- Author: Lynn M. Sosnoskie
While soil health is currently a big buzzword in CA agriculture, UC Davis researchers want to dig into how managing CA soils to build soil health indicators impacts a grower's crop management decisions, productivity, and economic bottom line.
To further investigate these issues, UC Davis soil scientists are looking for processing tomato growers interested in participating in a soil health survey in summer 2019. The research will provide insight into the relationship between soil health indicators (which include soil chemical, biological, and structural/physical factors) and crop management, including how certain aspects of soil health impact fertility management and tomato yields.
To do this, researchers will ask participating growers to choose 2-3 fields for researchers to survey, including what they view as their "best" and "worst" fields, in either subsurface drip or furrow irrigation. Growers will also be asked to provide information on the history of the fields sampled, including crop rotation, duration in drip irrigation (if applicable), a general description of inputs management, as well as their own perspectives on soil management. Soil collected from growers' fields will be analyzed for soil texture, N, P, K, Ca, Mg, Na, pH, organic matter, cation exchange capacity, electrical conductivity, and aggregate stability. Soil microbiological factors will also be measured, including bacterial and fungal biomass, mycorrhizal biomass, and microbial carbon and nitrogen pools.
Each participant will receive a detailed report on test results of their fields and overall findings from the study, though all results from individual fields and farms will be anonymized with all identifying information removed when being shared with anyone other than the grower.
UC Davis researchers hope that this study will contribute to knowledge of how soil health status impacts management decisions for annual vegetable growers on the ground, including how soil health can contribute to agroecosystem productivity, prosperity, and sustainability for California farms.
Please contact Nicole Tautges, UCD crop scientist, for more information or to request sampling on your farm.
Nicole Tautges
Cropping Systems Research Manager
Russell Ranch Sustainable Agriculture Facility
University of California, Davis, CA 95616
Ph: 530-219-5380
Email: netautges@ucdavis.edu
- Author: Sara Tiffany
- Author: Dr. Martin Burger
The Solution Center for Nutrient Management brought together growers, advisors and university researchers for a breakfast meeting to discuss nitrogen management in processing tomatoes. A number of growers attended to share their experiences and learn about research including a new protocol for accurate soil nitrate sampling, and the latest updates on agricultural greenhouse gas emissions research. Cooperative Extension Specialist Daniel Geisseler also presented the CDFA-FREP website that provides Fertilization Guidelines for California's Major Crops, including tomato: https://apps1.cdfa.ca.gov/FertilizerResearch/docs/Guidelines.html
Research Highlights:
Soil nitrate sampling protocol
For maximum accuracy that can reliably predict nitrate availability in the soil, growers should sample according to the following protocol:
- For fields with 60-inch beds: soil cores should be taken at 3 lateral distances from drip tape, in at least 4 locations within a field.
- For fields with 80-inch beds: soil cores should be taken 2 lateral distances from drip tape, in at least 3 locations within a field.
click here to read full summary (or scroll down)
Research on agricultural greenhouse gas emissions in tomatoes
- The adoption of subsurface drip irrigation substantially reduces greenhouse gas emissions in tomato production (compared to furrow irrigation).
- Use of nitrification inhibitors lowers nitrous oxide emissions in tomato fields with subsurface drip irrigation.
click here to read full summary (or scroll down)
Full Summaries:
Soil nitrate sampling protocol
UC Davis researcher Dr. Martin Burger presented the results of a survey conducted by post-doctoral scholar Cristina Lazcano on pre-plant nitrate, phosphorus (Olson-P), and exchangeable potassium levels in 16 processing tomato fields in Yolo, San Joaquin and Fresno counties. The purpose of the study was to develop an economical sampling protocol that reliably predicts nitrate availability and allows growers to adjust fertilizer rates taking the residual soil nitrate into account.
While the conversion to subsurface drip irrigation has enabled growers to precisely deliver water and nutrients close to plant roots, there is still pressure for growers to increase nitrogen use efficiency, for example to reduce the risk of nitrate leaching. Previously, the spatial distribution of macronutrients in fields under drip irrigation was not well known. One concern has been that nitrate may accumulate at the periphery of the wetted soil volume, whereas the less mobile nutrients phosphorus and potassium may be depleted near the drip tape where roots can be expected to proliferate.
According to the survey encompassing more than 1000 soil analyses, pre-plant nitrate levels in the 16 fields varied widely, ranging from 45 – 438 lbs NO3- - N per acre in the top 20 inches of soil, with higher levels of nitrate found in fields under consecutive tomato cultivation. No depletion of Olsen-P or potassium in the root feeding areas close to the drip tape was detected. The majority of the fields showed phosphorus concentrations lower than 15 ppm, which based on earlier research is the threshold below which a yield response can be expected from a P addition. In contrast, potassium levels were higher than previously reported values, ranging from 293 ppm on average in Yolo County to 468 ppm in Fresno County.
The nitrate sampling protocol was based on a Minimax analysis by selecting the minimum number of samples within the field and locations within the beds (i.e. lateral distance from the drip tape). The combination of samples with the lowest relative error across all fields (< 5% from the field average) and the lowest number of samples taken was selected as the best sampling procedure to estimate average soil NO3-N. The analysis showed that soil cores should be taken at three (60-inch beds) or two (80-inch beds) lateral distances in at least four (60-inch beds) or three (80-inch beds) locations within a field.
Table 1. Pre-plant nitrate sampling protocol for 60-inch beds in Yolo (Y), San Joaquin (SJ), and Fresno (F) County SDI tomato fields.
Table 2. Pre-plant nitrate sampling protocol for 80-inch beds in Yolo (Y), San Joaquin (SJ), and Fresno (F) County SDI tomato fields.
