Lab 5 Soil organic matter and soil nitrogen

Benefits of SOM

You already know about many of the benefits of soil organic matter, but here is a little review from the Food and Agriculture Organization (FAO) of the United Nations. 

Policies to incentivize building SOM

Governments, non-governmental organizations, grassroots groups, and innovative farmers and ranchers worldwide are increasingly attracted to management practices aimed to build soil organic matter and soil health. To support farmers and ranchers in building soil organic matter, California has rolled out the Healthy Soils Initiative. Learn from Karen Ross, Secretary of the California Department of Food & Agriculture, what the Healthy Soils Initiative is about. 

Building SOM and measuring success

Now that you know all about the importance of soil organic matter, you might be interested in helping a farmer or rancher in building soil organic matter. In order to be successful at building soil organic matter, you need to know which practices promote soil organic matter stabilization. Additionally, you will want to measure the current soil organic matter level, and monitor changes in soil organic matter over time, in response to your management actions. Read the bulletin by Sullivan et al. 2019 to learn more about building and measuring soil organic matter.  

1. List at least 4 benefits of building soil organic matter
2. The FAO video highlights the importance of measuring and monitoring soil organic carbon levels on a global scale. List two ecosystems highlighted in the video as having high importance for protecting soil organic carbon stocks and explain why. 
3. The video on soil organic matter was produced by the Food and Agriculture Organization of the United Nations (FAO). The FAO is a world leader in agriculture and food systems and sets the stage for many international, national and regional programs, policies and developments related to food and agriculture. Navigate the FAO website (http://www.fao.org/home/en/) and write a short paragraph on a story that interested you. 
4. What is the connection between the Healthy Soils Initiative and CalRecycle?
5. More details on the Healthy Soils Initiative are provided o the Healthy Soils Initiative webpage. Navigate to the website and find out which two funding sources are available to California farmers and ranchers to improve soil health.
6. In the bulletin by Sullivan et al. 2019, which management practices are recommended to build soil organic matter? List at least 4 examples. 
7. The soil organic matter levels shown on your standard soil test report are typically determined by the Loss-on-Ignition method (LOI). Describe how this method works in your own words. 
8. A common method for measuring soil organic carbon is the dry combustion method. What is the difference between results from the LOI versus the combustion method? How do results from the LOI and the combustion method relate?
9. How fast can you expect to measure a change in soil organic matter, following a change in management? 
10. How do soil properties affect the capacity of the soil to build soil organic matter?
11. At Cal Poly, we have a combustion analyzer to measure soil organic carbon levels. You take a soil sample from the lemon orchard, and find that the soil organic carbon content is 2.3%. You assume that soil organic matter contains 58% carbon. What is the percent soil organic matter in that soil?
12. Sullivan et al. 2019 briefly discuss the measurement of indicators of active soil organic matter. In your own words, describe why this is important, and list some of the novel measurements that are proposed.

Revisiting some key concepts of the N cycle

Nitrogen can take various forms in the soil. Most of the N in the soil is part of the soil organic matter. Nitrogen contained in soil organic matter is not plant available. Plant available forms of N are ammonium (NH₄⁺) and nitrate (NO₃^-) ions dissolved in the soil solution. During decomposition, N that is tied up in organic molecules is released in the form of NH₄⁺, a process named mineralization. Ammonium can be oxidized to NO₃^- by nitrifiers, and further reduced to nitrogen gas (N₂) by denitrifiers. Nitrification typically occurs when oxygen availability is high, while denitrification occurs at low oxygen availability. During both nitrification and denitrification, nitrous oxide (N₂O), a potent greenhouse gas and ozone-depleting substance, can be produced. Besides denitrification and N₂O emissions, N can be lost from the soil-plant system through NH₃ volatilization, NO₃^- leaching, or runoff. Microorganisms take up NH₄⁺ and NO₃^- from the soil solution to build biomass. This process is referred to as immobilization. The rate of N mineralization vs. immobilization affects the amount of plant available N in the soil. Refresh your memory on the concept of mineralization and immobilization by watching the video below. 

Soil N testing

In agricultural soils, concentrations of NO₃^- are typically an order of magnitude greater compared to concentrations of NH₄⁺. Therefore, agronomists will typically measure soil NO₃^- concentrations as an indicator of plant available N. 

