are surface-applied in the fall/winter to plots that are two border checks wide (120 ft) and approximately 1000 ft long. Two green waste compost rates – 3 tons/ac and 6 tons/ac – are being compared to the untreated (non-composted) control. The first compost application was made in Fall 2020 following the first cutting season of the alfalfa stand. The second application was made in Winter 2021, and the final will occur in fall/winter 2022. Baseline soil samples were collected at the beginning of the study (October 2020), and annual sampling is done every fall season before compost application. Alfalfa yield is assessed 3-4 times per year by taking quadrat samples from the grower's windrows. Greenhouse gas samples are collected on a monthly basis.
Preliminary results. Yield was measured from three cuttings in 2021, and so far, from two cuttings in 2022. (We anticipate measuring yield from two more cuttings in 2022.) Our preliminary results from these five cuttings indicate that compost can improve alfalfa yield over the untreated control but that a rate of 6 tons/ac does not improve yield over the 3 tons/ac rate (Fig. 1). We are also testing forage quality, and those results will be available in the fall.
I recently held a field day at the trial location. If you were not able to make it, please visit my website for the handouts. The handout “Compost for Soil Improvement in Alfalfa” shows other preliminary results from this trial, including soil carbon and nitrogen and greenhouse gas emissions. In addition, there are handouts describing other organic matter amendments in alfalfa and forages.
Figure 1. Preliminary yield results over five cuttings in 2021 and 2022. The compost rate of 3 tons/ac improved alfalfa yield over the untreated control.
Conclusions. Organic matter amendments, as from compost, can improve soil functioning, but changes take time to observe, let alone be realized financially. We estimate that compost (material plus hauling) costs approximately $27/ton, with an additional $10/ton for spreading (Fig. 2). To help offset the costs, the CDFA HSP provides incentives grants for farmers, and more funding may be available later this year. UC ANR Technical Service Providers Hope Zabronsky or Caddie Bergren are available to help growers with the application. And please don't hesitate to reach out to me if you would like more information on this trial or on the CDFA incentives programs.
Figure 2. Compost spreading at the San Joaquin County trial. Compost is not a small expense, but it may help improve soil functioning and alfalfa yield over the long-term.
UC Cooperative Extension and UC Davis will host a Healthy Soils Program field meeting on compost. The meeting will take place on Thursday, July 28th from 10:00am to 11:30am. The meeting will take place off of S. Landi Road, on Roberts Island in the Delta. Presentation topics include how to acquire compost, different types of compost, how compost can improve soil health and mitigate greenhouse gas emissions, and how to apply for cost-share funding. The meeting location is where we are trialing different rates of green waste compost application for potential soil health and alfalfa yield benefits. Preliminary results will be described. Attendance is free, and registration is not required. Continuing education credits will be offered (CCA and N management applications pending). The agenda is pasted below, and a downloadable version is attached. Thanks for your interest in UC Cooperative Extension programming, and we hope to see you later this month!
Over the last few years, I have been working on a project to characterize a suite of soil health properties in alfalfa receiving full and deficit irrigation. Soil health has been described as the ability of soil to function and is characterized by biological, chemical, and physical soil properties that are sensitive to changes in management. The idea for this project developed after the 2012-2015 drought when water shortages and regulatory curtailments meant that growers had to make tough decisions on how to apply scarce water resources. Some growers opted to cut irrigation to alfalfa since it is a deep-rooted crop that can scavenge water and nutrients from the soil profile. (See this recent blog post by UC Alfalfa and Forage Specialist Dan Putnam, and Farm Advisor Rachael Long on the resiliency of alfalfa during drought.) I had a hunch, however, that while alfalfa may be adapted to survive drought conditions, soil health properties might be negatively impacted because water is essential to life in the soil, facilitates nutrient movement and availability, and influences soil physical characteristics, among other things. Fortunate for me, there was a research trial at UC Davis where I could test this idea.
I view alfalfa as a model crop for studying soil health under restricted water conditions because practices like crop rotation and tillage do not occur over the four or more years of an alfalfa stand. Therefore, those practices would not confound the results. From this experiment, we are learning how imposing varying levels of deficit at different stages of the cropping season impact soil properties, which will help us optimize deficit irrigation strategies for alfalfa. Additionally, the deficit treatments serve as a proxy for drought and could potentially demonstrate how prioritization of water uses during drought may impact soil conservation outcomes.
Data analysis is ongoing, but preliminary results suggest that soil health may not be resilient under deficit irrigation or drought, even if alfalfa is. When the trial began in Spring 2019, there were no differences in rootzone salinity among treatments, which averaged 0.41 dS/m. After two cropping seasons where deficits were imposed, the 60 percent ETc treatment with the water cut-off toward the end of the season (CT) resulted in significantly higher rootzone salinity down to the 36-inch depth (Figure 2). The salinity in that treatment was higher than even the 40 percent ETc treatment that had the sustained deficit (SD) throughout the entire season. In other words, it appears that the timing of the deficit is more important than the amount of deficit, and applying water throughout the season – even if the amount is severely reduced – appears to mitigate salinity build-up in the rootzone. Of note, salinity is not high enough to be problematic at this site. The overall ECe of the soil is low, and water quality is generally good at this location. I would expect, however, that in locations where soil and/or water has higher salinity to begin with, then deficit irrigation that includes a water cut-off could be problematic.