***The full article about this study will appear in the Oct-Nov-Dec 2015 issue of California Agriculture.
Research on agricultural greenhouse gas emissions in tomatoes
An update on agricultural greenhouse gas emissions research included results of field studies testing a nitrification inhibitor for mitigation of nitrous oxide in subsurface drip irrigated tomato.
Nitrous oxide (N2O) is arguably the most important greenhouse gas produced in the agriculture sector, with its global warming potential 300 times that of Carbon Dioxide. N2O is produced by soil microbes during N transformations. N2O is a by-product of nitrification and denitrification.
Recent studies have shown that N2O produced during nitrification can be as important as that resulting from denitrification (Zhu et al., 2013). The highest rates of N2O emissions typically occur shortly after N fertilizer applications when soils are re-wet. The main regulatory factor is the availability of oxygen since microbes use nitrate (denitrification) and nitrite (nitrification) as electron acceptors of respiration when oxygen is in short supply. Soil processes that consume oxygen, such as the presence of a carbon source, and conditions that limit replenishment of oxygen levels in the soil, such as high soil water content, promote N2O production in soil. Compacted soils lead to rapid depletion of oxygen because of the reduced air spaces and greater tortuosity of pathways of oxygen diffusion.
Although the use of the nitrification inhibitor significantly lowered nitrous oxide emissions in SDI tomato in one of the two years of the study, the reduction in absolute values is rather small (64 lbs carbon dioxide per acre) to make a significant contribution to California's greenhouse gas inventory. With the adoption of subsurface drip irrigation, tomato growers have already lowered the impact of greenhouse gas emissions from tomato production substantially as furrow irrigation generated leads to greater nitrous oxide emissions than SDI.
References
Zhu, X., Burger, M., Doane, T.A., Horwath, W.R., 2013. Ammonia oxidation pathways and nitrifier denitrification are significant sources of N2O and NO under low oxygen availability. Proceedings of the National Academy of Sciences of the United States of America 110, 6328-6333.
- Author: Rob York
Article reviewed: Subsurface carbon contents: Some case studies in forest soils
By D.W. Johnson, J.D. Murphy, B.M. Rau, and W.W. Miller, published in the journal Forest Science, Vol 57, 3-10
The plot line: This is an article that was part of a special issue of Forest Science that was born from a conference on the importance of carbon in deep forest soils (it is not surprising that, yes, soil scientists think it is important!). This article emphasizes the need for understanding the pattern of carbon content as one goes deeper into forest soils. The pattern is highly relevant because deep soils are rarely sampled because of the physical difficulty involved in getting to deep soils (it’s a lot of digging). If one knows the pattern pretty well, then one can sample shallow soils and then estimate how much carbon is deeper if they are confident of the pattern. They found two basic shapes- linear (total carbon increases at a constant rate with depth) and asymptotic (total carbon increases at lower rates as you go deeper). The linear soils tended to have about 50% of their carbon below 20 cm (the common depth of sampling), while asymptotic tended to have about 35% below 20cm. The conclusions seemed to be that, it is reasonable to sample only part of the soil horizon and then extrapolate for estimating lower depths, but the correct extrapolation equation (i.e. the mathematical representation of the pattern) has to be used.
Relevant quote: “…there is a tendency to either ignore C and nutrient stores in deeper soil horizons, perhaps producing significant bias in soil C in global scale modeling efforts or to resort to modeling soil C contents of deeper soil horizons. ”
Relevance to landowners and stakeholders:
This article is largely for academics, so it is difficult to find direct relevance for landowners and stakeholders. Instead, I refer readers to the previous discussion of carbon accounting on September 4, 2009. The main point of that discussion- that we are a long ways off from being confident in estimating carbon in forests with great accuracy- still seems to be the case today. Studies like this, however, move us closer to the “gold standard” in carbon accounting that will someday be needed for true carbon markets to develop.
Relevance to managers:
I recently tried to bone up on the issue of carbon in forest soils because foresters are now required to address impacts of forest treatments upon carbon when conducting environmental impact reports. I liked this article because it focused on missing data- specifically data estimating the carbon in the deep soil profiles that are usually not sampled. For managers, the entire amount of carbon in soils is often virtually unknown compared to above ground carbon. It has always been part of a foresters job to understand how much wood volume (i.e. carbon) is standing above-ground in the forest, but it has only recently become part of our job to also understand how much carbon is below-ground.
One item of relevance from this article seems to be that there are two major sources of variability when it comes to carbon in soils: one is the pattern at which carbon content changes as one gets deeper in soils; the other is the total amount of carbon that exists from location to location. The latter can be estimated, but only if the former is understood with relatively good certainty. When applying estimates of belowground carbon to total forest carbon budgets, it probably makes sense to check to see how deep soil carbon is estimated, if at all.
Another relevant point is that there is likely to be carbon in very deep horizons that is unaccounted for when below ground carbon is estimated. In this article, soil carbon was estimated down to between ½ and 1 meter. Where deeper soils exist, a significant amount of carbon is not being estimated if carbon amounts are extrapolated to only ½ or 1 meter. This is less of a problem for soils with asymptotic patterns of soil carbon (the amount of carbon declines with depth). But even for these soils, a true asymptote was not reached within ½ to 1 meter depth so carbon would still remain unaccounted.
Critique (I always have one, no matter how good the article is):
My only critique is that the selection of the two models used to represent the pattern of how carbon changes with soil were not justified in a statistical sense. They fit patterns to either a logarithmic or asymptotic (a “Langmuir” equation) model, but don’t explain how one or the other model was selected. Often it is the correlation coefficient or a model selection criteria that is used to pick the “best” model. It is only a minor critique since both models are relatively parsimonious and do not have as much need for a model selection approach.