Soil samples for nitrate analysis are either collected near planting time (generally called pre-plant nitrate test; PPNT), or just before the main N application is due (pre-sidedress nitrate test; PSNT). In both cases, samples should be taken at least 3 weeks after the last nitrogen (N) application. 

Nutrient concentrations are typically stratified in the soil, with the highest nutrient concentrations in the top few inches, and lower nutrient concentrations as you go deeper down the soil profile. For optimizing your N fertilizer program, you want your sampling depth the match the expected rooting zone of your crop.

To determine soil N concentrations as shown on soil test reports, soil test labs will typically extract NO₃^- from the soil using a KCl extraction. Subsequently, NO₃^- concentrations are measured using an ion selective electrode or a colorimetric method. 

Pay attention to the units!

Soil N concentrations may be reported as ppm NO₃^-, ppm NO₃^--N, pounds N per acre furrow slice (lbs N/AFS) or pounds N per acre foot (lbs N/AF). Remember, ppm stands for parts per million, on a weight basis. Therefore, ppm N can be interpreted as mg N/kg soil or lbs N/1 000 000 lbs soil.

To convert from ppm NO₃^- to ppm NO₃^--N, you need to divide by the molecular mass of NO₃^- (14 + 3x16 = 62) and multiply by the atomic mass of N (14). I recommend always working with units of N, rather than units of NO₃^-. Units of N will more directly relate to crop requirements and fertilizer inputs. 

To convert from ppm NO₃^--N to lbs N/AFS, you multiply by 2, because there is approximately 2 000 000 lbs of soil/AFS. To convert from ppm NO₃^--N to lbs/AF, you multiply by 4. Note, 1 AFS is an acre of soil, to a depth of 6"; 1 AF is an acre of soil to a depth of 1 foot. If you wish to express your N concentration in lbs N/AF, you should take a soil sample to 1 foot deep. Likewise, the calculation of N concentrations in lbs N/AFS  should be based on the N concentration in a soil sample taken to a depth of 6". 

Interpretation of soil NO₃^- test results

The soil nitrogen concentration on a standard soil test report reflects the amount of NO₃^- present in the soil at the time of the soil sample was taken.   

Nitrate is very mobile and dynamic in soil. Heavy rainfall or application of excess irrigation water after taking soil samples may leach nitrate below the rooting zone or result in denitrification losses, making the N unavailable for the crops. In addition, crop residue management can affect how much N remains in plant available form. In the absence of crop residue management and excessive rainfall or irrigation water application between the time of sampling and the crop stage of major N uptake, the soil nitrate-N can be subtracted from the recommended N fertilization rate. 

It is important to note that the soil nitrate test is not a measure for the amount of N that will be made available by microbial activity during the cropping season. During the growing season, soil microorganisms mineralize soil organic material, constantly replenishing the pool of available N. 

It is important to have sufficient NO₃^- in the soil to support crop demand. However, NO₃^- is easily lost from the soil through leaching or denitrification, upon which it can cause environmental degradation. Therefore, it is important to avoid accumulation of soil NO₃^- concentrations beyond reasonable levels. UC Cooperative extension specialists recommend postponing and adjusting fertilizer N inputs if soil NO₃^- N levels exceed 20 ppm.  

1. Why do leaves turn yellow-ish when corn is grown in a soil that received a straw amendment?
2. Give three examples of management practices that affect immobilization and mineralization rates, and explain why
3. Which N pool is targeted on soil test reports and why?
4. At what time should soil samples be collected for analysis of soil N?
5. What is the best soil sampling depth for N analaysis?
6. A soil has a N concentration of 23 ppm NO₃^--N. How many pounds of NO₃^--N are there in an acre foot. 
7. You have 34 lbs NO₃^--N/AFS. This measure is based on a soil sample taken to a depth of 6". What was the concentration of N in that soil sample, expressed in ppm NO₃^-. (Note, the question asks to calculate the ppm NO₃^-, not NO₃^--N!).
8. Name 3 limitations associated with the standard N test results for planning your crop's N fertility program.
9. You are out in the field and take a field moist soil sample in a clay soil. You perform the nitrate quick test (we will learn how to do a quick nitrate test in the in-person lab activity -> see content for in person lab to convert from ppm NO₃^- on test strip to ppm NO₃^--N in soil), and find a reading of 50ppm on your soil test strip. Will you decide to apply additional fertilizer N to your crop at this time or not? Explain why.