There will be a lot more information to come about this project in the near future, but the salinity information seemed timely to share given our current water year. In addition to Dan, Isaya, and Umair, I want to acknowledge Daniel Geisseler (UC Nutrient Management Specialist), Will Horwath (Professor of Soil Biogeochemistry), and graduate student Veronica Suarez Romero who have helped on soil nitrogen and carbon testing. I also want to acknowledge the South Delta Water Agency for financial support of the project.
Soil health has been described as the ability of soil to function and is characterized by biological, chemical, and physical soil properties that are sensitive to changes in management. Studies have shown that reducing tillage, increasing rotational crop diversity, and employing cover crops during the fallow season can improve soil health characteristics. In turn, improvements in soil health may enhance various soil functions, like improving crop productivity, reducing input costs, and buffering plant health from living and non-living stresses.
Over the last year and a half, I have been working on a project to characterize a suite of soil health properties in alfalfa receiving full and deficit irrigation. When I was developing the project, I had two hunches. The first was that alfalfa production may improve certain soil health characteristics. Alfalfa provides soil coverage for several years. Alfalfa is also a deep-rooted crop that can scavenge water and nutrients deep in the soil profile. Additionally, alfalfa has been shown to provide a nitrogen benefit to subsequent crops. For reasons like these, I hypothesized that certain soil health characteristics might improve over the years of an alfalfa stand.
My second hunch, however, was that deficit irrigation could negatively impact soil health properties. In recent years, California alfalfa production has received negative press for water usage that exceeds that of other crops. Alfalfa does represent an important footprint in California's agricultural water use. Alfalfa has a high water demand (i.e. crop evapotranspiration, ETc) that is directly related to yield. All else being equal, as ETc increases, alfalfa yield also increases up to maximum ETc. Studies conducted by UC Alfalfa and Forage Specialist, Dan Putnam, have demonstrated, however, that alfalfa is resilient under water deficit conditions. While alfalfa may be resilient under deficit irrigation, water facilitates soil microbe functioning and nutrient availability. Therefore, I also hypothesize that soil health may degrade under deficit irrigation. This is critical knowledge to develop not only for deficit irrigation strategies but also in the event of drought, where growers may be asked to sacrifice crop irrigation for water transfers to other uses. Such knowledge could demonstrate how prioritization of water uses may impact soil conservation outcomes.
This project is being conducted at UC Davis on a Yolo silt loam and was initiated in Spring 2019. The treatments, which are replicated four times, are: 1) full irrigation (100 percent ETc), 2) full irrigation at the beginning of the season with a sudden cutoff toward the end of the season (60 percent ETc), 3) gradual deficit where each irrigation imposes restriction (60 percent ETc), and 4) more-severe gradual deficit (40 percent ETc). The treatments are applied using overhead irrigation – an 8000 series Valley 500 feet, four span linear-move system. The site allows us to observe soil characteristics under different levels of deficit, imposed at different stages of the cropping season. Soil sampling occurs twice each year – in the spring before irrigation begins and in the fall after the last irrigation. We are testing a comprehensive nutrient analysis, organic matter, total carbon and nitrogen, salinity, compaction, bulk density, N mineralization, and particulate organic carbon.
This project is ongoing, but we have interesting preliminary results. One example is with particulate organic carbon. Particulate organic carbon (POC) is a biological indicator of soil health because it is the fraction of soil organic matter that is readily available as an energy source for soil microorganisms. Though not statistically significant (Fig. 1, P = 0.066), there is a trend for the higher irrigation rates – and sustained irrigation throughout the season – to have higher POC. Between the two irrigation treatments providing approximately 60 percent of ETc, the third treatment which provides a sustained deficit throughout the season tends to have higher POC than the 60 percent ETc treatment that has a sudden cut-off of water about two-thirds of the way through the season. These results suggest the importance of water in sustaining soil biological activity.
Likewise, there is a similar trend for higher irrigation rates and sustained irrigation through the season to result in lower soil compaction readings (Fig. 2, P = 0.2691). At the 6-inch depth, the full irrigation rate trended toward having the lowest compaction, followed by the 60 percent sustained deficit treatment. All treatments, however, had average readings below 300 psi, which is the pressure above which root growth is believed to be constrained.
These are preliminary results. We will soil sample again this fall, and hopefully again next spring and fall, to see if these trends continue. I want to acknowledge my UC Davis collaborators on this project: Dan Putnam, Isaya Kisekka, Daniel Geisseler, Umair Gull, and Veronica Romero. I also want to acknowledge the South Delta Water Agency for funding support.
Figure 1. Average particulate organic carbon (top 12 inches) across four irrigation treatments and three seasonal readings (Spring 2019, Fall 2019, Spring 2020).
Figure 2. Average soil compaction at 6-inch depth across four irrigation treatments and three seasonal readings (Spring 2019, Spring 2020, Summer 2020).