Carbon and nitrogen cycles are tightly coupled. Isotope-based research shows that, even when we match our fertilizer nitrogen input with crop nitrogen requirements, crop plants will only take up about half of their N from the fertilizer applied in the current year. The rest of the N found in the crop comes from N that was present in the soil before fertilization took place. Despite this knowledge, current N rate recommendations are based on N budget approaches that disregard interactions between fertilizer N and SOM. Now, what happens to the fertilizer that wasn't taken up by the plant? Well, some of it may be lost to the environment, and some of it will be immobilized into the soil organic matter. To maintain soil fertility, it is important the maintain or build soil organic matter and the nitrogen contained in it. This illustrates the important link between good fertilizer stewardship and soil health. 

In the video below, the interaction between soil health management and plant nutrition is shown based on a long-term experiment in high value vegetable crops under organic management in the Salinas valley. This video provides a great example of how our understanding of SOM dynamics and the N cycle can greatly improve crop yield. 

You could be one of the pioneers unlocking how to optimize N fertilizer strategies to increase crop yield and reduce environmental pollution, taking into account the complex and fascinating world of soil organic matter dynamics and the many processes involved in the N cycle. 

1. In your own words, explain why dr. Brennan thinks yields were higher in years with a cover crop compared to years when no cover crop was grown? Refer to the processes in the N cycle that are relevant to this finding. 

This part of the lab is to be completed during the in-person activity

Nitrate quick test

Given the dynamic nature of soil NO₃^-, you may want to measure your soil NO3- levels more often than other nutrients on the soil test report. Commercial soil testing labs typically charge around $15 per sample for N analysis alone, and it may take a few days or weeks to get your test results back. The nitrate quick test, developed by Richard Smith and Michael Cahn at the UCCE in Salinas, provides a solution for testing soil NO₃^- levels in the field. While it is generally less accurate compared to the lab test, it can provide an economic and fast alternative for optimizing in-season N management. When you are joining virtually, you can learn how to do a nitrate quick test in 5 minutes!

During the lab, you will have the opportunity to try the quick nitrate test for yourself. 

Procedure

1. Fill a 50 ml plastic centrifuge tube tube to the 30 ml level with the 0.01 molar calcium chloride extracting solution.
2. Add the soil to the tube until the level of the solution rises to 40 ml; cap tightly and shake vigorously until thoroughly dispersed.
3. Let the sample sit for 15 minutes until the soil particles settle out and a clear zone of solution forms at the top of the tube.
4. Dip a EM Quant® nitrate test strip into the clear zone of solution, shake off excess solution, and wait 60 seconds. Compare the color that has developed on the strip with the color chart provided. When the strip color is between two color samples on the chart, interpolate the nitrate concentration of the strip as closely as possible. The strip color will continue to darken with time, so make the determination between 60 and 70 seconds after dipping the strip.

Interpretation of results

The nitrate test strips are calibrated in parts per million (ppm) NO₃^-. A conversion factor is applied to refine the measurement based on soil texture.

Strip reading ÷ correction factor = ppm NO₃^-

Table with correction factors based on soil texture and moisture status of the soil at the time of analysis. 

Next, another calculation step is required to convert ppm NO3- to ppm NO₃^--N. 

ppm NO₃^--N in soil = ppm NO₃^- in soil x 0.22 = _____________

If soil samples were collected at the beginning of the growing season, soil N less than 10 ppm NO₃^--N on a dry soil basis has limited N supply, and fertilization is usually justified. Soils between 10-20 ppm NO₃^--N have enough N to meet immediate plant needs but a modest amount of sidedress N may be appropriate. In soil with NO₃^--N greater than 20 ppm, additional N application should be postponed until retesting shows that residual soil NO₃^--N has declined. In tree crops, leaf tissue N typically guides N fertilization requirements. However, using soil test data in conjunction with leaf tissue analyses can help you further refine your fertilizer program. 

More details on the nitrate quick test and how to interpret results from this test are provided in this CDFA pamphlet. 

In this lab, we will use the nitrate quick test to determine nitrate concentrations in the lemon orchard.

1. What is the soil nitrate concentration in the tree row vs the tractor row in the lemon orchard. 
2. Explain any differences you may find in nitrate levels between the tree vs the tractor row using a systems thinking approach
3. Based on your results for the nitrate levels in the lemon orchard, would you make any modifications to the management?